Neurosteroid hydroxylase CYP7B: vivid reporter activity in dentate gyrus of gene-targeted mice and abolition of a widespread pathway of steroid and oxysterol hydroxylation

27-Hydroxycholesterol acts on estrogen receptor α expressed by POMC neurons in the arcuate nucleus to modulate feeding behavior

Oxysterols are metabolites of cholesterol that regulate cholesterol homeostasis. Among these, the most abundant oxysterol is 27-hydroxycholesterol (27HC), which can cross the blood-brain barrier. Because 27HC functions as an endogenous selective estrogen receptor modulator, we hypothesize that 27HC binds to the estrogen receptor α (ERα) in the brain to regulate energy balance. Supporting this view, we found that delivering 27HC to the brain reduced food intake and activated proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (POMCARH) in an ERα-dependent manner. In addition, we observed that inhibiting brain ERα, deleting ERα in POMC neurons, or chemogenetic inhibition of POMCARH neurons blocked the anorexigenic effects of 27HC. Mechanistically, we further revealed that 27HC stimulates POMCARH neurons by inhibiting the small conductance of the calcium-activated potassium (SK) channel. Together, our findings suggest that 27HC, through its interaction with ERα and modulation of the SK channel, inhibits food intake as a negative feedback mechanism against a surge in circulating cholesterol.

Cholestenoic acid as endogenous epigenetic regulator decreases hepatocyte lipid accumulation in vitro and in vivo

Cholestenoic acid (CA) has been reported as an important biomarker of many severe diseases, but its physiological and pathological roles remain unclear. This study aimed to investigate the potential role of CA in hepatic lipid homeostasis. Enzyme kinetic studies revealed that CA specifically activates DNA methyltransferases 1 (DNMT1) at low concentration with EC50 = 1.99 × 10-6 M and inhibits the activity at higher concentration with IC50 = 9.13 × 10-6 M, and specifically inhibits DNMT3a, and DNMT3b activities with IC50= 8.41 × 10-6 M and IC50= 4.89 × 10-6 M, respectively. In a human hepatocyte in vitro model of high glucose (HG)-induced lipid accumulation, CA significantly increased demethylation of 5mCpG in the promoter regions of over 7,000 genes, particularly those involved in master signaling pathways such as calcium-AMPK and 0.0027 at 6 h. RNA sequencing analysis showed that the downregulated genes are affected by CA encoding key enzymes, such as PCSK9, MVK, and HMGCR, which are involved in cholesterol metabolism and steroid biosynthesis pathways. In addition, untargeted lipidomic analysis showed that CA significantly reduced neutral lipid levels by 60% in the cells cultured in high-glucose media. Administration of CA in mouse metabolic dysfunction-associated steatotic liver disease (MASLD) models significantly decreases lipid accumulation, suppresses the gene expression involved in lipid biosynthesis in liver tissues, and alleviates liver function. This study shows that CA as an endogenous epigenetic regulator decreases lipid accumulation via epigenetic regulation. The results indicate that CA can be considered a potential therapeutic target for the treatment of metabolic disorders.NEW & NOTEWORTHY To our knowledge, this study is the first to identify the mitochondrial monohydroxy bile acid cholestenoic acid (CA) as an endogenous epigenetic regulator that regulates lipid metabolism through epigenome modification in human hepatocytes. The methods used in this study are all big data analysis, and the results of each part show the global regulation of CA on human hepatocytes rather than narrow point effects.

Different effects of CYP27A1 and CYP7B1 on cognitive function: Two mouse models in comparison

The oxysterol 27-hydroxycholesterol (27OHC) is produced by the enzyme sterol 27-hydroxylase (Cyp27A1) and is mainly catabolized to 7α-Hydroxy-3-oxo-4-cholestenoic acid (7-HOCA) by the enzyme cytochrome P-450 oxysterol 7α-hydroxylase (Cyp7B1). 27OHC is mostly produced in the liver and can reach the brain by crossing the blood-brain barrier. A large body of evidence shows that CYP27A1 overexpression and high levels of 27OHC have a detrimental effect on the brain, causing cognitive and synaptic dysfunction together with a decrease in glucose uptake in mice. In this work, we analyzed two mouse models with high levels of 27OHC: Cyp7B1 knock-out mice and CYP27A1 overexpressing mice. Despite the accumulation of 27OHC in both models, Cyp7B1 knock-out mice maintained intact learning and memory capacities, neuronal morphology, and brain glucose uptake over time. Neurons treated with the Cyp7B1 metabolite 7-HOCA did not show changes in synaptic genes and 27OHC-treated Cyp7B1 knock-out neurons could not counteract 27OHC detrimental effects. This suggests that 7-HOCA and Cyp7B1 deletion in neurons do not mediate the neuroprotective effects observed in Cyp7B1 knock-out animals. RNA-seq of neuronal nuclei sorted from Cyp7B1 knock-out brains revealed upregulation of genes likely to confer neuroprotection to these animals. Differently from Cyp7B1 knock-out mice, transcriptomic data from CYP27A1 overexpressing neurons showed significant downregulation of genes associated with synaptic function and several metabolic processes. Our results suggest that the differences observed in the two models may be mediated by the higher levels of Cyp7B1 substrates such as 25-hydroxycholesterol and 3β-Adiol in the knock-out mice and that CYP27A1 overexpressing mice may be a more suitable model for studying 27-OHC-specific signaling. We believe that future studies on Cyp7B1 and Cyp27A1 will contribute to a better understanding of the pathogenic mechanisms of neurodegenerative diseases like Alzheimer's disease and may lead to potential new therapeutic approaches.

Role of Oxysterols in the Activation of the NLRP3 Inflammasome as a Potential Pharmacological Approach in Alzheimer's Disease

Alzheimer's disease (AD), the most prevalent form of dementia, is a complex clinical condition with multifactorial origin posing a major burden to health care systems across the world. Even though the pathophysiological mechanisms underlying the disease are still unclear, both central and peripheral inflammation has been implicated in the process. Piling evidence shows that the nucleotide-binding domain, leucine-rich repeat and pyrin domain-containing protein 3 (NLRP3) inflammasome is activated in AD. As dyslipidemia is a risk factor for dementia, and cholesterol can also activate the inflammasome, a possible link between lipid levels and the NLRP3 inflammasome has been proposed in Alzheimer's. It is also speculated that not only cholesterol but also its metabolites, the oxysterols, may be involved in AD pathology. In this context, mounting data suggest that NLRP3 inflammasome activity can be modulated by different peripheral nuclear receptors, including liver-X receptors, which present oxysterols as endogenous ligands. In light of this, the current review explores whether the activation of NLRP3 by nuclear receptors, mediated by oxysterols, may also be involved in AD and could serve as a potential pharmacological avenue in dementia.

Hippocampal 7α-Hydroxylated Neurosteroids Are Raised by Training and Bolster Remote Spatial Memory with Increase of the Spine Densities

Neuroactive steroids, termed neurosteroids, are synthesized locally in the brain and influence biological functions including cognition and behavior. These neurosteroids are synthesized from cholesterol by a series of cytochrome P450 enzymes, among which a member of P450 hydroxylase, cytochrome P450-7b1 (CYP7B1), catalyzes the formation of 7α-hydroxylated neurosteroids, 7α-hydroxypregnenolone (7α-OH-Preg) and 7α-hydroxydehydroepiandrosterone (7α-OH-DHEA). Here we demonstrated the occurrence of these neurosteroids in the mouse hippocampus after spatial-learning tasks. Cyp7b1 deficiency impaired remote spatial memory with recent memory mostly unaffected. The hippocampal dendritic spine densities were reduced in Cyp7b1-deficient mice, and they were no more increased by the training. Furthermore, chronic intracerebroventricular administration of a mixture of 7α-OH-Preg and 7α-OH-DHEA rescued the deteriorated remote memory performance in Cyp7b1-deficient mice. It is concluded that the 7α-hydroxylated neurosteroids are required for long-term maintenance of spatial memory, and we suggest that these neurosteroids may induce synaptic remodeling to maintain the hippocampal function.

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LC-MS/MS analysis identified 7α-hydroxylated neurosteroids in the mouse hippocampus

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The hippocampal neurosteroids were induced by spatial water maze training

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KO of 7α-hydroxylating enzyme impaired remote memory and hippocampal spine density

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Infusion of the 7α-hydroxylated steroids to the KO rescued impaired remote memory

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

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

The interoceptive hippocampus: Mouse brain endocrine receptor expression highlights a dentate gyrus (DG)-cornu ammonis (CA) challenge-sufficiency axis

The primeval function of the mammalian hippocampus (HPC) remains uncertain. Implicated in learning and memory, spatial navigation, and neuropsychological disorders, evolutionary theory suggests that the HPC evolved from a primeval chemosensory epithelium. Deficits in sensing of internal body status ('interoception') in patients with HPC lesions argue that internal sensing may be conserved in higher vertebrates. We studied the expression patterns in mouse brain of 250 endocrine receptors that respond to blood-borne ligands. Key findings are (i) the proportions and levels of endocrine receptor expression in the HPC are significantly higher than in all other comparable brain regions. (ii) Surprisingly, the distribution of endocrine receptor expression within mouse HPC was found to be highly structured: receptors signaling 'challenge' are segregated in dentate gyrus (DG), whereas those signaling 'sufficiency' are principally found in cornu ammonis (CA) regions. Selective expression of endocrine receptors in the HPC argues that interoception remains a core feature of hippocampal function. Further, we report that ligands of DG receptors predominantly inhibit both synaptic potentiation and neurogenesis, whereas CA receptor ligands conversely promote both synaptic potentiation and neurogenesis. These findings suggest that the hippocampus acts as an integrator of body status, extending its role in context-dependent memory encoding from 'where' and 'when' to 'how I feel'. Implications for anxiety and depression are discussed.

Changes of BMI, steroid metabolome and psychopathology in patients with anorexia nervosa during hospitalization

Anorexia nervosa (AN) is associated with various alterations including the dysfunction of the HPA axis and consequently the hypercortisolemia and deficit in sex hormones but the comprehensive evaluation of changes in circulating steroids during the hospitalization of AN patients is lacking. We investigated the effect of realimentation of women with AN during hospitalization on 45 circulating steroids, the relationships between BMI, its change during hospitalization and physical activity, on one side and initial levels and their changes for two adipokines, circulating steroids, anorexia-specific (hunger, appetite and satiety), and anorexia non-specific symptoms (anxiety, depression fatigue, sleep, and body pain) on the other side. We included 33 women with anorexia who were hospitalized for 38(35, 44) days (median with quartiles). The increase of BMI from the initial value 15.2 (13.2, 16.6) kg/m² was 1.69 (1.37, 2.66) kg/m². The patients with more severe anorexia showed higher activity in 7β-, and 16α-hydroxylation of androgen precursors, which declined during hospitalization. Otherwise, the 7α-hydroxylation activity is higher in AN patients with less severe malnutrition and the ratio of 5-androstene-3β,7α,17β-triol to 5-androstene-3β,7β,17β-triol increased during the realimentation. Our data allow to speculate that the intensive 7β-, and 16α- and possibly also the 7α-hydroxylation of C19 Δ⁵ steroids participate in the pathophysiology of anorexia by additional catabolism of substrates available for synthesis of active androgens and estrogens. However, the question remains whether the synthetic analogues of 7α/β- and 16α-hydroxy-steroids prevent the catabolism of the sex steroid precursors, or further activate the "energy wasting" mitochondrial thermogenic metabolism.

mRNA as a Novel Treatment Strategy for Hereditary Spastic Paraplegia Type 5

Hereditary spastic paraplegia type 5 is a neurodegenerative disease caused by loss-of-function mutations in the CYP7B1 gene encoding the oxysterol 7-α-hydroxylase involved in bile acid synthesis in the liver. Lack of CYP7B1 leads to an accumulation of its oxysterol substrates, in particular 25-hydroxycholesterol and 27-hydroxycholesterol that are able to cross the blood-brain barrier and have neurotoxic properties. A potential therapeutic strategy for SPG5 is the replacement of CYP7B1 by administration of mRNA. Here, we studied the intravenous application of formulated mouse and human CYP7B1 mRNA in mice lacking the endogenous Cyp7b1 gene. A single-dose injection of either mouse or human CYP7B1 mRNA led to a pronounced degradation of oxysterols in liver and serum within 2 days of treatment. Pharmacokinetics indicate a single injection of human CYP7B1 mRNA to be effective in reducing oxysterols for at least 5 days. Repetitive applications of mRNA were safe for at least 17 days and resulted in a significant reduction of neurotoxic oxysterols not only in liver and serum but also to some extent in the brain. Our study highlights the potential to use mRNA as a novel therapy to treat patients with SPG5 disease.

Mitochondrial oxysterol biosynthetic pathway gives evidence for CYP7B1 as controller of regulatory oxysterols

The aim of this paper was to more completely study the mitochondrial CYP27A1 initiated acidic pathway of cholesterol metabolism. The mitochondrial CYP27A1 initiated pathway of cholesterol metabolism (acidic pathway) is known to synthesize two well-described vital regulators of cholesterol/lipid homeostasis, (25R)-26-hydroxycholesterol (26HC) and 25-hydroxycholesterol (25HC). Both 26HC and 25HC have been shown to be subsequently 7α-hydroxylated by Cyp7b1; reducing their regulatory abilities and furthering their metabolism to chenodeoxycholic acid (CDCA). Cholesterol delivery into the inner mitochondria membrane, where CYP27A1 is located, is considered the pathway's only rate-limiting step. To further explore the pathway, we increased cholesterol transport into mitochondrial CYP27A1 by selectively increased expression of the gene encoding the steroidogenic acute transport protein (StarD1). StarD1 overexpression led to an unanticipated marked down-regulation of oxysterol 7α-hydroxylase (Cyp7b1), a marked increase in 26HC, and the formation of a third vital regulatory oxysterol, 24(S)-hydroxycholesterol (24HC), in B6/129 mice livers. To explore the further metabolism of 24HC, as well as, 25HC and 26HC, characterizations of oxysterols and bile acids using three murine models (StarD1 overexpression, Cyp7b1^-/-, Cyp27a1^-/-) and human Hep G2 cells were conducted. This report describes the discovery of a new mitochondrial-initiated pathway of oxysterol/bile acid biosynthesis. Just as importantly, it provides evidence for CYP7B1 as a key regulator of three vital intracellular regulatory oxysterol levels.

Mining for Oxysterols in Cyp7b1-/- Mouse Brain and Plasma: Relevance to Spastic Paraplegia Type 5

Deficiency in cytochrome P450 (CYP) 7B1, also known as oxysterol 7α-hydroxylase, in humans leads to hereditary spastic paraplegia type 5 (SPG5) and in some cases in infants to liver disease. SPG5 is medically characterized by loss of motor neurons in the corticospinal tract. In an effort to gain a better understanding of the fundamental biochemistry of this disorder, we have extended our previous profiling of the oxysterol content of brain and plasma of Cyp7b1 knockout (-/-) mice to include, amongst other sterols, 25-hydroxylated cholesterol metabolites. Although brain cholesterol levels do not differ between wild-type (wt) and knockout mice, we find, using a charge-tagging methodology in combination with liquid chromatography-mass spectrometry (LC-MS) and multistage fragmentation (MSⁿ), that there is a build-up of the CYP7B1 substrate 25-hydroxycholesterol (25-HC) in Cyp7b1-/- mouse brain and plasma. As reported earlier, levels of (25R)26-hydroxycholesterol (26-HC), 3β-hydroxycholest-5-en-(25R)26-oic acid and 24S,25-epoxycholesterol (24S,25-EC) are similarly elevated in brain and plasma. Side-chain oxysterols including 25-HC, 26-HC and 24S,25-EC are known to bind to INSIG (insulin-induced gene) and inhibit the processing of SREBP-2 (sterol regulatory element-binding protein-2) to its active form as a master regulator of cholesterol biosynthesis. We suggest the concentration of cholesterol in brain of the Cyp7b1-/- mouse is maintained by balancing reduced metabolism, as a consequence of a loss in CYP7B1, with reduced biosynthesis. The Cyp7b1-/- mouse does not show a motor defect; whether the defect in humans is a consequence of less efficient homeostasis of cholesterol in brain has yet to be uncovered.

Dehydroepiandrosterone and Its CYP7B1 Metabolite 7α-Hydroxydehydroepiandrosterone Regulates 11β-Hydroxysteroid Dehydrogenase 1 Directions in Rat Leydig Cells

Background: The purpose of this study was to investigate cytochrome P450-7B1 (CYP7B1) in the human and rat testes to regulate 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) activity. We hypothesized that dehydroepiandrosterone (DHEA) and its product 7α-hydroxydehydroepiandrosterone (7αOHD) after catalysis of CYP7B1 played a critical role in driving the direction of 11β-HSD1, because 7αOHD is an alternative substrate for 11β-HSD1. Methods: We examined the influence of DHEA and 7αOHD on 11β-HSD1 activities in both intact Leydig cells and microsomes using radioactive substrates and identified the location of CYP7B1 in Leydig cells using immunohistochemical staining, Western blot, and qPCR. Results: We found that DHEA stimulated 11β-HSD1 oxidase activity in intact cells (EC₅₀ = 0.97 ± 0.11 μM) and inhibited its reductase activity (IC₅₀ = 1.04 ± 0.06 μM). In microsomes, DHEA was a competitive inhibitor of the reductase activity. The 11β-HSD1 oxidase activity in intact cells was inhibited by 7αOHD (IC₅₀ = 1.18 ± 0.12 μM), and the reductase activity was enhanced (EC₅₀ = 0.7 ± 0.04 μM). 7αOHD was a competitive inhibitor of 11β-HSD1 oxidase. CYP7B1 was present in rat Leydig cells, as shown by immunohistochemistry, Western blotting, and qPCR analysis. Conclusion: Our results are consistent with a conclusion that DHEA in the circulation driving 11β-HSD1 toward an oxidase in Leydig cells mainly through inhibiting the reductase of the enzyme, while 7αOHD (CYP7B1 catalytic product of DHEA) drives the enzyme toward the opposite direction.

Mammalian Cells Engineered To Produce New Steroids

Steroids can be difficult to modify through traditional organic synthesis methods, but many enzymes regio- and stereoselectively process a wide variety of steroid substrates. We tested whether steroid-modifying enzymes could make novel steroids from non-native substrates. Numerous genes encoding steroid-modifying enzymes, including some bacterial enzymes, were expressed in mammalian cells by transient transfection and found to be active. We made three unusual steroids by stable expression, in HEK293 cells, of the 7α-hydroxylase CYP7B1, which was selected because of its high native product yield. These cells made 7α,17α-dihydroxypregnenolone and 7β,17α-dihydroxypregnenolone from 17α-hydroxypregnenolone and produced 11α,16α-dihydroxyprogesterone from 16α-hydroxyprogesterone. The last two products were the result of CYP7B1-catalyzed hydroxylation at previously unobserved sites. A Rosetta docking model of CYP7B1 suggested that these substrates' D-ring hydroxy groups might prevent them from binding in the same way as the native substrates, bringing different carbon atoms close to the active ferryl oxygen atom. This new approach could potentially use other enzymes and substrates to produce many novel steroids for drug candidate testing.

Animal models to study bile acid metabolism

The use of animal models, particularly genetically modified mice, continues to play a critical role in studying the relationship between bile acid metabolism and human liver disease. Over the past 20 years, these studies have been instrumental in elucidating the major pathways responsible for bile acid biosynthesis and enterohepatic cycling, and the molecular mechanisms regulating those pathways. This work also revealed bile acid differences between species, particularly in the composition, physicochemical properties, and signaling potential of the bile acid pool. These species differences may limit the ability to translate findings regarding bile acid-related disease processes from mice to humans. In this review, we focus primarily on mouse models and also briefly discuss dietary or surgical models commonly used to study the basic mechanisms underlying bile acid metabolism. Important phenotypic species differences in bile acid metabolism between mice and humans are highlighted.

The Origin of 7α-Hydroxy-Dehydroepiandrosterone and Its Physiological Role: a History of Discoveries

Nearly 60 years has elapsed since the first isolation and identification of 7α-hydroxy-dehydroepiandrosterone, and in that time much information has been gained on its occurrence, metabolism, ontogeny, immunomodulatory activity, cell proliferation, cortisol control in local tissues and neuroactivity. Additional knowledge about this steroid may elucidate its role in obesity, neurodegenerative disturbances such as Alzheimer’s disease, or psychiatric disorders such as schizophrenia or depression. This review aims to provide a comprehensive summary of the available literature on 7α-hydroxy-dehydroepiandrosterone.

Biotransformation of Cholesterol and 16α,17α-Epoxypregnenolone and Isolation of Hydroxylase in Burkholderia cepacia SE-1

The metabolism of cholesterol is critical in eukaryotes as a precursor for vitamins, steroid hormones, and bile acids. Some steroid compounds can be transformed into precursors of steroid medicine by some microorganisms. In this study, the biotransformation products of cholesterol and 16α,17α-epoxypregnenolone produced by Burkholderia cepacia SE-1 were investigated, and a correlative enzyme, hydroxylase, was also studied. The biotransformation products, 7β-hydroxycholesterol, 7-oxocholesterol, and 20-droxyl-16α,17α-epoxypregn-1,4-dien-3-one, were purified by silica gel and Sephadex LH-20 column chromatography and identified by nuclear magnetic resonance and mass spectroscopy. The hydroxylase was isolated from the bacterium and the partial sequences of the hydroxylase, which belong to the catalases/peroxidase family, were analyzed using MS/MS analyses. The enzyme showed activity toward cholesterol and had a specific activity of 37.2 U/mg of protein at 30°C and pH 7.0.

Comparative aspects of neurosteroidogenesis: From fish to mammals

It is now clearly established that the central and peripheral nervous systems have the ability to synthesize de novo steroids referred to as neurosteroids. The major evidence for biosynthesis of neuroactive steroids by nervous tissues is based on the expression of enzymes implicated in the formation of steroids in neural cells. The aim of the present review is to summarize the current knowledge regarding the presence of steroidogenic enzymes in the brain of vertebrates and to highlight the very considerable contribution of Professor Kazuyoshi Tsutsui in this domain. The data indicate that expression of steroid-producing enzymes in the brain appeared early during vertebrate evolution and has been preserved from fish to mammals.

Cholesterol metabolites exported from human brain

The human brain contains approximately 25% of the body's cholesterol. The brain is separated from the circulation by the blood brain barrier. While cholesterol will not passes this barrier, oxygenated forms of cholesterol can cross the barrier. Here by measuring the difference in the oxysterol content of blood plasma in the jugular vein and in a forearm vein by mass spectrometry (MS) we were able to determine the flux of more than 20 cholesterol metabolites between brain and the circulation. We confirm that 24S-hydroxycholesterol is exported from brain at a rate of about 2-3mg/24h. Gas chromatography (GC)-MS data shows that the cholesterol metabolites 5α-hydroxy-6-oxocholesterol (3β,5α-dihydroxycholestan-6-one), 7β-hydroxycholesterol and 7-oxocholesterol, generally considered to be formed through reactive oxygen species, are similarly exported from brain at rates of about 0.1, 2 and 2mg/24h, respectively. Although not to statistical significance both GC-MS and liquid chromatography (LC)-MS methods indicate that (25R)26-hydroxycholesterol is imported to brain, while LC-MS indicates that 7α-hydroxy-3-oxocholest-4-enoic acid is exported from brain.

CYP7B1 Enzyme Deletion Impairs Reproductive Behaviors in Male Mice

In addition to androgenic properties mediated via androgen receptors, dihydrotestosterone (DHT) also regulates estrogenic functions via an alternate pathway. These estrogenic functions of DHT are mediated by its metabolite 5α-androstane-3β, 17β-diol (3β-diol) binding to estrogen receptor β (ERβ). CYP7B1 enzyme converts 3β-diol to inactive 6α- or 7α-triols and plays an important role as a regulator of estrogenic functions mediated by 3β-diol. Using a mutant mouse carrying a null mutation for the CYP7B1 gene (CYP7B1KO), we examined the contribution of CYP7B1 on physiology and behavior. Male, gonadectomized (GDX) CYP7B1KO and their wild type (WT) littermates were assessed for their behavioral phenotype, anxiety-related behavioral measures, and hypothalamic pituitary adrenal axis reactivity. No significant effects of genotype were evident in anxiety-like behaviors in open field (OFA), light-dark (L/D) exploration, and elevated plus maze (EPM). T significantly reduced open arm time on the EPM while not affecting L/D exploratory and OFA behaviors in CYP7B1KO and WT littermates. T also attenuated the corticosterone response to EPM in both genotypes. In GDX animals, T was able to reinstate male-specific reproductive behaviors (latencies and number of mounts, intromission, and ejaculations) in the WT but not in the CYP7B1KO mice. The male reproductive behavior defect in CYP7B1KO seems to be due to their inability to distinguish olfactory cues from a behavioral estrus female. CYP7B1KO mice also showed a reduction in androgen receptor mRNA expression in the olfactory bulb. Our findings suggest a novel role for the CYP7B1 enzyme in the regulation of male reproductive behaviors.

Updated survey of the steroid-converting enzymes in human adipose tissues

Over the past decade, adipose tissues have been increasingly known for their endocrine properties, that is, their ability to secrete a number of adipocytokines that may exert local and/or systemic effects. In addition, adipose tissues have long been recognized as significant sites for steroid hormone transformation and action. We hereby provide an updated survey of the many steroid-converting enzymes that may be detected in human adipose tissues, their activities and potential roles. In addition to the now well-established role of aromatase and 11β-hydroxysteroid dehydrogenase (HSD) type 1, many enzymes have been reported in adipocyte cell lines, isolated mature cells and/or preadipocytes. These include 11β-HSD type 2, 17β-HSDs, 3β-HSD, 5α-reductases, sulfatases and glucuronosyltransferases. Some of these enzymes are postulated to bear relevance for adipose tissue physiology and perhaps for the pathophysiology of obesity. This elaborate set of steroid-converting enzymes in the cell types of adipose tissue deserves further scientific attention. Our work on 20α-HSD (AKR1C1), 3α-HSD type 3 (AKR1C2) and 17β-HSD type 5 (AKR1C3) allowed us to clarify the relevance of these enzymes for some aspects of adipose tissue function. For example, down-regulation of AKR1C2 expression in preadipocytes seems to potentiate the inhibitory action of dihydrotestosterone on adipogenesis in this model. Many additional studies are warranted to assess the impact of intra-adipose steroid hormone conversions on adipose tissue functions and chronic conditions such as obesity, diabetes and cancer.

Cholestenoic acids regulate motor neuron survival via liver X receptors

Cholestenoic acids are formed as intermediates in metabolism of cholesterol to bile acids, and the biosynthetic enzymes that generate cholestenoic acids are expressed in the mammalian CNS. Here, we evaluated the cholestenoic acid profile of mammalian cerebrospinal fluid (CSF) and determined that specific cholestenoic acids activate the liver X receptors (LXRs), enhance islet-1 expression in zebrafish, and increase the number of oculomotor neurons in the developing mouse in vitro and in vivo. While 3β,7α-dihydroxycholest-5-en-26-oic acid (3β,7α-diHCA) promoted motor neuron survival in an LXR-dependent manner, 3β-hydroxy-7-oxocholest-5-en-26-oic acid (3βH,7O-CA) promoted maturation of precursors into islet-1+ cells. Unlike 3β,7α-diHCA and 3βH,7O-CA, 3β-hydroxycholest-5-en-26-oic acid (3β-HCA) caused motor neuron cell loss in mice. Mutations in CYP7B1 or CYP27A1, which encode enzymes involved in cholestenoic acid metabolism, result in different neurological diseases, hereditary spastic paresis type 5 (SPG5) and cerebrotendinous xanthomatosis (CTX), respectively. SPG5 is characterized by spastic paresis, and similar symptoms may occur in CTX. Analysis of CSF and plasma from patients with SPG5 revealed an excess of the toxic LXR ligand, 3β-HCA, while patients with CTX and SPG5 exhibited low levels of the survival-promoting LXR ligand 3β,7α-diHCA. Moreover, 3β,7α-diHCA prevented the loss of motor neurons induced by 3β-HCA in the developing mouse midbrain in vivo.Our results indicate that specific cholestenoic acids selectively work on motor neurons, via LXR, to regulate the balance between survival and death.

24S,25-Epoxycholesterol in mouse and rat brain

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24S,25-Epoxycholesterol identified and quantified in rodent brain.

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Knock out of Cyp27a1 leads to a decrease in 24S,25-epoxycholesterol.

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Knock out of Cyp7b1 leads to an increase in 24S,25-epoxycholesterol.

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24S,25-Epoxycholesterol is metabolised by Cyp7b1 but not Cyp27a1.

24S,25-Epoxycholesterol is formed in a shunt of the mevalonate pathway that produces cholesterol. It is one of the most potent known activators of the liver X receptors and can inhibit sterol regulatory element-binding protein processing. Until recently analysis of 24S,25-epoxycholesterol at high sensitivity has been precluded by its thermal lability and lack of a strong chromophore. Here we report on the analysis of 24S,25-epoxycholesterol in rodent brain where its level was determined to be of the order of 0.4–1.4 μg/g wet weight in both adult mouse and rat. For comparison the level of 24S-hydroxycholesterol in brain of both rodents was of the order of 20 μg/g, while that of cholesterol in mouse was 10–20 mg/g. By exploiting knockout mice for the enzyme oxysterol 7α-hydroxylase (Cyp7b1) we show that this enzymes is important for the subsequent metabolism of the 24S,25-epoxide.

Atherosclerosis and Alzheimer--diseases with a common cause? Inflammation, oxysterols, vasculature

BACKGROUND

Aging is accompanied by increasing vulnerability to pathologies such as atherosclerosis (ATH) and Alzheimer disease (AD). Are these different pathologies, or different presentations with a similar underlying pathoetiology?

DISCUSSION

Both ATH and AD involve inflammation, macrophage infiltration, and occlusion of the vasculature. Allelic variants in common genes including APOE predispose to both diseases. In both there is strong evidence of disease association with viral and bacterial pathogens including herpes simplex and Chlamydophila. Furthermore, ablation of components of the immune system (or of bone marrow-derived macrophages alone) in animal models restricts disease development in both cases, arguing that both are accentuated by inflammatory/immune pathways. We discuss that amyloid β, a distinguishing feature of AD, also plays a key role in ATH. Several drugs, at least in mouse models, are effective in preventing the development of both ATH and AD. Given similar age-dependence, genetic underpinnings, involvement of the vasculature, association with infection, Aβ involvement, the central role of macrophages, and drug overlap, we conclude that the two conditions reflect different manifestations of a common pathoetiology.

MECHANISM

Infection and inflammation selectively induce the expression of cholesterol 25-hydroxylase (CH25H). Acutely, the production of 'immunosterol' 25-hydroxycholesterol (25OHC) defends against enveloped viruses. We present evidence that chronic macrophage CH25H upregulation leads to catalyzed esterification of sterols via 25OHC-driven allosteric activation of ACAT (acyl-CoA cholesterol acyltransferase/SOAT), intracellular accumulation of cholesteryl esters and lipid droplets, vascular occlusion, and overt disease.

SUMMARY

We postulate that AD and ATH are both caused by chronic immunologic challenge that induces CH25H expression and protection against particular infectious agents, but at the expense of longer-term pathology.

7α-hydroxylation of dehydroepiandrosterone does not interfere with the activation of glucocorticoids by 11β-hydroxysteroid dehydrogenase in E(t)C cerebellar neurons

The neuroprotective action of dehydroepiandrosterone (DHEA) in the absence of a known specific receptor has been attributed to its metabolism by different cell types in the brain to various steroids, with a preference to its 7-hydroxylated products. The E(t)C cerebellar granule cell line converts DHEA almost exclusively to 7α-hydroxy-DHEA (7α-OH-DHEA). It has been postulated that DHEA's 7-OH and 7-oxo metabolites can decrease glucocorticoid levels by an interactive mechanism involving 11β-hydroxysteroid dehydrogenase (11β-HSD). In order to study the relationship of 7-hydroxylation of DHEA and glucocorticoid metabolism in intact brain cells, we examined whether E(t)C cerebellar neurons, which are avid producers of 7α-OH-DHEA, could also metabolize glucocorticoids. We report that E(t)C neuronal cells exhibit 11β-HSD1 reductase activity, and are able to convert 11-dehydrocorticosterone into corticosterone, whereas they do not demonstrate 11β-HSD2 dehydrogenase activity. Consequently, E(t)C cells incubated with DHEA did not yield 7-oxo- or 7β-OH-DHEA. Our findings are supported by the reductive environment of E(t)C cells through expression of hexose-6-phosphate dehydrogenase (H6PDH), which fosters 11β-HSD1 reductase activity. To further explore the role of 7α-OH-DHEA in E(t)C neuronal cells, we examined the effect of preventing its formation using the CYP450 inhibitor ketoconazole. Treatment of the cells with this drug decreased the yield of 7α-OH-DHEA by about 75% without the formation of alternate DHEA metabolites, and had minimal effects on glucocorticoid conversion. Likewise, elevated levels of corticosterone, the product of 11β-HSD1, had no effect on the metabolic profile of DHEA. This study shows that in a single population of whole-cells, with a highly reductive environment, 7α-OH-DHEA is unable to block the reducing activity of 11β-HSD1, and that 7-hydroxylation of DHEA does not interfere with the activation of glucocorticoids. Our investigation on the metabolism of DHEA in E(t)C neuronal cells suggest that other alternate mechanisms must be at play to explain the in vivo anti-glucocorticoid properties of DHEA and its 7-OH-metabolites.

Brain and pineal 7α-hydroxypregnenolone stimulating locomotor activity: identification, mode of action and regulation of biosynthesis

Biologically active steroids synthesized in the central and peripheral nervous systems are termed neurosteroids. However, the biosynthetic pathways leading to the formation of neurosteroids are still incompletely elucidated. 7α-Hydroxypregnenolone, a novel bioactive neurosteroid stimulating locomotor activity, has been recently identified in the brain of newts and quail. Subsequently, the mode of action and regulation of biosynthesis of 7α-hydroxypregnenolone have been determined. Moreover, recent studies on birds have demonstrated that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol. 7α-Hydroxypregnenolone is a major pineal neurosteroid that stimulates locomotor activity in juvenile chickens, connecting light-induced gene expression with locomotion. This review summarizes the advances in our understanding of the identification, mode of action and regulation of biosynthesis of brain and pineal 7α-hydroxypregnenolone, a potent stimulator of locomotor activity.

Cytochromes p450: roles in diseases

The cytochrome P450 superfamily consists of a large number of heme-containing monooxygenases. Many human P450s metabolize drugs used to treat human diseases. Others are necessary for synthesis of endogenous compounds essential for human physiology. In some instances, alterations in specific P450s affect the biological processes that they mediate and lead to a disease. In this minireview, we describe medically significant human P450s (from families 2, 4, 7, 11, 17, 19, 21, 24, 27, 46, and 51) and the diseases associated with these P450s.

EBI2-mediated bridging channel positioning supports splenic dendritic cell homeostasis and particulate antigen capture

Splenic dendritic cells (DCs) present blood-borne antigens to lymphocytes to promote T cell and antibody responses. The cues involved in positioning DCs in areas of antigen exposure in the spleen are undefined. Here we show that CD4⁺ DCs highly express EBI2 and migrate to its oxysterol ligand, 7α,25-OHC. In mice lacking EBI2 or the enzymes needed for generating normal distributions of 7α,25-OHC, CD4⁺ DCs are reduced in frequency and the remaining cells fail to situate in marginal zone bridging channels. The CD4⁺ DC deficiency can be rescued by LTβR agonism. EBI2-mediated positioning in bridging channels promotes DC encounter with blood-borne particulate antigen. Upon exposure to antigen, CD4⁺ DCs move rapidly to the T-B zone interface and promote induction of helper T cell and antibody responses. These findings establish an essential role for EBI2 in CD4⁺ DC positioning and homeostasis and in facilitating capture and presentation of blood-borne particulate antigens.

DOI:http://dx.doi.org/10.7554/eLife.00757.001

One of the main roles of the spleen is to make the antibodies that protect the body against viruses, bacteria and other microorganisms. Antibodies are made by B cells, which are a type of white blood cell, after they have been exposed to antigens. For most antibody responses, it is also necessary for the B cells to get help from other white blood cells called T cells that have been exposed to antigens. Specialized cells called dendritic cells have a central role in bringing the antigens—which are usually fragments of the infectious agents that have invaded the body—to the T cells.

One subset of dendritic cells, called CD4⁺ dendritic cells, are found in large numbers in a part of the spleen called the bridging channel, but the process by which these cells become localized in this channel has not been fully understood. Now, Yi and Cyster show that a receptor called EBI2, which is found on the surface of CD4⁺ dendritic cells, binds to a type of organic molecule called an oxysterol that is produced in the bridging channel.

In mice that had been genetically engineered to lack EBI2 or the enzymes needed to make this particular oxysterol—which is known as 7α,25-dihydroxycholesterol, or 7α,25-OHC for short—the CD4⁺ dendritic cells were no longer clustered in the bridging channel and their number was markedly decreased. This showed that the interaction between EBI2 and the oxysterol was essential for ensuring that the CD4⁺ dendritic cells were in the right place. The correct positioning of the CD4⁺ dendritic cells was, in turn, necessary for maintaining cell numbers. Moreover, these mice had a weakened immune response because of the very low number of antigens that were being presented to the T cells.

A number of autoimmune diseases, such as lupus, are caused by the body developing an immune response to its own cells and tissues. One implication of the work of Yi and Cyster is that if small molecule inhibitors of EBI2 could be designed, they might be able to suppress the onset of such autoimmune responses.

DOI:http://dx.doi.org/10.7554/eLife.00757.002

Oxysterol gradient generation by lymphoid stromal cells guides activated B cell movement during humoral responses

7α,25-dihydroxycholesterol (7α,25-OHC) is a ligand for the G protein-coupled receptor EBI2; however, the cellular sources of this oxysterol are undefined. 7α,25-OHC is synthesized from cholesterol by the stepwise actions of two enzymes, CH25H and CYP7B1, and is metabolized to a 3-oxo derivative by HSD3B7. We showed that all three enzymes control EBI2 ligand concentration in lymphoid tissues. Lymphoid stromal cells were the main CH25H- and CYP7B1-expressing cells required for positioning of B cells, and they also mediated 7α,25-OHC inactivation. CH25H and CYP7B1 were abundant at the follicle perimeter, whereas CH25H expression by follicular dendritic cells was repressed. CYP7B1, CH25H, and HSD3B7 deficiencies each resulted in defective T cell-dependent plasma cell responses. These findings establish that CYP7B1 and HSD3B7, as well as CH25H, have essential roles in controlling oxysterol production in lymphoid tissues, and they suggest that differential enzyme expression in stromal cell subsets establishes 7α,25-OHC gradients required for B cell responses.

Neurobehavioral abnormalities in a brain-specific NADPH-cytochrome P450 reductase knockout mouse model

The aim of the present study was to test a new hypothesis that brain cytochrome P450 reductase (CPR) and CPR-dependent enzymes play important roles in behavioral performance. A mouse model with brain neuron-specific deletion of the Cpr gene (brain-Cpr-null) was recently generated. Brain-Cpr-null mice and wild-type (WT) littermates were compared in a variety of behavioral assays. Notable differences were found in the exploratory behavior assay: for both males and females, activity in the center of the chamber was significantly higher for brain-Cpr-null than for WT mice on days 2 and 3 of the assay, although no significant difference was found between the two groups in anxiety-like behavior in the elevated zero maze. Furthermore, in the fear-conditioning assay, brain-Cpr-null mice exhibited significantly less activity suppression than did WT controls. This deficit in activity suppression was not accompanied by any difference between WT and brain-Cpr-null mice in nociceptive responses to foot shocks. Abnormal activity suppression was also observed in both male and female brain-Cpr-null mice during the contextual memory test. However, in the Morris water maze assay, the brain-Cpr-null and WT mice were indistinguishable, indicating normal spatial memory in the mutant mice. These data collectively indicate a novel role of the Cpr gene in fear conditioning and memory.

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

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

The 7-hydroxylation of dehydroepiandrosterone in rat brain

Dehydroepiandrosterone (DHEA) is an important neurosteroid with multiple functions in the central nervous system including neuroprotection. How DHEA exerts its neuroprotection function has not been fully elucidated. One possible mechanism is via its active metabolites, 7alpha-OH DHEA and 7beta-OH DHEA. The purpose of this research is to understand how DHEA is metabolized to 7alpha-OH DHEA and 7beta-OH DHEA by brain tissue. DHEA was incubated with rat brain microsomes and mitochondria and the 7alpha-OH DHEA and 7beta-OH DHEA formed by these fractions were analyzed by LC/MS. For the first time, we observed that DHEA could be metabolized to 7alpha-OH DHEA and 7beta-OH DHEA in mitochondria but the formation of 7alpha-OH DHEA and 7beta-OH DHEA demonstrated different enzymatic kinetic properties. Adding NADPH, an essential cofactor, to mitochondria incubation mixtures increased only the formation of 7alpha-OH DHEA, but not that of 7beta-OH DHEA. Addition of estradiol to the incubation mixtures inhibited only the formation of 7alpha-OH DHEA, but not that of 7beta-OH DHEA. Western blot analysis showed that both microsomes and mitochondria contained cytochrome P450 7B. We also found that 7alpha-OH DHEA could be converted to 7beta-OH DHEA by rat brain homogenates. Our data suggest that 7alpha-OH DHEA and 7beta-OH DHEA are formed by different enzymes and that 7beta-OH DHEA can be formed from both DHEA and 7alpha-OH DHEA, although the overall level of 7beta-OH DHEA was very low.

CYP7B1: one cytochrome P450, two human genetic diseases, and multiple physiological functions

The CYP7B1 cytochrome P450 enzyme hydroxylates carbons 6 and 7 of the B ring of oxysterols and steroids. Hydroxylation reduces the biological activity of these substrates and facilitates their conversion to end products that are readily excreted from the body. CYP7B1 is expressed in the liver, reproductive tract, and brain and performs different physiological functions in each tissue. Hepatic CYP7B1 activity is crucial for the inactivation of oxysterols and their subsequent conversion into bile salts. Loss of CYP7B1 activity is associated with liver failure in children. In the reproductive tract, the enzyme metabolizes androgens that antagonize estrogen action; mice without CYP7B1 have abnormal prostates and ovaries. The role of CYP7B1 in brain is under investigation; recent studies show that spastic paraplegia type 5, a progressive neuropathy, is caused by loss-of-function mutations in the human gene.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Analysis of neurosterols by GC-MS and LC-MS/MS

The term neurosteroid was coined by Baulieu and colleagues in Paris towards the end of the last century to describe steroids which are synthesised in the central or peripheral nervous system [E.E. Baulieu, Psychoneuroendocrinology 23 (1998) 963-87]. This definition was restricted to side-chain "shortened" steroids with 21 carbon atoms or less, and excluded sterols and their carboxylic acids with an intact side-chain. By analogy, we now use the term neurosterol to describe C(27) sterols synthesised in the nervous system. In this review we discuss the biological importance of neurosterols, and how they are extracted, isolated, and analysed by GC-MS and LC-MS/MS, from brain and relevant body fluids. We present applications of methodology employed for analysis of specific sterols and comment on the relative merits of the methods employed. Finally, the importance of future in-depth "sterolomic" investigations of brain is highlighted.

Targeted lipidomic analysis of oxysterols in the embryonic central nervous system

In this study two regions of embryonic (E11) mouse central nervous system (CNS) have been profiled for their unesterified sterol content. Using high-performance liquid chromatography (HPLC)-mass spectrometry (MS) and tandem mass spectrometry (MS(n)) low levels of oxysterols (estimated 2-165 ng g(-1) wet weight) were identified in cortex (Ctx) and spinal cord (Sc). The identified oxysterols include 7 alpha-, 7 beta-, 22R-, 24S-, 25- and 27-hydroxycholesterol; 24,25- and 24,27-dihydroxycholesterol; and 24S,25-epoxycholesterol. Of these, 24S-hydroxycholesterol is biosynthesised exclusively in brain. In comparison to adult mouse where the 24S-hydroxycholesterol level is about 40 microg g(-1) in brain the level of 24S-hydroxycholesterol reported here (estimated 26 ng g(-1) in Ctx and 13 ng g(-1) in Sc) is extremely low. Interestingly, the level of 24S,25-epoxycholesterol in both CNS regions (estimated 165 ng g(-1) in Ctx and 91 ng g(-1) in Sc) is somewhat higher than the levels of the hydroxycholesterols. This oxysterol is formed in parallel to cholesterol via a shunt of the mevalonate pathway and its comparatively high abundance may be a reflection of a high rate of cholesterol synthesis at this stage of development. Levels of cholesterol (estimated 1.25 mg g(-1) in Ctx and 1.15 mg g(-1) in Sc) and its precursors were determined by gas chromatography-mass spectrometry (GC-MS). In both CNS regions cholesterol levels were found to be lower than those reported in the adult, but in relation to cholesterol the levels of cholesterol precursors were higher than found in adult indicating a high rate of cholesterol synthesis. In summary, our data provide evidence for the presence of endogenous oxysterols in two brain regions of the developing CNS. Moreover, while most of the enzymes involved in hydroxysterol synthesis are minimally active at E11, our results suggest that the mevalonate pathway is significantly active, opening up the possibility for a function of 24S,25-epoxycholesterol during brain development.

Discovering oxysterols in plasma: a window on the metabolome

While the proteome defines the expressed gene products, the metabolome results from reactions controlled by such gene products. Plasma represents an accessible "window" to the metabolome both in regard of availability and content. The wide range of the plasma metabolome, in terms of molecular diversity and abundance, makes its comprehensive analysis challenging. Here we demonstrate an analytical method designed to target one region of the metabolome, that is, oxysterols. Since the discovery of their biological activity as ligands to nuclear receptors there has been a reawakening of interest in oxysterols and their analysis. In addition, the oxysterols, 24S- and 27-hydroxycholesterol, are currently under investigation as potential biomarkers associated with neurodegenerative disorders such as Alzheimer's disease and multiple sclerosis; widespread analysis of these lipids in clinical studies will require the development of robust, sensitive and rapid analytical techniques. In this communication we present results of an investigation of the oxysterols content of human plasma using a newly developed high-performance liquid chromatography-mass spectrometry (HPLC-MS) method incorporating charge-tagging and high-resolution MS. The method has allowed the identification in plasma of monohydroxylated cholesterol molecules, 7alpha-, 24S-, and 27-hydroxycholesterol; the cholestenetriol 7alpha,27-dihydroxycholesterol; and 3beta-hydroxycholest-5-en-27-oic acid and its metabolite 3beta,7alpha-dihydroxycholest-5-en-27-oic acid. The methodology described is also applicable for the analysis of other sterols in plasma, that is, cholesterol, 7-dehydrocholesterol, and desmosterol, as well as cholesterol 5,6- seco-sterols and steroid hormones. Although involving derivatization, sample preparation is straightforward and chromatographic analysis rapid (17 min), while the MS method offers high sensitivity (ng/mL of sterol in plasma, or pg on-column) and specificity. The methodology is suitable for targeted metabolomic analysis of sterols, oxysterols, and steroid hormones opening a "window" to view this region of the metabolome.

Hexose-6-phosphate dehydrogenase modulates 11beta-hydroxysteroid dehydrogenase type 1-dependent metabolism of 7-keto- and 7beta-hydroxy-neurosteroids

BACKGROUND

The role of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) in the regulation of energy metabolism and immune system by locally reactivating glucocorticoids has been extensively studied. Experiments determining initial rates of enzyme activity revealed that 11beta-HSD1 can catalyze both the reductase and the dehydrogenase reaction in cell lysates, whereas it predominantly catalyzes the reduction of cortisone to cortisol in intact cells that also express hexose-6-phosphate dehydrogenase (H6PDH), which provides cofactor NADPH. Besides its role in glucocorticoid metabolism, there is evidence that 11beta-HSD1 is involved in the metabolism of 7-keto- and 7-hydroxy-steroids; however the impact of H6PDH on this alternative function of 11beta-HSD1 has not been assessed.

METHODOLOGY

We investigated the 11beta-HSD1-dependent metabolism of the neurosteroids 7-keto-, 7alpha-hydroxy- and 7beta-hydroxy-dehydroepiandrosterone (DHEA) and 7-keto- and 7beta-hydroxy-pregnenolone, respectively, in the absence or presence of H6PDH in intact cells. 3D-structural modeling was applied to study the binding of ligands in 11beta-HSD1.

PRINCIPAL FINDINGS

We demonstrated that 11beta-HSD1 functions in a reversible way and efficiently catalyzed the interconversion of these 7-keto- and 7-hydroxy-neurosteroids in intact cells. In the presence of H6PDH, 11beta-HSD1 predominantly converted 7-keto-DHEA and 7-ketopregnenolone into their corresponding 7beta-hydroxy metabolites, indicating a role for H6PDH and 11beta-HSD1 in the local generation of 7beta-hydroxy-neurosteroids. 3D-structural modeling offered an explanation for the preferred formation of 7beta-hydroxy-neurosteroids.

CONCLUSIONS

Our results from experiments determining the steady state concentrations of glucocorticoids or 7-oxygenated neurosteroids suggested that the equilibrium between cortisone and cortisol and between 7-keto- and 7-hydroxy-neurosteroids is regulated by 11beta-HSD1 and greatly depends on the coexpression with H6PDH. Thus, the impact of H6PDH on 11beta-HSD1 activity has to be considered for understanding both glucocorticoid and neurosteroid action in different tissues.

Dehydroepiandrosterone metabolites and their interactions in humans

Dehydroepiandrosterone (DHEA) is 7alpha-hydroxylated by the cytochrome P4507B1 in the liver, skin and brain, which are targets for glucocorticoids. 7alpha-Hydroxy-DHEA produced anti-glucocorticoid effects in vivo but the interference between the glucocorticoid hormone binding with its receptor could not be determined. In the organs mentioned above, circulating inactive cortisone is reduced to active cortisol by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1). 7alpha-Hydroxy-DHEA is also a substrate for this enzyme. Studies of 11beta-HSD1 action on 7alpha-hydroxy-DHEA show the reversible production of 7beta-hydroxy-DHEA through an intermediary 7-oxo-DHEA. Both the production of 7alpha-hydroxysteroids and their interference with the activation of cortisone into cortisol are basic to the concept of native anti-glucocorticoids. The cytochrome P4507B1 responsible for 7alpha-hydroxy-DHEA production and 11beta-HSD1 are key enzymes for the modulation of glucocorticoid action in humans. This is a promising new area for research.

Liquid chromatography-mass spectrometry utilizing multi-stage fragmentation for the identification of oxysterols

In humans, the brain accounts for about 20% of the body's free cholesterol, most of which is synthesized de novo in brain. To maintain cholesterol balance throughout life, cholesterol becomes metabolized to 24S-hydroxycholesterol, principally in neurons. In mouse, rat, and probably human, metabolism to 24S-hydroxycholesterol accounts for about 50% of cholesterol turnover; however, the route by which the remainder is turned over has yet to be elucidated. Here, we describe a novel liquid chromatography (LC) multi-stage fragmentation mass spectrometry (MS(n)) methodology for the identification, with high sensitivity (low pg), of cholesterol metabolites in rat brain. The methodology includes derivatization to enhance ionization, exact mass analysis at high resolution to identify potential metabolites, and LC-MS(n) (n=3) to allow their characterization. 24S-hydroxycholesterol was confirmed as a major oxysterol in rat brain, and other oxysterols identified for the first time in brain included 24,25-, 24,27-, 25,27-, 6,24,- 7alpha,25-, and 7alpha,27-dihydroxycholesterols. In addition, 3beta-hydroxy-5-oxo-5,6-secocholestan-6-al and its aldol, two molecules linked to amyloidogenesis of proteins, were characterized in rat brain.

Central administration of a cytochrome P450-7B product 7 alpha-hydroxypregnenolone improves spatial memory retention in cognitively impaired aged rats

Pregnenolone (PREG) and dehydroepiandrosterone (DHEA) have been reported to improve memory in aged rodents. In brain, these neurosteroids are transformed predominantly into 7alpha-hydroxylated metabolites by the cytochrome P450-7B1 (CYP7B). The biological role of steroid B-ring hydroxylation is unclear. It has been proposed to generate bioactive derivatives that enhance cognition, immune, and other physiological processes. In support, 7alpha-hydroxylated DHEA increases the immune response in mice with greater potency than the parent steroid. Whether the memory-enhancing effects of PREG in rats is mediated via its 7alpha-hydroxylated metabolite 7alpha-hydroxyPREG is not known. We investigated this by treating memory-impaired aged rats (identified by their spatial memory performances in the Morris water maze task compared with young controls) with 7alpha-hydroxyPREG or PREG administered intracerebroventricularly using osmotic minipumps and then tested the rats during week 2 of steroid treatment in the eight-arm radial-arm version of the water maze (RAWM) that allows repeated assessment of learning. CYP7B bioactivity in hippocampal tissue (percentage conversion of [14C]DHEA to [14C]7alpha-hydroxyDHEA) was decreased selectively in memory-impaired aged rats compared with both young and memory-intact aged rats. 7alpha-hydroxyPREG (100 ng/h) but not PREG (100 ng/h) administration to memory-impaired aged rats for 11 d enhanced spatial memory retention (after a 30 min delay between an exposure trial 1 and test trial 2) in the RAWM. These data provide evidence for a biologically active enzyme product 7alpha-hydroxyPREG and suggests that reduced CYP7B function in the hippocampus of memory-impaired aged rats may, in part, be overcome by administration of 7alpha-hydroxyPREG.

Analysis of neurosterols and neurosteroids by mass spectrometry

In man the brain represents about 2% of the body weight, but contains 25% of the body's cholesterol. Cholesterol itself does not cross the blood-brain barrier and is synthesised in situ. Excess cholesterol from brain is exported in the form of oxysterols, or metabolised to steroids, which in contrast to cholesterol can cross the blood-brain barrier. Steroids and oxysterols may be synthesised in brain, but can also be transported into brain from peripheral tissue. Both oxysterols and steroids have biological activity in brain. They can behave as ligands for classical nuclear receptors, and exert their effects over hours to days, or interact with neurotransmitter gated ion channels and modulate neural transmission exerting their effects in milliseconds. The exact sterol and steroid content of brain has yet to be thoroughly characterised. In this mini-review we will discuss mass spectrometry methods for the analysis of steroids and sterols in brain, and propose methods suitable for the profiling of different brain regions with high sensitivity (sub pg) and specificity.

Up-Regulated PAR-2-Mediated Salivary Secretion in Mice Deficient in Muscarinic Acetylcholine Receptor Subtypes

Protease-activated receptor-2 (PAR-2) is expressed in the salivary glands and is expected to be a new target for the treatment of exocrine dysfunctions, such as dry mouth; however, the salivary secretory mechanism mediated by PAR-2 remains to be elucidated. Therefore, mechanism of the PAR-2-mediated salivary secretion was investigated in this study. We found that a PAR-2 agonist peptide, SLIGRL-OH, induced salivary flow in vivo and dose-dependent increase in [Ca(2+)](i) submandibular gland (SMG) acinar cells in wild-type (WT) mice and mice lacking M(3) or both M(1) and M(3) muscarinic acetylcholine receptors (mAChRs), whereas secretions in PAR-2 knockout (PAR-2KO) mice were completely abolished. The saliva composition secreted by SLIGRL-OH was similar to that secreted by mAChR stimulation. Ca(2+) imaging in WT acinar cells and beta-galactosidase staining in PAR-2KO mice, in which the beta-galactosidase gene (LacZ) was incorporated into the disrupted gene, revealed a nonubiquitous, sporadic distribution of PAR-2 in the SMG. Furthermore, compared with the secretion in WT mice, PAR-2-mediated salivary secretion and Ca(2+) response were enhanced in mice lacking M(3) or both M(1) and M(3) mAChRs, in which mAChR-stimulated secretion and Ca(2+) response in acinar cells were severely impaired. Although the mechanism underlying the enhanced PAR-2-mediated salivary secretion in M(3)-deficient mice is not clear, the result suggests the presence of some compensatory mechanism involving PAR-2 in the salivary glands deficient in cholinergic activation. These results indicate that PAR-2 present in the salivary glands mediates Ca(2+)-dependent fluid secretion, demonstrating potential usefulness of PAR-2 as a target for dry mouth treatment.

Convergence of genes implicated in Alzheimer's disease on the cerebral cholesterol shuttle: APP, cholesterol, lipoproteins, and atherosclerosis

Polymorphic genes associated with Alzheimer's disease (see ) delineate a clearly defined pathway related to cerebral and peripheral cholesterol and lipoprotein homoeostasis. They include all of the key components of a glia/neurone cholesterol shuttle including cholesterol binding lipoproteins APOA1, APOA4, APOC1, APOC2, APOC3, APOD, APOE and LPA, cholesterol transporters ABCA1, ABCA2, lipoprotein receptors LDLR, LRP1, LRP8 and VLDLR, and the cholesterol metabolising enzymes CYP46A1 and CH25H, whose oxysterol products activate the liver X receptor NR1H2 and are metabolised to esters by SOAT1. LIPA metabolises cholesterol esters, which are transported by the cholesteryl ester transport protein CETP. The transcription factor SREBF1 controls the expression of most enzymes of cholesterol synthesis. APP is involved in this shuttle as it metabolises cholesterol to 7-betahydroxycholesterol, a substrate of SOAT1 and HSD11B1, binds to APOE and is tethered to LRP1 via APPB1, APBB2 and APBB3 at the cytoplasmic domain and via LRPAP1 at the extracellular domain. APP cleavage products are also able to prevent cholesterol binding to APOE. BACE cleaves both APP and LRP1. Gamma-secretase (PSEN1, PSEN2, NCSTN) cleaves LRP1 and LRP8 as well as APP and their degradation products control transcription factor TFCP2, which regulates thymidylate synthase (TS) and GSK3B expression. GSK3B is known to phosphorylate the microtubule protein tau (MAPT). Dysfunction of this cascade, carved out by genes implicated in Alzheimer's disease, may play a major role in its pathology. Many other genes associated with Alzheimer's disease affect cholesterol or lipoprotein function and/or have also been implicated in atherosclerosis, a feature of Alzheimer's disease, and this duality may well explain the close links between vascular and cerebral pathology in Alzheimer's disease. The definition of many of these genes as risk factors is highly contested. However, when polymorphic susceptibility genes belong to the same signaling pathway, the risk associated with multigenic disease is better related to the integrated effects of multiple polymorphisms of genes within the same pathway than to variants in any single gene [Wu, X., Gu, J., Grossman, H.B., Amos, C.I., Etzel, C., Huang, M., Zhang, Q., Millikan, R.E., Lerner, S., Dinney, C.P., Spitz, M.R., 2006. Bladder cancer predisposition: a multigenic approach to DNA-repair and cell-cycle-control genes. Am. J. Hum. Genet. 78, 464-479.]. Thus, the fact that Alzheimer's disease susceptibility genes converge on a clearly defined signaling network has important implications for genetic association studies.

The native anti-glucocorticoid paradigm

Circulating 3beta-hydroxysteroids including dehydroepiandrosterone (DHEA) are 7alpha-hydroxylated by the cytochrome P450-7B1 in the liver, skin and brain, which are the target organs of glucocorticoids. Anti-glucocorticoid effects with 7alpha-hydroxy-DHEA were observed in vivo without an interference with glucocorticoid binding to its receptor. In the organs mentioned above, the circulating inactive cortisone was reduced into active cortisol by the 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1). We demonstrated that 7alpha-hydroxy-DHEA was also a substrate for this enzyme. Studies of the 11beta-HSD1 action on 7alpha-hydroxy-DHEA showed the reversible production of 7beta-hydroxy-DHEA through an intermediary 7-oxo-DHEA, and the kinetic parameters favored this production over that of active glucocorticoids. Both the production of 7alpha-hydroxysteroids and their interference with the activation of cortisone into cortisol are basic to the concept of native anti-glucocorticoids efficient at their production site. This opens a promising new area for research.

DHEA Metabolism in Arthritis: A Role for the p450 Enzyme Cyp7b at the Immune-Endocrine Crossroad

For dehydroepiandrosterone (DHEA) both immunosuppressive and immuno-stimulating properties have been described. The immunosuppressive effects may be explained by the conversion of DHEA into androgens and/or estrogens. The described immuno-stimulating effects of DHEA may be due to the conversion of DHEA into 7alpha-hydroxy-DHEA (7alpha-OH-DHEA) by the activity of the p450 enzyme, Cyp7b. 7alpha-OH-DHEA is thought to have anti-glucocoticoid activity preventing the anti-inflammatory action of endogenous glucocorticoids. To investigate a putative role of Cyp7b in the arthritic process, tissues from both the murine collagen-induce arthritis (CIA) model and from patients with rheumatoid arthritis (RA) were studied. We determined the Cyp7b expression levels in synovial tissue and the level of 7alpha-OH-DHEA in both serum and arthritic joints of mice with CIA. Our studies showed that the severity of arthritis correlates with increased Cyp7b activity. Next, we investigated Cyp7b expression and activity in RA patients where the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) are known to control the disease process. Fibroblast-like synoviocytes (FLS), isolated from RA synovial biopsies were found to express Cyp7b mRNA. In addition, Cyp7b enzymatic activity was detected in these cells. We also investigated whether Cyp7b activity is regulated by cytokines. Proinflammatory (e.g., TNF-alpha and IL-1beta) cytokines were found to stimulate Cyp7b activity and the anti-inflammatory cytokine transforming growth factor-beta (TGF-beta) was found to suppress Cyp7b activity in FLS. Next, we studied which signal transduction pathway is involved in the TNF-alpha-mediated induction of Cyp7b activity in human FLS. The results show a role for nuclear factor kappa B (NFkappaB) and activator protein-1 (AP-1) in the regulation of Cyp7b expression. Finally, we established that the effects of DHEA or 7alpha-OH-DHEA on the immune system can not be explained by glucocorticoid receptor (GR) engagement. The role of the p450 enzyme Cyp7b in DHEA metabolism and its relevance in the arthritic process will be discussed.

Synthesis of 7alpha-hydroxy-dehydroepiandrosterone and 7beta-hydroxy-dehydroepiandrosterone

The fermentation of dehydroepiandrosterone synthesized from the starting material diosgenin using Mucor racemosus produced 7alpha-hydroxy-dehydroepiandrosterone and 7beta-hydroxy-dehydroepiandrosterone. The bioactivity of the microbial metabolites is also discussed. The species M. racemosus was isolated by screening among stains from soil samples collected from various parts of China.

CYP7B expression and activity in fibroblast-like synoviocytes from patients with rheumatoid arthritis: regulation by proinflammatory cytokines

OBJECTIVE

The cytochrome P450 enzyme CYP7B catalyzes the conversion of dehydroepiandrosterone (DHEA) into 7alpha-hydroxy-DHEA (7alpha-OH-DHEA). This metabolite can stimulate the immune response. We previously reported that the severity of murine collagen-induced arthritis is correlated with CYP7B messenger RNA (mRNA) expression and activity in the arthritic joint. The purpose of this study was to investigate the presence of 7alpha-OH-DHEA in synovial samples and the cytokine regulation of CYP7B activity in fibroblast-like synoviocytes (FLS) from patients with rheumatoid arthritis (RA).

METHODS

The presence of 7alpha-OH-DHEA was examined in synovial biopsy tissues, synovial fluid, and serum by radioimmunoassay. The effect of cytokines on CYP7B mRNA expression and CYP7B activity in FLS was examined by determining the formation of the CYP7B enzyme product 7alpha-OH-DHEA with the use of high-performance liquid chromatography.

RESULTS

The CYP7B enzyme product 7alpha-OH-DHEA was found in synovial biopsy tissues, synovial fluid, and serum from RA patients. The proinflammatory cytokines tumor necrosis factor alpha (TNFalpha), interleukin-1alpha (IL-1alpha), IL-1beta, and IL-17 up-regulated CYP7B activity in an FLS cell line 2-10-fold. Enhanced CYP7B activity was correlated with an increase in CYP7B mRNA. The cytokine transforming growth factor beta inhibited CYP7B activity. Moreover, CYP7B activity was detected in 10 of 13 unstimulated synovial fibroblast cell lines. Stimulation with TNFalpha increased CYP7B activity in all cell lines tested.

CONCLUSION

Exposure to the proinflammatory cytokines TNFalpha, IL-1alpha, IL-1beta, and IL-17 increases CYP7B activity in synovial tissue. Increased CYP7B activity leads to higher levels of the DHEA metabolite 7alpha-OH-DHEA in synovial fluid, which may contribute to the maintenance of the chronic inflammation observed in RA patients.