Key regulatory oxysterols in liver: analysis as Δ4-3-ketone derivatives by HPLC and response to physiological perturbations

Over-Expression of Intestinal Alkaline Phosphatase Attenuates Atherosclerosis

[Figure: see text].

Cholesterol metabolism pathways - are the intermediates more important than the products?

Every cell in vertebrates possesses the machinery to synthesise cholesterol and to metabolise it. The major route of cholesterol metabolism is conversion to bile acids. Bile acids themselves are interesting molecules being ligands to nuclear and G protein‐coupled receptors, but perhaps the intermediates in the bile acid biosynthesis pathways are even more interesting and equally important. Here, we discuss the biological activity of the different intermediates generated in the various bile acid biosynthesis pathways. We put forward the hypothesis that the acidic pathway of bile acid biosynthesis has primary evolved to generate signalling molecules and its utilisation by hepatocytes provides an added bonus of producing bile acids to aid absorption of lipids in the intestine.

7α,25-Dihydroxycholesterol Suppresses Hepatocellular Steatosis through GPR183/EBI2 in Mouse and Human Hepatocytes

Nonalcoholic fatty liver disease is a chronic inflammatory liver disease. It is associated with obesity and type 2 diabetes. Oxycholesterols are metabolites of cholesterol, and several of them can act on the G protein-coupled receptor, G protein-coupled receptor 183 (GPR183)/Epstein-Barr virus-induced gene 2. We found expression of GPR183 in human hepatoma cell lines and in vivo induction of GPR183 expression in mouse livers after high-fat diet feeding. Therefore, the role of oxycholesterols and GPR183 in hepatocytes was studied using a model of hepatic steatosis induced by liver X receptor (LXR) activation. LXR activation by T0901317 resulted in fat accumulation in Hep3B human hepatoma cells. This lipid accumulation was inhibited by 7α,25-dihydroxycholesterol, the most potent agonist of GPR183. The protective effects of 7α,25-dihydroxycholesterol were suppressed by a specific GPR183 antagonist, NIBR189 [(2E)-3-(4-Bromophenyl)-1-[4-4-methoxybenzoyl)-1-piperazinyl]-2-propene-1-one]. T0901317 treatment induced expression of the major transcription factor for lipogenesis, sterol regulatory element-binding protein 1c (SREBP-1c). 7α,25-Dihydroxycholesterol inhibited the induction of SREBP-1c proteins in a GPR183-dependent manner. Using inhibitors specific for intracellular signaling molecules, 7α,25-dihydroxycholesterol-induced suppression of hepatocellular steatosis was shown to be mediated through G_i/o proteins, p38 mitogen-activated protein kinases, phosphoinositide 3-kinase, and AMP-activated protein kinase. In addition, the inhibitory effect of 7α,25-dihydroxycholesterol was validated in HepG2 cells and primary mouse hepatocytes. Therefore, the present report suggests that 7α,25-dihydroxycholesterol-GPR183 signaling may suppress hepatocellular steatosis in the liver. SIGNIFICANCE STATEMENT: Oxycholesterols, which are metabolites of cholesterol, act on the G protein-coupled receptor, G protein-coupled receptor 183 (GPR183)/Epstein-Barr virus-induced gene 2, which is expressed in human hepatoma cell lines, and its expression is induced in vivo in mouse livers after high-fat diet feeding. Activation of GPR183 inhibits fat accumulation in primary mouse hepatocytes and HepG2 cells through G_i/o proteins, p38 mitogen-activated protein kinases, phosphoinositide 3-kinase, and AMP-activated protein kinase.

Targeting the transcription factor receptor LXR to treat clear cell renal cell carcinoma: agonist or inverse agonist?

Growing evidence indicates that clear cell renal cell carcinoma (ccRCC) is a metabolism-related disease. Changes in fatty acid (FA) and cholesterol metabolism play important roles in ccRCC development. As a nuclear transcription factor receptor, Liver X receptor (LXR) regulates a variety of key molecules associated with FA synthesis and cholesterol transport. Therefore, targeting LXR may provide new therapeutic targets for ccRCC. However, the potential regulatory effect and molecular mechanisms of LXR in ccRCC remain unknown. In the present study, we found that both an LXR agonist and an XLR inverse agonist could inhibit proliferation and colony formation and induce apoptosis in ccRCC cells. We observed that the LXR agonist LXR623 downregulated the expression of the low-density lipoprotein receptor (LDLR) and upregulated the expression of ABCA1, which resulted in reduced intracellular cholesterol and apoptosis. The LXR inverse agonist SR9243 downregulated the FA synthesis proteins sterol regulatory element-binding protein 1c (SREBP-1c), fatty acid synthase (FASN) and stearoyl-coA desaturase 1 (SCD1), causing a decrease in intracellular FA content and inducing apoptosis in ccRCC cells. SR9243 and LXR623 induced apoptosis in ccRCC cells but had no killing effect on normal renal tubular epithelial HK2 cells. We also found that SRB1-mediated high-density lipoprotein (HDL) in cholesterol influx is the cause of high cholesterol in ccRCC cells. In conclusion, our data suggest that an LXR inverse agonist and LXR agonist decrease the intracellular FA and cholesterol contents in ccRCC to inhibit tumour cells but do not have cytotoxic effects on non-malignant cells. Thus, LXR may be a safe therapeutic target for treating ccRCC patients.

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.

Quantification of Oxysterol Nuclear Receptor Ligands by LC/MS/MS

Oxidative derivatives of cholesterol such as 22(R)-hydroxycholesterol, 24(S)-hydroxycholesterol, 25-hydroxycholesterol, and (25S),26-hydroxycholesterol are endogenous ligands for the liver X receptors (LXRα and LXRβ). The LXRs are nuclear hormone receptors known as "intracellular cholesterol sensors" because of their ability to bind to and be activated by oxysterols at circulating concentrations. Oxysterols are expressed in a tissue-specific manner and are generally at least 10⁴ to 10⁶-fold less abundant than cholesterol. Thus, the extraction and measurement of oxysterols from plasma and tissues are facilitated by the removal of bulk sterols by solid phase extraction prior to quantitative analysis by mass spectrometry. In this chapter we describe step by step methods for extracting and quantitating oxysterols from biological samples using electrospray ionization LC/MS/MS.

GPR40 agonist ameliorates liver X receptor-induced lipid accumulation in liver by activating AMPK pathway

Hepatic steatosis is strongly linked to insulin resistance and type 2 diabetes. GPR40 is a G protein-coupled receptor mediating free fatty acid-induced insulin secretion and thus plays a beneficial role in the improvement of diabetes. However, the impact of GPR40 agonist on hepatic steatosis still remains to be elucidated. In the present study, we found that activation of GPR40 by its agonist GW9508 attenuated Liver X receptor (LXR)-induced hepatic lipid accumulation. Activation of LXR in the livers of C57BL/6 mice fed a high-cholesterol diet and in HepG2 cells stimulated by chemical agonist caused increased expression of its target lipogenic genes and subsequent lipid accumulation. All these effects of LXR were dramatically downregulated after GW9508 supplementation. Moreover, GPR40 activation was accompanied by upregulation of AMPK pathway, whereas the inhibitive effect of GPR40 on the lipogenic gene expression was largely abrogated by AMPK knockdown. Taken together, our results demonstrated that GW9508 exerts a beneficial effect to ameliorate LXR-induced hepatic steatosis through regulation of AMPK signaling pathway.

Cholestane-3β,5α,6β-triol: high levels in Niemann-Pick type C, cerebrotendinous xanthomatosis, and lysosomal acid lipase deficiency

Niemann-Pick type C (NPC) is a progressive neurodegenerative disease characterized by lysosomal/endosomal accumulation of unesterified cholesterol and glycolipids. Recent studies have shown that plasma cholestane-3β,5α,6β-triol (CT) and 7-ketocholesterol (7-KC) could be potential biomarkers for the diagnosis of NPC patients. We aimed to know the sensitivity and specificity of these biomarkers for the diagnosis of NPC compared with other diseases that can potentially lead to oxysterol alterations. We studied 107 controls and 122 patients including 16 with NPC, 3 with lysosomal acid lipase (LAL) deficiency, 8 with other lysosomal diseases, 5 with galactosemia, 11 with cerebrotendinous xanthomatosis (CTX), 3 with Smith-Lemli-Opitz, 14 with peroxisomal biogenesis disorders, 19 with unspecific hepatic diseases, 13 with familial hypercholesterolemia, and 30 with neurological involvement and no evidence of an inherited metabolic disease. CT and 7-KC were analyzed by HPLC-ESI-MS/MS as mono-dimethylglycine derivatives. Levels of 7-KC were high in most of the studied diseases, whereas those of CT were only high in NPC, LAL, and CTX patients. Consequently, although CT is a sensitive biomarker of NPC disease, including those cases with doubtful filipin staining, it is not specific. 7-KC is a very unspecific biomarker.

Quantitative detection of free 24S-hydroxycholesterol, and 27-hydroxycholesterol from human serum

Background

Cholesterol metabolism is important for the maintenance of myelin and neuronal membranes in the central nervous system. Blood concentrations of the brain specific cholesterol metabolite 24S-hydroxysterol to the peripheral metabolite 27-hydroxycholesterol may be useful surrogate markers for neurodegenerative diseases including Alzheimer’s disease, Huntington’s disease, HIV-Associated Neurocognitive Disorders, and Multiple Sclerosis. However, current methods to isolate hydroxycholesterols are labor intensive, prone to produce variable extraction efficiencies and do not discriminate between free and esterfied forms of hydroxycholesterols. Since free hydroxycholesterols are the biologically active form of these sterols, separating free from esterfied forms may provide a sensitive measure to identify disease-associated differences in brain sterol metabolism.

Results

We found that average human serum concentrations were 12.3 ± 4.79 ng/ml for free 24(s)-hydroxycholesterol and 17.7 ± 8.5 ng/ml for 27-hydroxycholesterol.

Conclusion

Serum measurements of these biologically active oxysterols may be useful surrogate measures for brain health in a variety of neurodegenerative conditions.

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.

5-cholesten-3β,25-diol 3-sulfate decreases lipid accumulation in diet-induced nonalcoholic fatty liver disease mouse model

Sterol regulatory element-binding protein-1c (SREBP-1c) increases lipogenesis at the transcriptional level, and its expression is upregulated by liver X receptor α (LXRα). The LXRα/SREBP-1c signaling may play a crucial role in the pathogenesis of nonalcoholic fatty liver disease (NAFLD). We previously reported that a cholesterol metabolite, 5-cholesten-3β,25-diol 3-sulfate (25HC3S), inhibits the LXRα signaling and reduces lipogenesis by decreasing SREBP-1c expression in primary hepatocytes. The present study aims to investigate the effects of 25HC3S on lipid homeostasis in diet-induced NAFLD mouse models. NAFLD was induced by feeding a high-fat diet (HFD) in C57BL/6J mice. The effects of 25HC3S on lipid homeostasis, inflammatory responses, and insulin sensitivity were evaluated after acute treatments or long-term treatments. Acute treatments with 25HC3S decreased serum lipid levels, and long-term treatments decreased hepatic lipid accumulation in the NAFLD mice. Gene expression analysis showed that 25HC3S significantly suppressed the SREBP-1c signaling pathway that was associated with the suppression of the key enzymes involved in lipogenesis: fatty acid synthase, acetyl-CoA carboxylase 1, and glycerol-3-phosphate acyltransferase. In addition, 25HC3S significantly reduced HFD-induced hepatic inflammation as evidenced by decreasing tumor necrosis factor and interleukin 1 α/β mRNA levels. A glucose tolerance test and insulin tolerance test showed that 25HC3S administration improved HFD-induced insulin resistance. The present results indicate that 25HC3S as a potent endogenous regulator decreases lipogenesis, and oxysterol sulfation can be a key protective regulatory pathway against lipid accumulation and lipid-induced inflammation in vivo.

Methods for oxysterol analysis: past, present and future

Oxysterols are oxidised forms of cholesterol or its precursors. In this article we will concentrate specifically on those formed in mammalian systems. Oxidation may be catalysed by endogenous enzymes or through reactive oxygen species forming a myriad of potential products. A number of these products are biologically active, and oxysterols may have roles in cholesterol homeostasis, neurogenesis, protein prenylation and in the immune system. Oxysterols are also implicated in aetiology of disease states including atherosclerosis, neurodegenerative and inflammatory diseases. Reports indicating the levels of oxysterols in plasma, cerebrospinal fluid and various tissues are in many cases unrealistic owing to a lack of attention to the possibility of autoxidation, a process by which oxysterols are formed from cholesterol by oxygen in air. This article comprises a critical assessment of the technical difficulties of oxysterol analysis, highlights methodologies utilising best practise and discusses newer procedures.

Oxysterol sulfation by cytosolic sulfotransferase suppresses liver X receptor/sterol regulatory element binding protein-1c signaling pathway and reduces serum and hepatic lipids in mouse models of nonalcoholic fatty liver disease

Cytosolic sulfotransferase (SULT2B1b) catalyzes oxysterol sulfation. 5-Cholesten-3β-25-diol-3-sulfate (25HC3S), one product of this reaction, decreases intracellular lipids in vitro by suppressing liver X receptor/sterol regulatory element binding protein (SREBP)-1c signaling, with regulatory properties opposite to those of its precursor 25-hydroxycholesterol. Upregulation of SULT2B1b may be an effective strategy to treat hyperlipidemia and hepatic steatosis. The objective of the study was to explore the effect and mechanism of oxysterol sulfation by SULT2B1b on lipid metabolism in vivo. C57BL/6 and LDLR(-/-) mice were fed with high-cholesterol diet or high-fat diet for 10 weeks and infected with adenovirus encoding SULT2B1b. SULT2B1b expressions in different tissues were determined by immunohistochemistry and Western blot. Sulfated oxysterols in liver were analyzed by high-pressure liquid chromatography. Serum and hepatic lipid levels were determined by kit reagents and hematoxylin and eosin staining. Gene expressions were determined by real-time reverse transcriptase polymerase chain reaction and Western Blot. Following infection, SULT2B1b was successfully overexpressed in the liver, aorta, and lung tissues, but not in the heart or kidney. SULT2B1b overexpression, combined with administration of 25-hydroxycholesterol, significantly increased the formation of 25HC3S in liver tissue and significantly decreased serum and hepatic lipid levels, including triglycerides, total cholesterol, free cholesterol, and free fatty acids, as compared with controls in both C57BL/6 and LDLR(-/-) mice. Gene expression analysis showed that increases in SULT2B1b expression were accompanied by reduction in key regulators and enzymes involved in lipid metabolism, including liver X receptor α, SREBP-1, SREBP-2, acetyl-CoA carboxylase-1, and fatty acid synthase. These findings support the hypothesis that 25HC3S is an important endogenous regulator of lipid biosynthesis.

Identification of a new plasma biomarker of Alzheimer's disease using metabolomics technology

We performed unbiased analysis of steroid-related compounds to identify novel Alzheimer's disease (AD) plasma biomarkers using liquid chromatography-atmospheric pressure chemical ionization-mass spectroscopy. The analysis revealed that desmosterol was found to be decreased in AD plasma versus controls. To precisely quantify variations in desmosterol, we established an analytical method to measure desmosterol and cholesterol. Using this LC-based method, we discovered that desmosterol and the desmosterol/cholesterol ratio are significantly decreased in AD. Finally, the validation of this assay using 109 clinical samples confirmed the decrease of desmosterol in AD as well as a change in the desmosterol/cholesterol ratio in AD. Interestingly, we could also observe a difference between mild cognitive impairment and control. In addition, the decrease of desmosterol was somewhat more significant in females. Receiver operating characteristic (ROC) analysis between controls and AD, using plasma desmosterol shows a score of 0.80, indicating a good discrimination power for this marker in the two reference populations and confirms the potential usefulness of measuring plasma desmosterol levels for diagnosing AD. Further analysis showed a significant correlation of plasma desmosterol with Mini-Mental State Examination scores. Although larger sample populations will be needed to confirm this diagnostic marker sensitivity, our studies demonstrate a sensitive and accurate method of detecting plasma desmosterol concentration and suggest that plasma desmosterol could become a powerful new specific biomarker for early and easy AD diagnosis.

High-dose supplemental selenite to male Syrian hamsters fed hypercholesterolaemic diets alters Ldlr, Abcg8 and Npc1l1 mRNA expression and lowers plasma cholesterol concentrations

The aim of the present study was to elucidate possible cholesterol-lowering mechanism(s) of high-dose supplemental Se in the form of selenite, a known hypocholesterolaemic agent. Male Syrian hamsters (four groups, ten per group) were fed semi-purified diets for 4 weeks containing 0·1 % cholesterol and 15 % saturated fat with selenite corresponding to varying levels of Se: (1) Se 0·15 parts per million (ppm), control diet; (2) Se 0·85 ppm; (3) Se 1·7 ppm; (4) Se 3·4 ppm. Lipids were measured in the bile, faeces, liver and plasma. The mRNA expression of several known regulators of cholesterol homeostasis (ATP-binding cassette transporters g5 (Abcg5) and g8 (Abcg8), 7-hydroxylase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, LDL receptor (LdLr) and Nieman-Pick C1-like 1 protein (Npc1l1)) were measured in the liver and/or jejunum. Oxysterols including 24-(S)-hydroxycholesterol, 25-hydroxycholesterol and 27-hydroxycholesterol (27-OHC) were measured in the liver. Significantly lower total plasma cholesterol concentrations were observed in hamsters consuming the low (0·85 ppm) and high (3·4 ppm) Se doses. The two highest doses of Se resulted in decreased plasma LDL-cholesterol concentrations and increased mRNA levels of hepatic Abcg8, Ldlr and jejunal Ldlr. Higher hepatic 27-OHC and TAG concentrations and lower levels of jejunal Npc1l1 mRNA expression were noted in the 1·7 and 3·4 ppm Se-treated hamsters. Overall, Se-induced tissue changes in mRNA expression including increased hepatic Abcg8 and Ldlr, increased jejunal Ldlr and decreased jejunal Npc1l1, provide further elucidation regarding the hypocholesterolaemic mechanisms of action of Se in the form of selenite.

Highly sensitive and specific analysis of sterol profiles in biological samples by HPLC-ESI-MS/MS

High-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) is a powerful method for the microanalysis of compounds in biological samples. Compared with gas chromatography-mass spectrometry (GC-MS), this method is more broadly applicable to various compounds and usually does not require a derivatization step before analysis. However, when neutral sterols are analyzed, the sensitivities of usual HPLC-MS/MS method are not superior to those of GC-MS because the sterols are relatively resistant to ionization. In this review, we introduce the recent development of HPLC-MS/MS analysis for the quantification of non-cholesterol sterols. By adding an effective derivatization step to the conventional procedure, sterol analysis by HPLC-MS/MS surpassed that obtained by GC-MS in sensitivity. In addition, sufficient specificity of this method was achieved by selected reaction monitoring (SRM) and thorough chromatographic separation of each sterol.

Antioxidant Supplements Improve Profiles of Hepatic Oxysterols and Plasma Lipids in Butter-fed Hamsters

Hypercholesterolemic diets are associated with oxidative stress that may contribute to hypercholesterolemia by adversely affecting enzymatically-generated oxysterols involved in cholesterol homeostasis. An experiment was conducted to examine whether the cholesterol-lowering effects of the antioxidants selenium and α-tocopherol were related to hepatic oxysterol concentrations. Four groups of male Syrian hamsters (n = 7-8) were fed high cholesterol and saturated fat (0.46% cholesterol, 14.3% fat) hypercholesterolemic semi-purified diets: 1) Control; 2) Control + α-tocopherol (67 IU all-racemic-α-tocopheryl-acetate/kg diet); 3) Control + selenium (3.4 mg selenate/kg diet); and 4) Control + α-tocopherol + selenium. Antioxidant supplementation was associated with lowered plasma cholesterol concentrations, decreased tissue lipid peroxidation and higher hepatic oxysterol concentrations. A second experiment examined the effect of graded selenium doses (0.15, 0.85, 1.7 and 3.4 mg selenate/kg diet) on mRNA expression of the oxysterol-generating enzyme, hepatic 27-hydroxylase (CYP27A1, EC 1.14.13.15), in hamsters (n = 8-9) fed the hypercholesterolemic diets. Supplementation of selenium at 3.4 mg selenate/kg diet was not associated with increased hepatic 27-hydroxylase mRNA. In conclusion, the cholesterol lowering effects of selenium and α-tocopherol were associated with increased hepatic enzymatically generated oxysterol concentrations, which appears to be mediated via improved antioxidant status rather than increased enzymatic production.

Are side-chain oxidized oxysterols regulators also in vivo?

Oxsterols are oxygenated metabolites of cholesterol that are short-lived intermediates or end products in cholesterol excretion pathways. They are present in very low concentrations in mammalian systems, always accompanied by a high excess of cholesterol. According to current concepts, side-chain oxidized oxysterols may be mediators of many cholesterol-induced regulatory effects. When added to cultured cells in vitro, side-chain oxidized oxysterols limit intracellular cholesterol levels by at least three different mechanisms: 1) binding to Insig with subsequent block of the sterol regulatory element-binding proteins (SREBP)-mediated mechanism for regulation of sterol sensitive genes; 2) increasing degradation of hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase, eventually by a mechanism involving binding of Insig to the enzyme; 3) activation of LXR-mediated stimulation of cholesterol transporters and cholesterol metabolism. Addition of pure unesterified oxysterols to cell cultures is however highly unphysiological, and the in vivo relevance of such experiments is questionable. Transgenic mouse models with markedly reduced or increased concentration of some specific oxysterols do not present marked disturbances in cholesterol turnover and homeostasis. Oxysterol-binding proteins such as LXR have been conclusively shown to be of importance for cholesterol turnover in vivo, but their physiological ligands have not yet been defined with certainty. During the last few years, new experimental data has accumulated supporting the contention that side-chain oxysterols are involved in some LXR-mediated regulation in vivo, at least in some biological systems. The new findings will be critically reviewed here.

Regulation of CYP3A4 and CYP2B6 expression by liver X receptor agonists

The liver X receptor (LXR) agonists, 24(S),25-epoxycholesterol and T0901317, were previously shown to be capable of inducing CYP3A expression in primary cultured rodent hepatocytes through activation of the pregnane X receptor (PXR). In this study, the abilities of these two LXR agonists to regulate CYP3A4 and CYP2B6 mRNA expression in primary cultures of human hepatocytes were evaluated. Treatment with 10 or 30 microM of the endogenous oxysterol, 24(S),25-epoxycholesterol, had no effect on CYP3A4 mRNA content in five preparations of primary cultured human hepatocytes, while 30 microM 24(S),25-epoxycholesterol treatment increased CYP2B6 mRNA content by approximately two-fold. By comparison, treatment with the synthetic LXR agonist, T0901317, potently increased CYP3A4 and CYP2B6 mRNA levels in the human hepatocyte cultures, producing multi-fold increases at 10nM. Using a HepG2-based transactivation assay, T0901317 activated human PXR with an EC(50) approximately 20nM, which was more than 10-fold lower than that of the potent PXR ligand, SR-12813, while treatment with 24(S),25-epoxycholesterol failed to induce reporter expression in this assay. Therefore, while 24(S),25-epoxycholesterol-mediated PXR activation and CYP3A induction does not appear to be conserved from rodent to human, T0901317 is among the most potent known activators of human PXR.

Hypolipidemic effects and mechanisms of Panax notoginseng on lipid profile in hyperlipidemic rats

Maintenance of normal lipid levels has implicated the involvement of genes induced by liver X receptor alpha (LXRalpha) and Farnesoid X receptor (FXR). This study was designed to evaluate the hypolipidemic effects of n-butanol extract (NE3) of Panax notoginseng (Burk.) F.H. Chen root on lipid homeostasis and investigate the possible mechanisms in animal experiments. In the transactivation assays, NE3 was identified as a dual FXR/LXRalpha agonist. Subsequently, Sprague-Dawley male rats on a high-fat/high-cholesterol diet were treated orally with NE3 or vehicle alone. As expected, the concentrations of serum TC, TG and LDL-C in rats treated with various concentrations of NE3 showed significant (P<0.01) and dose-dependent decrease, respectively, accompanied with a significant (P<0.01) and dose-dependent decrease in the concentration of hepatic TC and TG. Express-level analysis indicated that both LXRalpha target genes including ABCA1, ABCG5, ABCG8 and FXR target genes including ApoCII and SHP were significantly induced by NE3 (P<0.01). Interestingly, LDLR mRNA level was significantly higher by NE3 (P<0.01), accompanied with the significantly decreased expression levels of CYP7A1, ApoCIII and SREBP1c genes (P<0.01). Based on these results, it can be concluded that NE3 as a dual FXR/LXRalpha agonist largely prevented the accumulation of abnormal lipid in the hyperlipidemic rats.

Cytotoxic derivatives of (22R,23R)-dihydroxystigmastane

(22R,23R)-22,23-dihydroxystigmast-4-en-3-one, (22R,23R)-22,23-dihydroxystigmast-4-en-3,6-dione, (22R,23R)-3beta,5alpha,6beta,22,23-pentahydroxystigmastane, (22R,23R)-5alpha,6alpha-oxido-3beta,22,23-trihydroxystigmastane, (22R,23R)-5beta,6beta-oxido-3beta,22,23-trihydroxystigmastane, and (22R,23R)-3beta,6beta,22,23-tetrahydroxystigmast-4-ene were synthesized. Their cytotoxicities were comparatively studied using the MCF-7 line of carcinoma cells of human mammary gland and cells of human hepatoma of the Hep G2 line.

Synthesis of the oxysterol, 24(S), 25-epoxycholesterol, parallels cholesterol production and may protect against cellular accumulation of newly-synthesized cholesterol

AIM

The effects of 24(S),25-epoxycholesterol (24,25EC) on aspects of cholesterol homeostasis is well-documented. When added to cells, 24,25EC decreases cholesterol synthesis and up-regulates cholesterol efflux genes, including ABCA1. Synthesis of 24,25EC occurs in a shunt of the mevalonate pathway which also produces cholesterol. Therefore, 24,25EC synthesis should be subject to the same negative feedback regulation as cholesterol synthesis. To date, no role has been ascribed to 24,25EC in light of the fact that increased accumulation of cholesterol should decrease formation of this oxysterol through feedback inhibition. This leads to the intriguing paradox: why inhibit production of an apparently important regulator of cholesterol homeostasis when it is needed most?

METHODS

We used a combination of pharmacological and genetic approaches in Chinese Hamster Ovary cell-lines to investigate this paradox. Endogenous synthesis of 24,25EC was manipulated using partial inhibition of the enzyme, Oxidosqualene Cyclase. Changes in cholesterol and 24,25EC synthesis were determined using metabolic labelling with [1-14C]-acetate, thin-layer chromatography and phosphorimaging. Transcriptional effects mediated via SREBP and LXR were analysed by luciferase reporter assays.

RESULTS

We showed that cholesterol addition to cells lead to a rapid and preferential inhibition of 24,25EC synthesis. Addition of 24,25EC resulted in parallel inhibition of 24,25EC and cholesterol synthesis. Furthermore, we used a variety of approaches to examine the relationship between cholesterol and 24,25EC synthesis, including cell-lines with different rates of cholesterol synthesis, varying cholesterol synthetic rates by pre-treatment with a statin, or lipoprotein cholesterol loading of macrophages. In all cases, we showed that 24,25EC synthesis faithfully tracked cholesterol synthesis. Moreover, changes in 24,25EC synthesis exerted downstream effects, reducing SREBP transcriptional activity whilst increasing ABCA1 and LXR transcriptional activity.

CONCLUSION

Our results show that 24,25EC synthesis parallels cholesterol synthesis, consistent with this oxysterol functioning as a safety valve to protect against the accumulation of newly-synthesised cholesterol (as opposed to exogenously-derived cholesterol). Considering that 24,25EC is capable of being produced in all cholesterogenic cells, we propose that production of 24,25EC may represent a ubiquitous defence mechanism.

Enzymatic reduction of oxysterols impairs LXR signaling in cultured cells and the livers of mice

Liver X receptors (LXRs) are nuclear receptors that play crucial roles in lipid metabolism in vivo and are activated by oxysterol ligands in vitro. The identity of the ligand that activates LXRs in vivo is uncertain. Here we provide two lines of evidence that oxysterols are LXR ligands in vitro and in vivo. First, overexpression of an oxysterol catabolic enzyme, cholesterol sulfotransferase, inactivates LXR signaling in several cultured mammalian cell lines but does not alter receptor response to the nonsterol agonist T0901317. Adenovirus-mediated expression of the enzyme in mice prevents dietary induction of hepatic LXR target genes by cholesterol but not by T0901317. Second, triple-knockout mice deficient in the biosynthesis of three oxysterol ligands of LXRs, 24S-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol, respond to dietary T0901317 by inducing LXR target genes in liver but show impaired responses to dietary cholesterol. We conclude that oxysterols are in vivo ligands for LXR.

Cholesterol gallstone disease

With a prevalence of 10-15% in adults in Europe and the USA, gallstones are the most common digestive disease needing admission to hospital in the West. The interplay between interprandial and postprandial physiological responses to endogenous and dietary lipids underscores the importance of coordinated hepatobiliary and gastrointestinal functions to prevent crystallisation and precipitation of excess biliary cholesterol. Indeed, identifying the metabolic and transcriptional pathways that drive the regulation of biliary lipid secretion has been a major achievement in the field. We highlight scientific advances in protein and gene regulation of cholesterol absorption, synthesis, and catabolism, and biliary lipid secretion with respect to the pathogenesis of cholesterol gallstone disease. We discuss the physical-chemical mechanisms of gallstone formation in bile and the active role of the gallbladder and the intestine. We also discuss gaps in our knowledge of the pathogenesis of gallstone formation and the potential for gene targeting in therapy.

Identification of ligands for DAF-12 that govern dauer formation and reproduction in C. elegans

In response to environmental and dietary cues, the C. elegans orphan nuclear receptor, DAF-12, regulates dauer diapause, reproductive development, fat metabolism, and life span. Despite strong evidence for hormonal control, the identification of the DAF-12 ligand has remained elusive. In this work, we identified two distinct 3-keto-cholestenoic acid metabolites of DAF-9, a cytochrome P450 involved in hormone production, that function as ligands for DAF-12. At nanomolar concentrations, these steroidal ligands (called dafachronic acids) bind and transactivate DAF-12 and rescue the hormone deficiency of daf-9 mutants. Interestingly, DAF-9 has a biochemical activity similar to mammalian CYP27A1 catalyzing addition of a terminal acid to the side chain of sterol metabolites. Together, these results define the first steroid hormones in nematodes as ligands for an invertebrate orphan nuclear receptor and demonstrate that steroidal regulation of reproduction, from biology to molecular mechanism, is conserved from worms to humans.

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.

LXR deficiency and cholesterol feeding affect the expression and phenobarbital-mediated induction of cytochromes P450 in mouse liver

Metabolic transformation by the superfamily of cytochromes P450 (CYPs) plays an important role in the detoxification of xenobiotics such as drugs, environmental pollutants, and food additives. Endogenous substrates of CYPs include fatty acids, sterols, steroids, and bile acids. Induction of CYPs via transcriptional activation by substrates and other xenobiotics is an important adaptive mechanism that increases the organism's defense capability against toxicity. Numerous in vivo and in vitro data have highlighted the concept that the molecular mechanism of hepatic drug induction is linked to endogenous regulatory pathways. In particular, in vitro data suggest that oxysterols via the liver X receptor (LXR) inhibit phenobarbital (PB)-mediated induction of CYPs. To study the link between LXR, cholesterol homeostasis, and drug induction in vivo, we designed experiments in wild-type, LXRalpha-, LXRbeta-, and LXRalpha/beta-deficient mice. Our data expose differential regulatory patterns for Cyp2b10 and Cyp3a11 dependent on the expression of LXR isoforms and on challenge of cholesterol homeostasis by excess dietary cholesterol. Our results suggest that, in the mouse, liver cholesterol status significantly alters the pattern of expression of Cyp3a11, whereas the absence of LXR leads to an increase in PB-mediated activation of Cyp2b10.

Pregnane X receptor prevents hepatorenal toxicity from cholesterol metabolites

Efficient detoxification and clearance of cholesterol metabolites such as oxysterols, bile alcohols, and bile acids are critical for survival because they can promote liver and cardiovascular disease. We report here that loss of the nuclear xenobiotic receptor PXR (pregnane X receptor), a regulator of enterohepatic drug metabolism and clearance, results in an unexpected acute lethality associated with signs of severe hepatorenal failure when mice are fed with a diet that elicits accumulation of cholesterol and its metabolites. Induction of a distinct drug clearance program by a high-affinity ligand for the related nuclear receptor, the constitutive androstane receptor, does not overcome the lethality, indicating the unique requirement of PXR for detoxification. We propose that the PXR signaling pathway protects the body from toxic dietary cholesterol metabolites, and, by extension, PXR ligands may ameliorate human diseases such as cholestatic liver diseases and the associating acute renal failure.

Prevention of cholesterol gallstone disease by FXR agonists in a mouse model

Cholesterol gallstone disease is characterized by several events, including cholesterol precipitation in bile, increased bile salt hydrophobicity and gallbladder inflammation. Here, we describe the same phenotype in mice lacking the bile acid receptor, FXR. Furthermore, in susceptible wild-type mice that recapitulate human cholesterol gallstone disease, treatment with a synthetic FXR agonist prevented sequelae of the disease. These effects were mediated by FXR-dependent increases in biliary bile salt and phospholipid concentrations, which restored cholesterol solubility and thereby prevented gallstone formation. Taken together, these results indicate that FXR is a promising therapeutic target for treating or preventing cholesterol gallstone disease.

Adipocytic differentiation and liver x receptor pathways regulate the accumulation of triacylglycerols in human vascular smooth muscle cells

Lipid accumulation by vascular smooth muscle cells (VSMC) is a feature of atherosclerotic plaques. In this study we describe two mechanisms whereby human VSMC foam cell formation is driven by de novo synthesis of fatty acids leading to triacylglycerol accumulation in intracellular vacuoles, a process distinct from serum lipoprotein uptake. VSMC cultured in adipogenic differentiation medium accumulated lipids and were induced to express the adipocyte marker genes adipsin, adipocyte fatty acid-binding protein, C/EBPalpha, PPARgamma, and leptin. However, complete adipocyte differentiation was not observed as numerous genes present in mature adipocytes were not detected, and the phenotype was reversible. The rate of lipid accumulation was not affected by PPARgamma agonists, but screening for the effects of other nuclear receptor agonists showed that activation of the liver X receptors (LXR) dramatically promoted lipid accumulation in VSMC. Both LXRalpha and LXRbeta were present in VSMC, and their activation with TO901317 resulted in induction of the lipogenic genes fatty acid synthetase, sterol regulatory element binding protein (SREBP1c), and stearoyl-CoA desaturase. 27-Hydroxycholesterol, an abundant oxysterol synthesized by VSMC acted as an LXR antagonist and, therefore, may have a protective role in preventing foam cell formation. Immunohistochemistry showed that VSMC within atherosclerotic plaques express adipogenic and lipogenic markers, suggesting these pathways are present in vivo. Moreover, the development of an adipogenic phenotype in VSMC is consistent with their known phenotypic plasticity and may contribute to their dysfunction in atherosclerotic plaques and, thus, impinge on plaque growth and stability.

CYP3A induction by liver x receptor ligands in primary cultured rat and mouse hepatocytes is mediated by the pregnane X receptor

The effects of oxysterol and drug ligands of the liver X receptor (LXR) on cytochrome P450 expression were evaluated in primary cultured rodent hepatocytes. Treatment of rat hepatocyte cultures with either 25-hydroxycholesterol or 24(S),25-epoxycholesterol (10(-5) to 5 x 10(-5) M) produced concentration-dependent elevations in CYP3A mRNA and immunoreactive protein levels but did not increase the amounts of CYP1A1, CYP2B, or CYP4A gene products. The effects of 24(S),25-epoxycholesterol on CYP3A content were much greater than were those of 25-hydroxycholesterol, consistent with the relative abilities of these sterols to bind and activate LXR. To understand the mechanistic basis of these observations, experiments were performed using primary cultured hepatocytes prepared from LXRalpha/beta- or pregnane X receptor (PXR)-null mice. CYP3A mRNA levels were increased after treatment with 24(S),25-epoxycholesterol in both wild-type and LXR-null mouse hepatocytes. In contrast, neither 24(S),25-epoxycholesterol nor either of two additional potent LXR ligands, 22(R)-hydroxycholesterol and N-(2,2,2-trifluoroethyl)-N-[4-[2,2,2-trifluoro-1-hydroxy-1(trifluoromethyl)ethyl-]phenyl]-benzenesulfonamide (T0901317), altered CYP3A mRNA levels in hepatocytes prepared from PXR-null mice, although these agents induced CYP3A mRNA content in wild-type cultures. As evidence that the LXR ligands also activated PXR in rat hepatocytes, cotransfection of primary cultures with a dominant negative PXR abolished reporter gene induction after treatment with any of the test agents. These results indicate that selected LXR ligands are capable of activating PXR, probably as a defensive measure to prevent the accumulation of these potentially toxic endogenous molecules.

FXR-mediated down-regulation of CYP7A1 dominates LXRalpha in long-term cholesterol-fed NZW rabbits

We investigated how cholesterol feeding regulates cholesterol 7alpha-hydroxylase (CYP7A1) via the nuclear receptors farnesoid X receptor (FXR) and liver X receptor alpha (LXRalpha) in New Zealand white rabbits. After 1 day of 2% cholesterol feeding, when the bile acid pool size had not expanded, mRNA levels of the FXR target genes short-heterodimer partner (SHP) and sterol 12alpha-hydroxylase (CYP8B) were unchanged, indicating that FXR activation remained constant. In contrast, the mRNA levels of the LXRalpha target genes ATP binding cassette transporter A1 (ABCA1) and cholesteryl ester transfer protein (CETP) increased 5-fold and 2.3-fold, respectively, associated with significant increases in hepatic concentrations of oxysterols. Activity and mRNA levels of CYP7A1 increased 2.4 times and 2.2 times, respectively. After 10 days of cholesterol feeding, the bile acid pool size increased nearly 2-fold. SHP mRNA levels increased 4.1-fold while CYP8B declined 64%. ABCA1 mRNA rose 8-fold and CETP mRNA remained elevated. Activity and mRNA of CYP7A1 decreased 60% and 90%, respectively. Feeding cholesterol for 1 day did not enlarge the ligand pool size or change FXR activation, while LXRalpha was activated highly secondary to increased hepatic oxysterols. As a result, CYP7A1 was up-regulated. After 10 days of cholesterol feeding, the bile acid (FXR ligand) pool size increased, which activated FXR and inhibited CYP7A1 despite continued activation of LXRalpha. Thus, in rabbits, when FXR and LXRalpha are activated simultaneously, the inhibitory effect of FXR overrides the stimulatory effect of LXRalpha to suppress CYP7A1 mRNA expression.

X-ray crystal structure of the liver X receptor beta ligand binding domain: regulation by a histidine-tryptophan switch

The x-ray crystal structures of the human liver X receptor beta ligand binding domain complexed to sterol and nonsterol agonists revealed a perpendicular histidinetryptophan switch that holds the receptor in its active conformation. Hydrogen bonding interactions with the ligand act to position the His-435 imidazole ring against the Trp-457 indole ring, allowing an electrostatic interaction that holds the AF2 helix in the active position. The neutral oxysterol 24(S),25-epoxycholesterol accepts a hydrogen bond from His-435 that positions the imidazole ring of the histidine above the pyrrole ring of the tryptophan. In contrast, the acidic T0901317 hydroxyl group makes a shorter hydrogen bond with His-435 that pulls the imidazole over the electron-rich benzene ring of the tryptophan, possibly strengthening the electrostatic interaction. Point mutagenesis of Trp-457 supports the observation that the ligand-histidine-tryptophan coupling is different between the two ligands. The lipophilic liver X receptor ligand-binding pocket is larger than the corresponding steroid hormone receptors, which allows T0901317 to adopt two distinct conformations. These results provide a molecular basis for liver X receptor activation by a wide range of endogenous neutral and acidic ligands.

Chromatographic behavior of oxygenated derivatives of cholesterol

Oxygenated derivatives of cholesterol have important functions in many biochemical processes. These oxysterols are difficult to study because of their low physiological concentrations, the facile formation of cholesterol autoxidation artifacts, and lack of information on their chromatographic behavior. Focusing on metabolites and autoxidation products of cholesterol, we have documented the chromatographic mobilities of 35 oxysterols under a variety of conditions: eight solvent systems for thin-layer chromatography on silica gel, several mobile phases for reversed-phase high-performance liquid chromatography (HPLC), and two types of stationary phase for capillary gas chromatography (GC) using trimethylsilyl derivatives. Notable differences in selectivity could be obtained by modifying the stationary or mobile phases. Separations of oxysterol pairs isomeric at side-chain carbons or C-7 were achieved on normal-phase, reversed-phase, chiral, or silver-ion HPLC columns. Chromatographic behavior is also described for side-chain hexadeuterated and heptafluorinated oxysterols, which are useful as standards in isotope dilution analyses and autoxidation studies, respectively. The overall results are relevant to many problems of oxysterol analysis, including the initial separation of oxysterols from cholesterol, determination of highly polar and nonpolar oxysterols, separation of isomeric pairs, selection of derivatization conditions for GC analysis, and quantitation of the extent of cholesterol autoxidation.

Human sterol 27-hydroxylase (CYP27) overexpressor transgenic mouse model. Evidence against 27-hydroxycholesterol as a critical regulator of cholesterol homeostasis

CYP27-overexpressed transgenic mice were generated with the use of a human full-length CYP27 coding region cloned into a ubiquitous expression vector. Positive transgenic mice were identified by tail DNA genotyping and high fecal 27-hydroxycholesterol content. The levels of 27-hydroxycholesterol were found to be 3-5 times higher in the circulation and the tissues of the overexpressed mice when compared with littermate controls. There were no gross morphological differences between the overexpressed mice and their controls. Total cholesterol and triglyceride levels were not affected by overexpression of CYP27. Serum lathosterol was also normal, suggesting a normal rate of cholesterol synthesis. Serum levels of 7alpha-hydroxycholesterol were unaffected, suggesting a normal rate of bile acid formation in the pathway involving cholesterol 7alpha-hydroxylase. Biliary bile acid composition was slightly affected by CYP27 overexpression in female but not in male mice. Fecal levels of neutral steroids were slightly but significantly increased in overexpressor female mice but not in male mice. Levels of 24-hydroxycholesterol in the circulation were significantly reduced in the overexpressed mice, probably as a consequence of a recently described catabolic pathway involving CYP27. Combined with the results of our previous work on mice with a disruption of the CYP27 gene, the present results suggest that the levels of 27-hydroxycholesterol are not of critical importance for cholesterol homeostasis in mice.

Methods for measuring lipid metabolism in vivo

PURPOSE OF REVIEW

This review discusses diverse methods that have been used in several recent papers for the qualitative and quantitative analysis of lipids, studies of lipid oxidation, lipoprotein fractionation, and studies of lipid metabolism and metabolic kinetics using tracers. Papers for this review were selected on the basis of their timeliness, novelty, and/or their potential impact on diverse fields of lipid metabolism.

RECENT FINDINGS

Many methods used for studies of lipid metabolism employ advanced chromatographic and mass spectrometric techniques to characterize lipids. In particular, the use of stable isotopically labeled tracers has become increasingly important to study metabolic kinetics.

SUMMARY

Such developments in methodology will continue to advance studies of lipid metabolism in many areas of clinical interest, including heart disease, obesity, and diabetes.

Oxysterols in human circulation: which role do they have?

Oxysterols are oxygenated derivatives of cholesterol that are intermediates in cholesterol excretion pathways. They may also be regarded as transport forms of cholesterol and introduction of an additional hydroxyl group facilitates flux of cholesterol across cell membranes and the blood-brain barrier. According to current concepts, oxysterols are also mediating a number of cholesterol-induced metabolic effects. The recent discovery of nuclear receptors with an affinity for oxysterols has given support to this concept. Nuclear receptors such as liver X receptor alpha do have a role in cholesterol homeostasis, but there is still only indirect evidence that oxysterols are the physiological ligands. In this overview we report some recent advancements in our knowledge about the origin and metabolic fate of the quantitatively most important oxysterols occurring in the circulation. In addition, we discuss the possibility that some of these oxysterols may activate liver X receptors and regulate cholesterol homeostasis.