Metabolism of 27-, 25- and 24-hydroxycholesterol in rat glial cells and neurons

GC-MS analysis of oxysterols and their formation in cultivated liver cells (HepG2)

Oxysterols play a key role in many (patho)physiological processes and they are potential biomarkers for oxidative stress in several diseases. Here we developed a rapid gas chromatographic-mass spectrometry-based method for the separation and quantification of 11 biologically relevant oxysterols bearing hydroxy, epoxy, and dihydroxy groups. Efficient chromatographic separation (resolution ≥ 1.9) was achieved using a medium polarity 35%-diphenyl/65%-dimethyl polysiloxane stationary phase material (30 m × 0.25 mm inner diameter and 0.25 μm film thickness). Based on thorough analysis of the fragmentation during electron ionization we developed a strategy to deduce structural information of the oxysterols. Optimized sample preparation includes (i) extraction with a mixture of n-hexane/iso-propanol, (ii) removal of cholesterol by solid phase extraction with unmodified silica, and (iii) trimethylsilylation. The method was successfully applied on the analysis of brain samples, showing consistent results with previous studies and a good intra- and interday precision of ≤20%. Finally, we used the method for the investigation of oxysterol formation during oxidative stress in HepG2 cells. Incubation with tert-butyl hydroperoxide led to a massive increase in free radical formed oxysterols (7-keto-chol > 7β-OH-chol >> 7α-OH-chol), while 24 h incubation with the glutathione peroxidase 4 inhibitor RSL3 showed no increase in oxidative stress based on the oxysterol pattern. Overall, the new method described here enables the robust analysis of a biologically meaningful pattern of oxysterols with high sensitivity and precision allowing us to gain new insights in the biological formation and role of oxysterols.

Cholesterol homeostasis in the vertebrate retina: biology and pathobiology

Cholesterol is a quantitatively and biologically significant constituent of all mammalian cell membrane, including those that comprise the retina. Retinal cholesterol homeostasis entails the interplay between de novo synthesis, uptake, intraretinal sterol transport, metabolism, and efflux. Defects in these complex processes are associated with several congenital and age-related disorders of the visual system. Herein, we provide an overview of the following topics: (a) cholesterol synthesis in the neural retina; (b) lipoprotein uptake and intraretinal sterol transport in the neural retina and the retinal pigment epithelium (RPE); (c) cholesterol efflux from the neural retina and the RPE; and (d) biology and pathobiology of defects in sterol synthesis and sterol oxidation in the neural retina and the RPE. We focus, in particular, on studies involving animal models of monogenic disorders pertinent to the above topics, as well as in vitro models using biochemical, metabolic, and omic approaches. We also identify current knowledge gaps and opportunities in the field that beg further research in this topic area.

Structural and biochemical investigation of MARK4 inhibitory potential of cholic acid: Towards therapeutic implications in neurodegenerative diseases

Microtubule affinity regulating kinase (MARK4) is considered as a potential drug target for diabetes, cancer, and neurodegenerative diseases. Since the role of MARK4 in the phosphorylation of tau protein and subsequently Alzheimer's disease has been established, therefore, we have investigated the binding affinity and MARK4 inhibitory potential of cholic acid (CHA) using both computational and spectroscopic methods. Molecular docking suggested a strong binding of CHA to the functionally important residues of MARK4. We further performed 500 ns molecular dynamics simulation which suggested the MARK4-CHA system was quite stable throughout the simulation trajectory. CHA potential binds to the MARK4 with a binding constant (K) of 10⁷ M^-1 at 288 K. Further, MARK4 activity was inhibited by CHA with an IC₅₀ = 5.5 μM. Further insights into the thermodynamic parameters suggested that MARK4-CHA complex formation is driven by both electrostatic and van der Waals interactions. Overall study provides a rationale to use CHA in the drug development via MARK4 inhibition, towards possible therapeutic implications in neurodegenerative diseases.

Serum bile acids profiling by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and its application on pediatric liver and intestinal diseases

Background

A method for bile acid profiling measuring 21 primary and secondary bile acids in serum samples was developed and validated with liquid chromatography-tandem mass spectrometry (LC-MS/MS). Sample preparation included spiking with internal standards followed by protein precipitation, centrifugation, drying under nitrogen gas and reconstitution. Extracted samples were injected onto a Phenomenex Kinetex C18 column (150 × 4.60 mm, 2.6 μm).

Methods

Data was collected with LC-MS/MS operated in negative ion mode with multiple reaction monitoring (MRM) and single reaction monitoring (SRM). The analytical run time was 12 min.

Results

The method showed excellent linearity with high regression coefficients (>0.99) over a range of 0.05 and 25 μM for all analytes tested. The method also showed acceptable intra-day and inter-day accuracy and precision. As a proof of concept, the analytical method was applied to patients with neonatal intrahepatic cholestasis caused by citrin deficiency (NICCD), biliary atresia (BA), and necrotizing enterocolitis (NEC), and distinct bile acids profiles were demonstrated.

Conclusions

The method could be poised to identify possible biomarkers for non-invasive early diagnosis of these disorders.

Chronic infusion of taurolithocholate into the brain increases fat oxidation in mice

Bile acids can function in the postprandial state as circulating signaling molecules in the regulation of glucose and lipid metabolism via the transmembrane receptor TGR5 and nuclear receptor FXR. Both receptors are present in the central nervous system, but their function in the brain is unclear. Therefore, we investigated the effects of intracerebroventricular (i.c.v.) administration of taurolithocholate (tLCA), a strong TGR5 agonist, and GW4064, a synthetic FXR agonist, on energy metabolism. We determined the effects of chronic i.c.v. infusion of tLCA, GW4064, or vehicle on energy expenditure, body weight and composition as well as tissue specific fatty acid uptake in mice equipped with osmotic minipumps. We found that i.c.v. administration of tLCA (final concentration in cerebrospinal fluid: 1 μM) increased fat oxidation (tLCA group: 0.083 ± 0.006 vs control group: 0.036 ± 0.023 kcal/h, F = 5.46, P = 0.04) and decreased fat mass (after 9 days of tLCA infusion: 1.35 ± 0.13 vs controls: 1.96 ± 0.23 g, P = 0.03). These changes were associated with enhanced uptake of triglyceride-derived fatty acids by brown adipose tissue and with browning of subcutaneous white adipose tissue. I.c.v. administration of GW4064 (final concentration in cerebrospinal fluid: 10 μM) did not affect energy metabolism, body composition nor bile acid levels, negating a role of FXR in the central nervous system in metabolic control. In conclusion, bile acids such as tLCA may exert metabolic effects on fat metabolism via the brain.

25-Hydroxycholesterol is involved in the pathogenesis of amyotrophic lateral sclerosis

This study aimed to evaluate the levels of three major hydroxycholesterols (24-, 25-, and 27-hydroxycholesterols) in the serum and cerebrospinal fluid (CSF) of patients with amyotrophic lateral sclerosis (ALS), as well as to show their role in the pathogenesis of ALS experimental models. The level of 25-hydroxycholesterol were higher in untreated ALS patients (n = 30) than in controls without ALS (n = 33) and ALS patients treated with riluzole (n = 9) both in their serum and CSF. The level of 25-hydroxycholesterol in the serum of ALS patients were significantly associated with their disease severity and rate of progression. In the motor neuron-like cell line (NSC34) with the human mutant G93A superoxide dismutase 1 gene (mSOD1-G93A), 25-hydroxycholesterol induced motor neuronal death/ apoptosis via glycogen synthase kinase-3β and liver X receptor pathways; riluzole treatment attenuated these effects. The expressions of enzymes that synthesize 25-hydroxycholesterol were significantly increased in the brains of early symptomatic mSOD1G93A mice. Our data, obtained from patients with ALS, a cellular model of ALS, and an animal model of ALS, suggests that 25-hydroxycholesterol could be actively involved in the pathogenesis of ALS, mostly in the early symptomatic disease stage, by mediating neuronal apoptosis.

Reduced Plasma Levels of 25-Hydroxycholesterol and Increased Cerebrospinal Fluid Levels of Bile Acid Precursors in Multiple Sclerosis Patients

Multiple sclerosis (MS) is an autoimmune, inflammatory disease of the central nervous system (CNS). We have measured the levels of over 20 non-esterified sterols in plasma and cerebrospinal fluid (CSF) from patients suffering from MS, inflammatory CNS disease, neurodegenerative disease and control patients. Analysis was performed following enzyme-assisted derivatisation by liquid chromatography–mass spectrometry (LC–MS) exploiting multistage fragmentation (MSⁿ). We found increased concentrations of bile acid precursors in CSF from each of the disease states and that patients with inflammatory CNS disease classified as suspected autoimmune disease or of unknown aetiology also showed elevated concentrations of 25-hydroxycholestertol (25-HC, P < 0.05) in CSF. Cholesterol concentrations in CSF were not changed except for patients diagnosed with amyotrophic lateral sclerosis (P < 0.01) or pathogen-based infections of the CNS (P < 0.05) where they were elevated. In plasma, we found that 25-HC (P < 0.01), (25R)26-hydroxycholesterol ((25R)26-HC, P < 0.05) and 7α-hydroxy-3-oxocholest-4-enoic acid (7αH,3O-CA, P < 0.05) were reduced in relapsing-remitting MS (RRMS) patients compared to controls. The pattern of reduced plasma levels of 25-HC, (25R)26-HC and 7αH,3O-CA was unique to RRMS. In summary, in plasma, we find that the concentration of 25-HC in RRMS patients is significantly lower than in controls. This is consistent with the hypothesis that a lower propensity of macrophages to synthesise 25-HC will result in reduced negative feedback by 25-HC on IL-1 family cytokine production and exacerbated MS. In CSF, we find that the dominating metabolites reflect the acidic pathway of bile acid biosynthesis and the elevated levels of these in CNS disease is likely to reflect cholesterol release as a result of demyelination or neuronal death. 25-HC is elevated in patients with inflammatory CNS disease probably as a consequence of up-regulation of the type 1 interferon-stimulated gene cholesterol 25-hydroxylase in macrophages.

Bile Acids in Neurodegenerative Disorders

Bile acids, a structurally related group of molecules derived from cholesterol, have a long history as therapeutic agents in medicine, from treatment for primarily ocular diseases in ancient Chinese medicine to modern day use as approved drugs for certain liver diseases. Despite evidence supporting a neuroprotective role in a diverse spectrum of age-related neurodegenerative disorders, including several small pilot clinical trials, little is known about their molecular mechanisms or their physiological roles in the nervous system. We review the data reported for their use as treatments for neurodegenerative diseases and their underlying molecular basis. While data from cellular and animal models and clinical trials support potential efficacy to treat a variety of neurodegenerative disorders, the relevant bile acids, their origin, and the precise molecular mechanism(s) by which they confer neuroprotection are not known delaying translation to the clinical setting.

Defective cholesterol metabolism in amyotrophic lateral sclerosis

As neurons die, cholesterol is released in the central nervous system (CNS); hence, this sterol and its metabolites may represent a biomarker of neurodegeneration, including in amyotrophic lateral sclerosis (ALS), in which altered cholesterol levels have been linked to prognosis. More than 40 different sterols were quantified in serum and cerebrospinal fluid (CSF) from ALS patients and healthy controls. In CSF, the concentration of cholesterol was found to be elevated in ALS samples. When CSF metabolite levels were normalized to cholesterol, the cholesterol metabolite 3β,7α-dihydroxycholest-5-en-26-oic acid, along with its precursor 3β-hydroxycholest-5-en-26-oic acid and product 7α-hydroxy-3-oxocholest-4-en-26-oic acid, were reduced in concentration, whereas metabolites known to be imported from the circulation into the CNS were not found to differ in concentration between groups. Analysis of serum revealed that (25R)26-hydroxycholesterol, the immediate precursor of 3β-hydroxycholest-5-en-26-oic acid, was reduced in concentration in ALS patients compared with controls. We conclude that the acidic branch of bile acid biosynthesis, known to be operative in-part in the brain, is defective in ALS, leading to a failure of the CNS to remove excess cholesterol, which may be toxic to neuronal cells, compounded by a reduction in neuroprotective 3β,7α-dihydroxycholest-5-en-26-oic acid.

Estrogen-mediated regulation of steroid metabolism in rat glial cells; effects on neurosteroid levels via regulation of CYP7B1-mediated catalysis

Many neuroactive steroids, including dehydroepiandrosterone (DHEA), pregnenolone, 27-hydroxycholesterol and 17β-estradiol, are known to affect development and function of the brain and nervous system. These and other steroids can undergo tissue and/or cell-specific enzymatic conversions into steroid metabolites. Carefully regulated production of steroids with various physiological effects is important for cells of the nervous system. Astrocytes express many steroidogenic enzymes and are considered important producers of brain steroids. The quantitative roles of different pathways for steroid metabolism in rat astrocytes are not clear. In the current study we examined effects of estrogens on steroid metabolism catalyzed by CYP7B1 and other enzymes in primary cultures of rat astrocytes. The CYP7B1 enzyme, which has been linked to neurodegenerative disease, is involved in the metabolism of several important neurosteroids. In the present study, we found that 7α-hydroxylation, performed by CYP7B1, is the quantitatively most important pathway for DHEA metabolism in rat astrocytes. In addition, our present experiments on catalytic steroid conversions revealed that estrogens significantly suppress the CYP7B1-catalyzed metabolism of not only DHEA but also of pregnenolone and 27-hydroxycholesterol in rat astrocytes. These novel findings point to a regulatory mechanism for control of the cellular levels of these neurosteroids via CYP7B1. Our hypothesis that estrogens can regulate neurosteroid levels via this enzymatic reaction was supported by experiments using ELISA to assay levels of DHEA and pregnenolone in the presence or absence of estrogen. Furthermore, the present results show that estrogen suppresses CYP7B1-catalyzed 7α-hydroxylation also in primary cultures of rat Schwann cells, indicating that regulation by estrogen via this enzyme may be of relevance in both the CNS and the PNS.

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.

Dietary intake of plant sterols stably increases plant sterol levels in the murine brain

Plant sterols such as sitosterol and campesterol are frequently administered as cholesterol-lowering supplements in food. Recently, it has been shown in mice that, in contrast to the structurally related cholesterol, circulating plant sterols can enter the brain. We questioned whether the accumulation of plant sterols in murine brain is reversible. After being fed a plant sterol ester-enriched diet for 6 weeks, C57BL/6NCrl mice displayed significantly increased concentrations of plant sterols in serum, liver, and brain by 2- to 3-fold. Blocking intestinal sterol uptake for the next 6 months while feeding the mice with a plant stanol ester-enriched diet resulted in strongly decreased plant sterol levels in serum and liver, without affecting brain plant sterol levels. Relative to plasma concentrations, brain levels of campesterol were higher than sitosterol, suggesting that campesterol traverses the blood-brain barrier more efficiently. In vitro experiments with brain endothelial cell cultures showed that campesterol crossed the blood-brain barrier more efficiently than sitosterol. We conclude that, over a 6-month period, plant sterol accumulation in murine brain is virtually irreversible.

Nano-liquid chromatography-tandem mass spectrometry analysis of oxysterols in brain: monitoring of cholesterol autoxidation

Oxysterols are present in mammalian brain at ng/g-μg/g levels while cholesterol is present at the mg/g level. This makes oxysterol analysis of brain challenging. In an effort to meet this challenge we have developed, and validated, an isolation method based on solid phase extraction and an analytical protocol involving oxidation/derivatisation (i.e., charge-tagging) followed by nano-flow liquid chromatography (nano-LC) combined with tandem mass spectrometry utilising multi-stage fragmentation (MS(n)). The oxidation/derivatisation method employed improves detection limits by two orders of magnitude, while nano-LC-MS(n) provides separation of isomers and allows oxysterol quantification. Using this method 13 different oxysterols have been identified in rat brain including 24S-hydroxycholesterol, 24S,25-epoxycholesterol and 7α,26-dihydroxycholest-4-en-3-one. The level of 24S-hydroxycholesterol in rat brain was determined to be 20.3±3.4 μg/g and quantitative estimates were made for the other oxysterols identified. The presence of a large excess of cholesterol over oxysterol in brain raises the problem of autoxidation during sterol isolation and sample preparation. Thus, in parallel to identification studies, the degree of cholesterol autoxidation occurring during sterol isolation and analysis has been evaluated with the aid of [(2)H(7)]-labelled cholesterol and cholesterol autoxidation products identified.

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood-brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.

Analysis of oxysterol metabolomes

Oxysterols are oxygenated forms of cholesterol. This definition can, however, be expanded to include oxygenated derivatives of plant sterols and also of cholesterol precursors. Oxysterols are formed in the first steps of cholesterol metabolism and also from cholesterol by reactive oxygen species. Oxysterols were once thought of as simple intermediates, or side-products, in the conversion of cholesterol to hormonal steroids and bile acids, however, they have subsequently been shown to be biologically active molecules in their own right. In this article we will discuss methods of oxysterol analysis including "classical" gas chromatography-mass spectrometry (GC-MS) methods and more recent liquid chromatography (LC)-MS methods. Our main focus, however, will be on analytical methods based on "charge-tagging" and LC-tandem mass spectrometry (MS/MS or MS(n)) which we have developed over the last decade in our laboratory. Examples will be given of oxysterol analysis in brain, cerebrospinal fluid (CSF) and blood. The advantages and disadvantages of the various methods of oxysterol analysis will be discussed.

Effects on DHEA levels by estrogen in rat astrocytes and CNS co-cultures via the regulation of CYP7B1-mediated metabolism

The neurosteroid dehydroepiandrosterone (DHEA) is formed locally in the CNS and has been implicated in several processes essential for CNS function, including control of neuronal survival. An important metabolic pathway for DHEA in the CNS involves the steroid hydroxylase CYP7B1. In previous studies, CYP7B1 was identified as a target for estrogen regulation in cells of kidney and liver. In the current study, we examined effects of estrogens on CYP7B1-mediated metabolism of DHEA in primary cultures of rat astrocytes and co-cultures of rat CNS cells. Astrocytes, which interact with neurons in several ways, are important for brain neurosteroidogenesis. We found that estradiol significantly suppressed CYP7B1-mediated DHEA hydroxylation in primary mixed CNS cultures from fetal and newborn rats. Also, CYP7B1-mediated DHEA hydroxylation and CYP7B1 mRNA were markedly suppressed by estrogen in primary cultures of rat astrocytes. Interestingly, diarylpropionitrile, a well-known agonist of estrogen receptor β, also suppressed CYP7B1-mediated hydroxylation of DHEA. Several previous studies have reported neuroprotective effects of estrogens. The current data indicate that one of the mechanisms whereby estrogen can exert protective effects in the CNS may involve increase of the levels of DHEA by suppression of its metabolism.

Changes in brain cholesterol metabolome after excitotoxicity

Excitotoxicity due to excess stimulation of glutamate receptors in neurons is accompanied by increased Ca(2+) influx, stimulation of Ca(2+)-dependent enzymes, ATP depletion, increase in lipid peroxidation products, and loss of glutathione. These changes resemble neurochemical alterations in acute neuronal injury (stroke, spinal cord injury, and traumatic brain injury) and chronic neurodegenerative diseases such as Alzheimer's disease. Intracerebroventricular injection of the potent glutamate analog kainate in rats results in increased cholesterol concentration in the hippocampus at short to medium time intervals, i.e., 3 days-1 week post-injection, as detected by gas chromatography-mass spectrometry in the lesioned hippocampus. This is accompanied by an early increase in levels of cholesterol biosynthetic precursors and increases in both enzymatically derived oxysterols such as 24-hydroxycholesterol and cholesterol oxidation products (COPs) generated by reactive oxygen species, including cholesterol epoxides and 7-ketocholesterol. In contrast to COPs, no change in concentration of the neurosteroid pregnenolone was found after KA injury. Cholesterol and COPs significantly increase exocytosis in cultured PC12 cells and neurons, and both oxysterols and COPs are able to induce cytotoxic and apoptotic injuries in different cell types, including neurons. Together, the findings suggest that increased cholesterol and COPs after KA excitotoxicity could themselves lead to disturbed neuronal ion homeostasis, increased neurotransmitter release, and propagation of excitotoxicity.

Cerebrospinal fluid steroidomics: are bioactive bile acids present in brain?

In this study we have profiled the free sterol content of cerebrospinal fluid by a combination of charge tagging and liquid chromatography-tandem mass spectrometry. Surprisingly, the most abundant cholesterol metabolites were found to be C(27) and C(24) intermediates of the bile acid biosynthetic pathways with structures corresponding to 7alpha-hydroxy-3-oxocholest-4-en-26-oic acid (7.170 +/- 2.826 ng/ml, mean +/- S.D., six subjects), 3beta-hydroxycholest-5-en-26-oic acid (0.416 +/- 0.193 ng/ml), 7alpha,x-dihydroxy-3-oxocholest-4-en-26-oic acid (1.330 +/- 0.543 ng/ml), and 7alpha-hydroxy-3-oxochol-4-en-24-oic acid (0.172 +/- 0.085 ng/ml), and the C(26) sterol 7alpha-hydroxy-26-norcholest-4-ene-3,x-dione (0.204 +/- 0.083 ng/ml), where x is an oxygen atom either on the CD rings or more likely on the C-17 side chain. The ability of intermediates of the bile acid biosynthetic pathways to activate the liver X receptors (LXRs) and the farnesoid X receptor was also evaluated. The acidic cholesterol metabolites 3beta-hydroxycholest-5-en-26-oic acid and 3beta,7alpha-dihydroxycholest-5-en-26-oic acid were found to activate LXR in a luciferase assay, but the major metabolite identified in this study, i.e. 7alpha-hydroxy-3-oxocholest-4-en-26-oic acid, was not an LXR ligand. 7Alpha-hydroxy-3-oxocholest-4-en-26-oic acid is formed from 3beta,7alpha-dihydroxycholest-5-en-26-oic acid in a reaction catalyzed by 3beta-hydroxy-Delta(5)-C(27)-steroid dehydrogenase (HSD3B7), which may thus represent a deactivation pathway of LXR ligands in brain. Significantly, LXR activation has been found to reduce the symptoms of Alzheimer disease (Fan, J., Donkin, J., and Wellington C. (2009) Biofactors 35, 239-248); thus, cholesterol metabolites may play an important role in the etiology of Alzheimer disease.

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.

Capillary liquid chromatography combined with tandem mass spectrometry for the study of neurosteroids and oxysterols in brain

Neurosteroids and neurosterols are found in brain at low levels (ng/g-microg/g) against a high background of cholesterol (mg/g). As such their analysis can be challenging. Traditionally, these molecules have been analysed by gas chromatography (GC)-mass spectrometry (MS), however, the absence of molecular ions in GC-MS spectra, even from derivatised molecules, can make the discovery and identification of novel neurosteroids/sterols difficult. To avoid this scenario, liquid chromatography (LC) combined with desorption ionisation methods are employed. In this review we discuss the application of LC-MS and LC-tandem mass spectrometry (MS/MS) for the identification of neurosteroids/sterols, paying particular attention to the use of low-flow-rate LC to maximise chromatographic and mass spectrometric performance.

The effect of 24S-hydroxycholesterol on cholesterol homeostasis in neurons: quantitative changes to the cortical neuron proteome

In humans, the brain represents only about 2% of the body's mass but contains about one-quarter of the body's free cholesterol. Cholesterol is synthesized de novo in brain and removed by metabolism to oxysterols. 24S-Hydoxycholesterol represents the major metabolic product of cholesterol in brain, being formed via the cytochrome P450 (CYP) enzyme CYP46A1. CYP46A1 is expressed exclusively in brain, normally by neurons. In this study, we investigated the effect of 24S-hydroxycholesterol on the proteome of rat cortical neurons. With the use of two-dimensional liquid chromatography linked to nanoelectrospray tandem mass spectrometry, over 1040 proteins were identified including members of the cholesterol, isoprenoid and fatty acid synthesis pathways. With the use of stable isotope labeling technology, the protein expression patterns of enzymes in these pathways were investigated. 24S-Hydroxycholesterol was found to down-regulate the expression of members of the cholesterol/isoprenoid synthesis pathways including 3-hydroxy-3-methylglutaryl-Coenzyme A synthase 1 (EC 2.3.3.10), diphosphomevalonate decarboxylase (EC 4.1.1.33), isopentenyl-diphosphate delta isomerase (EC 5.3.3.2), farnesyl-diphosphate synthase (Geranyl trans transferase, EC 2.5.1.10), and dedicated sterol synthesis enzymes, farnesyl-diphosphate farnesyltransferase 1 (squalene synthase, EC 2.5.1.21) and methylsterol monooxygenase (EC 1.14.13.72). The expression of many enzymes in the cholesterol/isoprenoid and fatty acid synthesis pathways are regulated by the membrane-bound transcription factors named sterol regulatory element-binding proteins (SREBPs), which themselves are both transcriptionally and post-transcriptionally regulated. The current proteomic data indicates that 24S-hydroxycholesterol down-regulates cholesterol synthesis in neurons, possibly, in a post-transcriptional manner through SREBP-2. In contrast to cholesterol metabolism, enzymes responsible for the synthesis of fatty acids were not found to be down-regulated in neurons treated with 24S-hydroxycholesterol, while apolipoprotein E (apo E), a cholesterol trafficking protein, was found to be up-regulated. Taken together, this data leads to the hypothesis that, in times of cholesterol excess, 24S-hydroxycholesterols signals down-regulation of cholesterol synthesis enzymes through SREBP-2, but up-regulates apo E synthesis (through the liver X receptor) leading to cholesterol storage and restoration of cholesterol balance.

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.

Crossing the barrier: oxysterols as cholesterol transporters and metabolic modulators in the brain

A normal brain function requires constant levels of cholesterol, and the need for constancy seems to be higher here than in any other organ. Nature has met this need by isolation of brain cholesterol by a highly efficient blood-brain barrier. As a low synthesis of cholesterol is present in the brain, a mechanism for compensatory elimination is required. A decade ago we made the unexpected finding that the favoured mechanism for this involves conversion into 24S-hydroxycholesterol, followed by diffusion over the blood-brain barrier. Recent studies by us and others on this new pathway have given new insights into the mechanisms by which cholesterol homeostasis is maintained in the brain. We recently demonstrated a flux of another oxygenated product of cholesterol, 27-hydroxycholesterol, in the opposite direction. The latter flux may be important for neurodegeneration, and may be the link between hypercholesterolaemia and Alzheimer's disease. An overview of the above studies is presented and the possibility that the cholesterol 24S-hydroxylase in the brain may be important for memory and learning and that it may be a new drug target is discussed.

Role of ABCG1 and ABCA1 in regulation of neuronal cholesterol efflux to apolipoprotein E discs and suppression of amyloid-beta peptide generation

Maintenance of an adequate supply of cholesterol is important for neuronal function, whereas excess cholesterol promotes amyloid precursor protein (APP) cleavage generating toxic amyloid-beta (Abeta) peptides. To gain insights into the pathways that regulate neuronal cholesterol level, we investigated the potential for reconstituted apolipoprotein E (apoE) discs, resembling nascent lipoprotein complexes in the central nervous system, to stimulate neuronal [3H]cholesterol efflux. ApoE discs potently accelerated cholesterol efflux from primary human neurons and cell lines. The process was saturable (17.5 microg of apoE/ml) and was not influenced by APOE genotype. High performance liquid chromatography analysis of cholesterol and cholesterol metabolites effluxed from neurons indicated that <25% of the released cholesterol was modified to polar products (e.g. 24-hydroxycholesterol) that diffuse from neuronal membranes. Thus, most cholesterol (approximately 75%) appeared to be effluxed from neurons in a native state via a transporter pathway. ATP-binding cassette transporters ABCA1, ABCA2, and ABCG1 were detected in neurons and neuroblastoma cell lines and expression of these cDNAs revealed that ABCA1 and ABCG1 stimulated cholesterol efflux to apoE discs. In addition, ABCA1 and ABCG1 expression in Chinese hamster ovary cells that stably express human APP significantly reduced Abeta generation, whereas ABCA2 did not modulate either cholesterol efflux or Abeta generation. These data indicate that ABCA1 and ABCG1 play a significant role in the regulation of neuronal cholesterol efflux to apoE discs and in suppression of APP processing to generate Abeta peptides.

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.

Cholestanol metabolism in patients with cerebrotendinous xanthomatosis: absorption, turnover, and tissue deposition

To study the metabolism of cholestanol in patients with cerebrotendinous xanthomatosis (CTX), we measured the cholestanol absorption, the cholesterol and cholestanol turnover, and the tissue content of sterols in two patients. Cholestanol absorption was approximately 5.0%. The rapid exchangeable pool of cholestanol was 233 mg, and the total exchangeable pool was 752 mg. The production rate of cholestanol in pool A was 39 mg/day. [4-14C]cholestanol was detected in the xanthomas, but neither [4-14C]cholestanol nor [4-14C]cholesterol was detected in peripheral nerves biopsied at 49 and 97 days after [4-14C]cholesterol given intravenously. Of the 18 tissues analyzed at biopsy and autopsy, the cholestanol content varied from 0.09 mg/g in psoas muscle to 76 mg/g in a cerebellar xanthoma. With the assumption that the cholestanol-to-cholesterol ratio is 1.0, the relative cholestanol-to-cholesterol ratio varied from 1.0 in plasma and liver to 30.0 in the cerebellar xanthoma; cholestanol was especially high in nerve tissue. Our data indicate that CTX patients absorb cholestanol from the diet. They have a higher than normal cholestanol production rate. Cholestanol was derived from cholesterol. In CTX patients, the blood-brain barrier was intact to the passage of [4-14C]cholesterol and [4-14C]cholestanol. The deposition of large amounts of cholestanol (up to 30% of total sterols in cerebellum) in nerve tissues must have an important role in the neurological symptoms in CTX patients. In view of the intact blood-brain barrier, several other explanations for the large amounts of cholestanol in the brain were postulated.

25-Hydroxycholesterol has a dual effect on the proliferation of cultured rat astrocytes

We examined the effects of 25-OH-cholesterol on the growth of cultured rat astrocytes in the presence of lipoprotein-deficient serum (LPDS). 25-OH-cholesterol at 0.5-8 microM induced an increase in DNA synthesis as measured by [3H]thymidine incorporation into DNA, staining the cells with crystal violet, or counting the number of cells in different phases of the cell cycle by flow cytometry; however, at higher doses, an inhibition of cell proliferation was produced. Similar dose-dependent effects were found in media containing albumin (alone or with added EGF, PDGF, IGF-I or insulin), fetal bovine serum (FBS), or cholesterol-enriched LPDS. Mevalonate, and partially 25-OH-cholesterol, reversed the decrease in cell viability caused by mevinolin (lovastatin). However, mevalonate did not have any effect on 25-OH-cholesterol-stimulated proliferation. Finally, in media with albumin alone or in the presence of fetal bovine serum, growth factors, insulin or forskolin, 25-OH-cholesterol did not affect the expression of either c-fos mRNA or c-fos protein, as measured by real-time quantitative PCR or by Western blot, respectively. These results suggest that 25-OH-cholesterol has a dual effect on the proliferation of cultured rat astrocytes through an AP-1-independent mechanism. This could be of interest for gaining a better knowledge of the pathophysiological processes occurring in these cells.

Analysis of oxysterols by electrospray tandem mass spectrometry

Oxysterols are oxygenated derivatives of cholesterol. They are intermediates in cholesterol excretion pathways and may also be regarded as transport forms of cholesterol. The introduction of additional hydroxyl groups to the cholesterol skeleton facilitates the flux of oxysterols across the blood brain barrier, and oxysterols have been implicated in mediating a number of cholesterol-induced metabolic effects. Oxysterols are difficult to analyze by atmospheric pressure ionization mass spectrometry on account of the absence of basic or acidic functional groups in their structures. In this communication, we report a method for the derivatization and analysis of oxysterols by electrospray mass spectrometry. Oxysterols with a 3beta-hydroxy-Delta5 structure were converted by cholesterol oxidase to 3-oxo-Delta4 steroids and then derivatized with the Girard P reagent to give Girard P hydrazones, which were subsequently analyzed by tandem mass spectrometry. The improvement in sensitivity for the analysis of 25-hydroxycholesterol upon oxidation and derivatization was over 1000.

Studies on the transcriptional regulation of cholesterol 24-hydroxylase (CYP46A1): marked insensitivity toward different regulatory axes

Mammalian CNS contains a disproportionally large and remarkably stable pool of cholesterol. Despite an efficient recycling there is some requirement for elimination of brain cholesterol. Conversion of cholesterol into 24S-hydroxycholesterol by the cholesterol 24-hydroxylase (CYP46A1) is the quantitatively most important mechanism. Based on the protein expression and plasma levels of 24S-hydroxycholesterol, CYP46A1 activity appears to be highly stable in adults. Here we have made a structural and functional characterization of the promoter of the human CYP46A1 gene. No canonical TATA or CAAT boxes were found in the promoter region. Moreover this region had a high GC content, a feature often found in genes considered to have a largely housekeeping function. A broad spectrum of regulatory axes using a variety of promoter constructs did not result in a significant transcriptional regulation. Oxidative stress caused a significant increase in transcriptional activity. The possibility of a substrate-dependent transcriptional regulation was explored in vivo in a sterol-deficient mouse model (Dhcr24 null) in which almost all cholesterol had been replaced with desmosterol, which is not a substrate for CYP46A1. Compared with heterozygous littermates there was no statistically significant difference in the mRNA levels of Cyp46a1. During the first 2 weeks of life in the wild-type mouse, however, a significant increase of Cyp46a1 mRNA levels was found, in parallel with an increase in 24S-hydroxycholesterol level and a reduction of cholesterol synthesis. The failure to demonstrate a significant transcriptional regulation under most conditions is discussed in relation to the turnover of brain and neuronal cholesterol.

Crossing the barrier: net flux of 27-hydroxycholesterol into the human brain

Side chain oxidized oxysterols have a unique ability to traverse lipophilic membranes. We tested the hypothesis that there is a net flux of 27-hydroxycholesterol from the circulation into the brain using plasma samples collected from the internal jugular vein and an artery of healthy male volunteers. Two independent studies were performed, one in which total levels of 27-hydroxycholesterol were measured and one in which the free fraction of 27-hydroxycholesterol was measured. In the majority of subjects studied, the level of 27-hydroxycholesterol was higher in the artery than in the vein, and uptake from the circulation was calculated to be about 5 mg/24 h. The distribution of 27-hydroxycholesterol in human brain was found to be consistent with an extracerebral origin, with a concentration gradient from the white to the gray matter--a situation opposite that of 24S-hydroxycholesterol, which os exclusively formed in brain. In view of the fact that the blood-brain barrier is impermeable to cholesterol and that 27-hydroxycholesterol is a potent regulator of several cholesterol-sensitive genes, the flux of 27-hydroxycholesterol into the brain may be and important link between intra- and extracerebral cholesterol homeostasis.

Thematic review series: brain Lipids. Cholesterol metabolism in the central nervous system during early development and in the mature animal

Unesterified cholesterol is an essential structural component of the plasma membrane of every cell. During evolution, this membrane came to play an additional, highly specialized role in the central nervous system (CNS) as the major architectural component of compact myelin. As a consequence, in the human the mean concentration of unesterified cholesterol in the CNS is higher than in any other tissue (approximately 23 mg/g). Furthermore, even though the CNS accounts for only 2.1% of body weight, it contains 23% of the sterol present in the whole body pool. In all animals, most growth and differentiation of the CNS occurs in the first few weeks or years after birth, and the cholesterol required for this growth apparently comes exclusively from de novo synthesis. Currently, there is no evidence for the net transfer of sterol from the blood into the brain or spinal cord. In adults, the rate of synthesis exceeds the need for new structural sterol, so that net movement of cholesterol out of the CNS must take place. At least two pathways are used for this excretory process, one of which involves the formation of 24(S)-hydroxycholesterol. Whether or not changes in the plasma cholesterol concentration alter sterol metabolism in the CNS or whether such changes affect cognitive function in the brain or the incidence of dementia remain uncertain at this time.

Fifty years with bile acids and steroids in health and disease

Cholesterol and its metabolites, e.g., steroid hormones and bile acids, constitute a class of compounds of great biological importance. Their chemistry, biochemistry, and regulation in the body have been intensely studied for more than two centuries. The author has studied aspects of the biochemistry and clinical chemistry of steroids and bile acids for more than 50 years, and this paper, which is an extended version of the Schroepfer Medal Award lecture, reviews and discusses part of this work. Development and application of analytical methods based on chromatography and mass spectrometry (MS) have been a central part of many projects, aiming at detailed characterization and quantification of metabolic profiles of steroids and bile acids under different conditions. In present terminology, much of the work may be termed steroidomics and cholanoidomics. Topics discussed are bile acids in human bile and feces, bile acid production, bacterial dehydroxylation of bile acids and steroids during the enterohepatic circulation, profiles of steroid sulfates in plasma of humans and other primates, development of neutral and ion-exchanging lipophilic derivatives of Sephadex for sample preparation and group separation of steroid and bile acid conjugates, profiles of steroids and bile acids in human urine under different conditions, hydroxylation of bile acids in liver disease, effects of alcohol-induced redox changes on steroid synthesis and metabolism, alcohol-induced changes of bile acid biosynthesis, compartmentation of bile acid synthesis studied with 3H-labeled ethanol, formation and metabolism of sulfated metabolites of progesterone in human pregnancy, abnormal patterns of these in patients with intrahepatic cholestasis of pregnancy corrected by ursodeoxycholic acid, inherited and acquired defects of bile acid biosynthesis and their treatment, conjugation of bile acids and steroids with N-acetylglucosamine, sulfate-glucuronide double conjugates of hydroxycholesterols, extrahepatic 7alpha-hydroxylation and 3-dehydrogenation of hydroxycholesterols, and extrahepatic formation of C27 bile acids. The final part discusses analysis of free and sulfated steroids in brain tissue by capillary liquid chromatography-electrospray MS and suggests a need for reevaluation of the function of steroid sulfates in rat brain.

Bioconversion of 3beta-hydroxy-5-cholenoic acid into chenodeoxycholic acid by rat brain enzyme systems

We have previously demonstrated that the rat brain contains three unconjugated bile acids, and chenodeoxycholic acid (CDCA) is the most abundantly present in a tight protein binding form. The ratio of CDCA to the other acids in rat brain tissue was significantly higher than the ratio in the peripheral blood, indicating a contribution from either a specific uptake mechanism or a biosynthetic pathway for CDCA in rat brain. In this study, we have demonstrated the existence of an enzymatic activity that converts 3beta-hydroxy-5-cholenoic acid into CDCA in rat brain tissue. To distinguish marked compounds from endogenous related compounds, 18O-labeled 3beta-hydroxy-5-cholenoic acid, 3beta,7alpha-dihydroxy-5-cholenoic acid, and 7alpha-hydroxy-3-oxo-4-cholenoic acid were synthesized as substrates for in vitro incubation studies. The results clearly suggest that 3beta-hydroxy-5-cholenoic acid was converted to 3beta,7alpha-dihydroxy-5-cholenoic acid by microsomal enzymes. The 7alpha-hydroxy-3-oxo-4-cholenoic acid was produced from 3beta,7alpha-dihydroxy-5-cholenoic acid by the action of microsomal enzymes, and Delta4-3-oxo acid was converted to CDCA by cytosolic enzymes. These findings indicate the presence of an enzymatic activity that converts 3beta-hydroxy-5-cholenoic acid into CDCA in rat brain tissue. Furthermore, this synthetic pathway for CDCA may relate to the function of 24S-hydroxycholesterol, which plays an important role in cholesterol homeostasis in the body.

Quantitation of two pathways for cholesterol excretion from the brain in normal mice and mice with neurodegeneration

Although the pool of cholesterol in the adult central nervous system (CNS) is large and of constant size, little is known of the process(es) involved in regulation of sterol turnover in this pool. In 7-week-old mice, net excretion of cholesterol from the brain equaled 1.4 mg/day/kg body weight, and from the whole animal was 179 mg/day/kg. Deletion of cholesterol 24-hydroxylase, an enzyme highly expressed in the CNS, did not alter brain growth or myelination, but reduced sterol excretion from the CNS 64% to 0.5 mg/day/kg. In mice with a mutation in the Niemann-Pick C gene that had ongoing neurodegeneration, sterol excretion from the CNS was increased to 2.3 mg/day/kg. Deletion of cholesterol 24-hydroxylase activity in these animals reduced net excretion only 22% to 1.8 mg/day/kg. Thus, at least two different pathways promote net sterol excretion from the CNS. One uses cholesterol 24-hydroxylase and may reflect sterol turnover in large neurons in the brain. The other probably involves the movement of cholesterol or one of its metabolites across the blood-brain barrier and may more closely mirror sterol turnover in pools such as glial cell membranes and myelin.

From brain to bile. Evidence that conjugation and omega-hydroxylation are important for elimination of 24S-hydroxycholesterol (cerebrosterol) in humans

The brain is the almost exclusive site of formation of 24S-hydroxycholesterol in man, and there is a continuous flux of this oxysterol across the blood-brain barrier into the circulation. The hepatic metabolism of 24S-hydroxycholesterol was studied here by three different approaches: incubation of tritium-labeled 24S-hydroxycholesterol with human primary hepatocytes, administration of tritium-labeled 24S-hydroxycholesterol to a human volunteer, and quantitation of free and conjugated 24S-hydroxycholesterol and its neutral metabolites in ileocecal fluid from patients with ileal fistulae. 24S-Hydroxycholesterol as well as 24R-hydroxycholesterol were converted into bile acids by human hepatocytes at a rate of about 40% of that of the normal intermediate in bile acid synthesis, 7 alpha-hydroxycholesterol. There was also a conversion of 24S-hydroxycholesterol into conjugate(s) of 5-cholestene-3 beta,24S,27-triol at a rate similar to the that of conversion into bile acids. When administered to a human volunteer, labeled 24S-hydroxycholesterol was converted into bile acids at about half the rate of simultaneously administered labeled 7 alpha-hydroxycholesterol. Free, sulfated, and glucuronidated 24S-hydroxycholesterol and 5-cholestene-3 beta,24,27-triol were identified in ileocecal fluid. The excretion of these steroids was about 3.5 mg/24 h, amounting to more than 50% of the total estimated flux of 24S-hydroxycholesterol from the brain. It is concluded that 24S-hydroxycholesterol is a less efficient precursor to bile acids and that about half of it is conjugated and eliminated in bile as such or as a conjugate of a 27-hydroxylated metabolite. The less efficient metabolism of 24S-hydroxycholesterol may explain the surprisingly high levels of this oxysterol in the circulation and is of interest in relation to the suggested role of 24S-hydroxycholesterol as a regulator of cholesterol homeostasis.

7alpha-Hydroperoxycholesterol causes CNS neuronal cell death

Brain cholesterol, which is synthesized in the central nervous system and also partly taken up from lipoproteins via the blood-brain barrier, is a major component of neuronal membranes. Oxidation of cholesterol leads to the formation of oxysterols, which have been shown to act cytotoxic. The influence of 7alpha-hydroperoxycholesterol, was investigated using the human neuroblastoma cell line SH-SY5Y. 7alpha-Hydroperoxycholesterol caused neuronal cell death; this neurotoxic effect was dose-dependent, within 48 h 10 microM led to 50%, 50 microM to 92% loss of cell viability, which was detected by cell morphology and Trypan blue exclusion. DNA-fragmentation or caspase-3 activity were not detectable, LDH release occurred rapidly and reactive oxygen species (ROS) were generated. Therefore we infer that 7alpha-hydroperoxycholesterol, apart from its role in atherosclerosis, leads to necrosis of neuronal cells.

Aminosterols from the dogfish shark Squalus acanthias

Seven new aminosterols related to squalamine (8) were isolated from the liver of the dogfish shark Squalus acanthias. Their structures (1-7) were determined using spectroscopic methods, including 2D NMR and HRFABMS. These aminosterols possess a relatively invariant cholestane skeleton with a trans AB ring junction, a spermidine or spermine attached equatorially at C3, and a steroidal side-chain that may be sulfated. The structure of the lone spermine conjugate, 7 (MSI-1436), was confirmed by its synthesis from (5alpha,7alpha, 24R)-7-hydroxy-3-ketocholestan-24-yl sulfate. Some members of this family of aminosterols exhibit a broad spectrum of antimicrobial activity comparable to squalamine.

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.

Paracrine glucocorticoid activity produced by mouse thymic epithelial cells

Previous data have suggested that glucocorticoids (GCs) are involved in the differentiation of thymocytes into mature T cells. In this report we demonstrate that the mouse thymic epithelial cells (TEC) express the cytochrome P450 hydroxylases Cyp11A1, Cyp21, and Cyp11B1. These enzymes, in combination with 3beta-hydroxysteroid dehydrogenase (3betaHSD), convert cholesterol into corticosterone, the major GC in rodents. In addition, when TEC were cocultured with 'reporter cells' containing the glucocorticoid receptor (GR) and a GR-dependent reporter gene, a specific induction of reporter gene activity was observed. Induction of reporter gene activity was blocked when the TEC and reporter cells were incubated in the presence of the Cyp11B1 inhibitor metyrapone or the 3betaHSD inhibitor trilostane, as well as by the GR antagonist RU486. Coculturing of TEC with thymocytes induced apoptosis in the latter, which was partially blocked by the enzyme inhibitors and RU486. We conclude that TEC secrete a GC hormone activity and suggest a paracrine role for this in thymocyte development.

Hepatic and extrahepatic dehydrogenation/isomerization of 5-cholestene-3 beta,7 alpha-diol: localization of 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase in pig tissues and subcellular fractions

Conversion of 5-cholestene-3 beta,7 alpha-diol (7 alpha-hydroxycholesterol) into 7 alpha-hydroxy-4-cholesten-3-one was studied with microsomes from different pig tissues and with liver subcellular fractions. Dehydrogenase/isomerase activity was efficient in microsomes from liver, ovary and lung, but less efficient in microsomes from adrenal gland and kidney. Microsomes from these tissues, with the exception of lung, were also active in dehydrogenation/isomerization of dehydroepiandrosterone and pregnenolone. Inhibition studies were carried out with trilostane, a competitive inhibitor of 3 beta-hydroxysteroid dehydrogenases active in steroid hormone biosynthesis (C19/C21-dehydrogenases), and a monoclonal antibody raised against a purified hepatic 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase. The results showed that the C27-dehydrogenase activity in the tissues was not dependent on the C19/C21 dehydrogenases, but was dependent on the 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase. Liver mitochondria, cytosol and peroxisomes lacked dehydrogenase/isomerase activity towards 7 alpha-hydroxycholesterol when microsomal contamination was taken into account. Immunoblotting experiments with monoclonal antibodies raised against the 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase showed immunoreactivity only with protein in liver microsomes. Immunohistochemical studies showed localization of the 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase in the bile duct epithelium. It is concluded that 7 alpha-hydroxycholesterol is converted into 7 alpha-hydroxy-4-cholesten-3-one by the microsomal 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase in liver and extrahepatic tissues.

Neurotoxicity of 25-OH-cholesterol on sympathetic neurons

Cultured rat sympathetic neurons derived from postnatal rat superior cervical ganglia (SCG) were used to compare the neurotoxicity of several cholesterol oxides. The cholesterol oxides tested included: 7-beta-OH-, 7-keto-, 19-OH-, 22(R)-OH-, 22(S)-OH-, and 25-OH-cholesterol. These agents caused an acute as well as a delayed toxicity in sympathetic neurons with 25-OH-cholesterol appearing to be the most toxic. A time-dependent experiment indicated that 25-OH-cholesterol at 4 microg/ml (10 microM) was able to kill 50% of the cells in 36 h. Morphological studies indicate that most of the cells do not exhibit a structural change similar to that observed in neuronal programmed cell death. Whole-cell patch clamp recording of untreated controls and 25-OH-cholesterol (2 microg/ml)-treated cells indicated that this toxicity was not accompanied by significant changes in voltage-dependent calcium channel activity. A number of pharmacological agents including ethylene glycolbis (beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA), cycloheximide, KCl, vitamin E, and methyl-beta-cyclodextrin were able to prevent the 25-OH-cholesterol-induced cell death to various degrees. These results suggest that, in addition to causing pathological changes in cells directly involved in atherosclerosis, cholesterol oxides may induce neurotoxicity in sympathetic neurons.