Bile acid synthesis in the Smith-Lemli-Opitz syndrome: effects of dehydrocholesterols on cholesterol 7α-hydroxylase and 27-hydroxylase activities in rat liver

Cholic acid increases plasma cholesterol in Smith-Lemli-Opitz syndrome: A pilot study

Background

Smith-Lemli-Opitz syndrome (SLOS) is an inherited disorder of cholesterol biosynthesis associated with congenital malformations, growth delay, intellectual disability and behavior problems. SLOS is caused by bi-allelic mutations in DHCR7, which lead to reduced activity of 7-dehydrocholesterol reductase that catalyzes the last step in cholesterol biosynthesis. Symptoms of SLOS are thought to be due to cholesterol deficiency and accumulation of its precursor 7-dehydrocholesterol (7-DHC) and 8-dehydrocholesterol (8-DHC), and toxic oxysterols. Therapy for SLOS often includes dietary cholesterol supplementation, but lipids are poorly absorbed from the diet, possibly due to impaired bile acid synthesis. We hypothesized that bile acid supplementation with cholic acid would improve dietary cholesterol absorption and raise plasma cholesterol levels.

Methods

Twelve SLOS subjects (10 M, 2F, ages 2-27 years) who had plasma cholesterol ≤125 mg/dL were treated with cholic acid (10 mg/kg/day) divided twice daily for 2 months. Plasma cholesterol, 7-DHC and 8-DHC were measured by GC-MS. Oxysterols were measured by ultra-high-performance LC-MS/MS. Data were analyzed using paired t-tests.

Results

At baseline, plasma cholesterol was 75 ± 24 mg/dL (mean ± SD; range 43-125, n = 12). After 2 months on cholic acid, mean plasma cholesterol increased to 97 ± 29 mg/dL (p = 0.011). Eleven of 12 subjects showed an increase in plasma cholesterol that varied from 3.8% to 85.7% (mean 38.7 ± 23.3%). 7-Hydroxycholesterol decreased by 20.6% on average (p = 0.013) but no significant changes were seen in 7-DHC or 8-DHC. Mean body weight tended to increase (3.6% p = 0.069). Subjects tolerated cholic acid well and experienced no drug-related adverse events.

Conclusions

In this pilot study, cholic acid supplementation was well tolerated and safe and resulted in an increase in plasma cholesterol in most SLOS subjects. Further controlled longitudinal studies are needed to look for the sustainability of the biochemical effect and possible clinical benefits.

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

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

Analysis of the ability of vitamin D3-metabolizing cytochromes P450 to act on vitamin D3 sulfate and 25-hydroxyvitamin D3 3-sulfate

25-Hydroxyvitamin D3 (25(OH)D3) is present in the human circulation esterified to sulfate with some studies showing that 25(OH)D3 3-sulfate levels are almost as high as unconjugated 25(OH)D3. Vitamin D3 is also present in human serum in the sulfated form as are other metabolites. Our aim was to determine whether sulfated forms of vitamin D3 and vitamin D3 metabolites can be acted on by vitamin D-metabolizing cytochromes P450 (CYPs), one of which (CYP11A1) is known to act on cholesterol sulfate. We used purified, bacterially expressed CYPs to test if they could act on the sulfated forms of their natural substrates. Purified CYP27A1 converted vitamin D3 sulfate to 25(OH)D3 3-sulfate with a catalytic efficiency (k_cat/K_m) approximately half that for the conversion of vitamin D3 to 25(OH)D3. Similarly, the rate of metabolism of vitamin D3 sulfate was half that of vitamin D3 for CYP27A1 in rat liver mitochondria. CYP2R1 which is also a vitamin D 25-hydroxylase did not act on vitamin D3 sulfate. CYP11A1 was able to convert vitamin D3 sulfate to 20(OH)D3 3-sulfate but at a considerably lower rate than for conversion of vitamin D3 to 20(OH)D3. 25(OH)D3 3-sulfate was not metabolized by the activating enzyme, CYP27B1, nor by the inactivating enzyme, CYP24A1. Thus, we conclude that 25(OH)D3 3-sulfate in the circulation may act as a pool of metabolically inactive vitamin D3 to be released by hydrolysis at times of need whereas vitamin D3 sulfate can be metabolized in a similar manner to free vitamin D3 by CYP27A1 and to a lesser degree by CYP11A1.

Sex-, age-, and organ-dependent improvement of bile acid hydrophobicity by ursodeoxycholic acid treatment: A study using a mouse model with human-like bile acid composition

Cyp2a12^-/-Cyp2c70^-/- double knockout (DKO) mice have a human-like hydrophobic bile acid (BA) composition and show reduced fertility and liver injury. Ursodeoxycholic acid (UDCA) is a hydrophilic and cytoprotective BA used to treat various liver injuries in humans. This study investigated the effects of orally administered UDCA on fertility and liver injury in DKO mice. UDCA treatment prevented abnormal delivery (miscarriage and preterm birth) in pregnant DKO mice, presumably by increasing the hydrophilicity of serum BAs. UDCA also prevented liver damage in six-week-old DKO mice, however liver injury emerged in UDCA-treated 20-week-old female, but not male, DKO mice. In 20-week-old male UDCA-treated DKO mice, conjugated plus unconjugated UDCA proportions in serum, liver, and bile were 71, 64, and 71% of the total BAs, respectively. In contrast, conjugated plus unconjugated UDCA proportions in serum, liver, and bile of females were 56, 34, and 58% of the total BAs, respectively. The UDCA proportion was considerably low in female liver only and was compensated by highly hydrophobic lithocholic acid (LCA). Therefore, UDCA treatment markedly reduced the BA hydrophobicity index in the male liver but not in females. This appears to be why UDCA treatment causes liver injury in 20-week-old female mice. To explore the cause of LCA accumulation in the female liver, we evaluated the hepatic activity of CYP3A11 and SULT2A1, which metabolize LCAs to more hydrophilic BAs. However, there was no evidence to suggest that either enzyme activity was lower in females than in males. As female mice have a larger BA pool than males, excessive loading of LCAs on the hepatic bile salt export pump (BSEP) may be the reason for the hepatic accumulation of LCAs in female DKO mice with prolonged UDCA treatment. Our results suggest that the improvement of BA hydrophobicity in DKO mice by UDCA administration is sex-, age-, and organ-dependent.

Diet, Sun, Physical Activity and Vitamin D Status in Children with Inflammatory Bowel Disease

In the course of inflammatory bowel disease (IBD) malabsorption may lead to a vitamin D deficiency and calcium–phosphate misbalance. However, the reports on the vitamin D status in children with IBD are few and ambiguous. Here, we are presenting complex analyses of multiple factors influencing 25OHD levels in IBD children (N = 62; Crohn’s disease n = 34, ulcerative colitis n = 28, mean age 14.4 ± 3.01 years, F/M 23/39) and controls (n = 47, mean age 13.97 ± 2.57, F/M 23/24). Additionally, calcium–phosphate balance parameters and inflammatory markers were obtained. In children with IBD disease, activity and location were defined. Information about therapy, presence of fractures and abdominal surgery were obtained from medical records. All subjects were surveyed on the frequency and extent of exposure to sunlight (forearms, partially legs for at least 30 min a day), physical activity (at least 30 min a day) and diet (3 days diary was analyzed with the program DIETA 5). The mean 25OHD level was higher in IBD patients compared to controls (18.1 ng/mL vs. 15.5 ng/mL; p = 0.03). Only 9.7% of IBD patients and 4.25% of controls had the optimal vitamin D level (30–50 ng/mL). Despite the higher level of 25OHD, young IBD patients showed lower calcium levels in comparison to healthy controls. There was no correlation between the vitamin D level and disease activity or location of gastrointestinal tract lesions. Steroid therapy didn’t have much influence on the vitamin D level while vitamin D was supplemented. Regular sun exposure was significantly more common in the control group compared to the IBD group. We found the highest concentration of vitamin D (24.55 ng/mL) with daily sun exposure. There was no significant correlation between the vitamin D level and frequency of physical activity. The analysis of dietary diaries showed low daily intake of vitamin D in both the IBD and the control group (79.63 vs. 85.14 IU/day). Pediatric patients, both IBD and healthy individuals, require regular monitoring of serum vitamin D level and its adequate supplementation.

Bile acid biosynthesis in Smith-Lemli-Opitz syndrome bypassing cholesterol: Potential importance of pathway intermediates

Bile acids are the end products of cholesterol metabolism secreted into bile. They are essential for the absorption of lipids and lipid soluble compounds from the intestine. Here we have identified a series of unusual Δ5-unsaturated bile acids in plasma and urine of patients with Smith-Lemli-Opitz syndrome (SLOS), a defect in cholesterol biosynthesis resulting in elevated levels of 7-dehydrocholesterol (7-DHC), an immediate precursor of cholesterol. Using liquid chromatography - mass spectrometry (LC-MS) we have uncovered a pathway of bile acid biosynthesis in SLOS avoiding cholesterol starting with 7-DHC and proceeding through 7-oxo and 7β-hydroxy intermediates. This pathway also occurs to a minor extent in healthy humans, but elevated levels of pathway intermediates could be responsible for some of the features SLOS. The pathway is also active in SLOS affected pregnancies as revealed by analysis of amniotic fluid. Importantly, intermediates in the pathway, 25-hydroxy-7-oxocholesterol, (25R)26-hydroxy-7-oxocholesterol, 3β-hydroxy-7-oxocholest-5-en-(25R)26-oic acid and the analogous 7β-hydroxysterols are modulators of the activity of Smoothened (Smo), an oncoprotein that mediates Hedgehog (Hh) signalling across membranes during embryogenesis and in the regeneration of postembryonic tissue. Computational docking of the 7-oxo and 7β-hydroxy compounds to the extracellular cysteine rich domain of Smo reveals that they bind in the same groove as both 20S-hydroxycholesterol and cholesterol, known activators of the Hh pathway.

Regulation of bile acid metabolism in mouse models with hydrophobic bile acid composition

The bile acid (BA) composition in mice is substantially different from that in humans. Chenodeoxycholic acid (CDCA) is an end product in the human liver; however, mouse Cyp2c70 metabolizes CDCA to hydrophilic muricholic acids (MCAs). Moreover, in humans, the gut microbiota converts the primary BAs, cholic acid and CDCA, into deoxycholic acid (DCA) and lithocholic acid (LCA), respectively. In contrast, the mouse Cyp2a12 reverts this action and converts these secondary BAs to primary BAs. Here, we generated Cyp2a12 KO, Cyp2c70 KO, and Cyp2a12/Cyp2c70 double KO (DKO) mice using the CRISPR-Cas9 system to study the regulation of BA metabolism under hydrophobic BA composition. Cyp2a12 KO mice showed the accumulation of DCAs, whereas Cyp2c70 KO mice lacked MCAs and exhibited markedly increased hepatobiliary proportions of CDCA. In DKO mice, not only DCAs or CDCAs but also DCAs, CDCAs, and LCAs were all elevated. In Cyp2c70 KO and DKO mice, chronic liver inflammation was observed depending on the hepatic unconjugated CDCA concentrations. The BA pool was markedly reduced in Cyp2c70 KO and DKO mice, but the FXR was not activated. It was suggested that the cytokine/c-Jun N-terminal kinase signaling pathway and the pregnane X receptor-mediated pathway are the predominant mechanisms, preferred over the FXR/small heterodimer partner and FXR/fibroblast growth factor 15 pathways, for controlling BA synthesis under hydrophobic BA composition. From our results, we hypothesize that these KO mice can be novel and useful models for investigating the roles of hydrophobic BAs in various human diseases.

Sterols and oxysterols in plasma from Smith-Lemli-Opitz syndrome patients

Smith-Lemli-Opitz syndrome (SLOS) is a severe autosomal recessive disorder resulting from defects in the cholesterol synthesising enzyme 7-dehydrocholesterol reductase (Δ⁷-sterol reductase, DHCR7, EC 1.3.1.21) leading to a build-up of the cholesterol precursor 7-dehydrocholesterol (7-DHC) in tissues and blood plasma. Although the underling enzyme deficiency associated with SLOS is clear there are likely to be multiple mechanisms responsible for SLOS pathology. In an effort to learn more of the aetiology of SLOS we have analysed plasma from SLOS patients to search for metabolites derived from 7-DHC which may be responsible for some of the pathology. We have identified a novel hydroxy-8-dehydrocholesterol, which is either 24- or 25-hydroxy-8-dehydrocholesterol and also the known metabolites 26-hydroxy-8-dehydrocholesterol, 4-hydroxy-7-dehydrocholesterol, 3β,5α-dihydroxycholest-7-en-6-one and 7α,8α-epoxycholesterol. None of these metabolites are detected in control plasma at quantifiable levels (0.5ng/mL).

New methods for analysis of oxysterols and related compounds by LC-MS

Oxysterols are oxygenated forms of cholesterol or its precursors. They are formed enzymatically and via reactive oxygen species. Oxysterols are intermediates in bile acid and steroid hormone biosynthetic pathways and are also bioactive molecules in their own right, being ligands to nuclear receptors and also regulators of the processing of steroid regulatory element-binding proteins (SREBPs) to their active forms as transcription factors regulating cholesterol and fatty acid biosynthesis. Oxysterols are implicated in the pathogenesis of multiple disease states ranging from atherosclerosis and cancer to multiple sclerosis and other neurodegenerative diseases including Alzheimer's and Parkinson's disease. Analysis of oxysterols is challenging on account of their low abundance in biological systems in comparison to cholesterol, and due to the propensity of cholesterol to undergo oxidation in air to generate oxysterols with the same structures as those present endogenously. In this article we review the mass spectrometry-based methods for oxysterol analysis paying particular attention to analysis by liquid chromatography-mass spectrometry (LC-MS).

On the formation of 7-ketocholesterol from 7-dehydrocholesterol in patients with CTX and SLO

A new mechanism for formation of 7-ketocholesterol was recently described involving cytochrome P-450 (CYP)7A1-catalyzed conversion of 7-dehydrocholesterol into 7-ketocholesterol with cholesterol-7,8-epoxide as a side product. Some patients with cerebrotendinous xanthomatosis (CTX) and all patients with Smith-Lemli-Opitz syndrome (SLO) have markedly increased levels of 7-dehydrocholesterol in plasma and tissues. In addition, the former patients have markedly upregulated CYP7A1. We hypothesized that these patients may produce 7-ketocholesterol from 7-dehydrocholesterol with formation of cholesterol-7,8-epoxide as a side product. In accord with this hypothesis, two patients with CTX were found to have increased levels of 7-ketocholesterol and 7-dehydrocholesterol, as well as a significant level of cholesterol-7,8-epoxide. The latter steroid was not detectable in plasma from healthy volunteers. Downregulation of CYP7A1 activity by treatment with chenodeoxycholic acid reduced the levels of 7-ketocholesterol in parallel with decreased levels of 7-dehydrocholesterol and cholesterol-7,8-epoxide. Three patients with SLO were found to have markedly elevated levels of 7-ketocholesterol as well as high levels of cholesterol-7,8-epoxide. The results support the hypothesis that 7-dehydrocholesterol is a precursor to 7-ketocholesterol in SLO and some patients with CTX.

7-Dehydrocholesterol metabolites produced by sterol 27-hydroxylase (CYP27A1) modulate liver X receptor activity

7-Dehydrocholesterol (7-DHC) is a common precursor of vitamin D3 and cholesterol. Although various oxysterols, oxygenated cholesterol derivatives, have been implicated in cellular signaling pathways, 7-DHC metabolism and potential functions of its metabolites remain poorly understood. We examined 7-DHC metabolism by various P450 enzymes and detected three metabolites produced by sterol 27-hydroxylase (CYP27A1) using high-performance liquid chromatography. Two were further identified as 25-hydroxy-7-DHC and 26/27-hydroxy-7-DHC. These 7-DHC metabolites were detected in serum of a patient with Smith-Lemli-Opitz syndrome. Luciferase reporter assays showed that 25-hydroxy-7-DHC activates liver X receptor (LXR) α, LXRβ and vitamin D receptor and that 26/27-hydroxy-7-DHC induces activation of LXRα and LXRβ, although the activities of both compounds on LXRs were weak. In a mammalian two-hybrid assay, 25-hydroxy-7-DHC and 26/27-hydroxy-7-DHC induced interaction between LXRα and a coactivator fragment less efficiently than a natural LXR agonist, 22(R)-hydroxycholesterol. These 7-DHC metabolites did not oppose agonist-induced LXR activation and interacted directly to LXRα in a manner distinct from a potent agonist. These findings indicate that the 7-DHC metabolites are partial LXR activators. Interestingly, 25-hydroxy-7-DHC and 26/27-hydroxy-7-DHC suppressed mRNA expression of sterol regulatory element-binding protein 1c, an LXR target gene, in HepG2 cells and HaCaT cells, while they weakly increased mRNA levels of ATP-binding cassette transporter A1, another LXR target, in HaCaT cells. Thus, 7-DHC is catabolized by CYP27A1 to metabolites that act as selective LXR modulators.

Treatment of Smith-Lemli-Opitz syndrome and other sterol disorders

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive genetic condition with a broad phenotype that results from deficiency of the final enzyme of the cholesterol synthesis pathway. This defect causes low or low-normal plasma cholesterol levels and increased 7- and 8-dehydrocholesterol (DHC) levels. Many therapies for SLOS and other disorders of sterol metabolism have been proposed, and a few of them have been undertaken in selected patients, but robust prospective clinical trials with validated outcome measures are lacking. We review the current literature and expert opinion on treatments for SLOS and other selected sterol disorders, including dietary cholesterol therapy, statin treatment, bile acid supplementation, medical therapies, and surgical interventions, as well as directions for future therapies and treatment research.

Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive, multiple congenital malformation and intellectual disability syndrome, with clinical characteristics that encompass a wide spectrum and great variability. Elucidation of the biochemical and genetic basis for SLOS, specifically understanding SLOS as a cholesterol deficiency syndrome caused by mutation in DHCR7, opened up enormous possibilities for therapeutic intervention. When cholesterol was discovered to be the activator of sonic hedgehog, cholesterol deficiency with inactivation of this developmental patterning gene was thought to be the cause of SLOS malformations, yet this explanation is overly simplistic. Despite these important research breakthroughs, there is no proven treatment for SLOS. Better animal models are needed to allow potential treatment testing and the study of disease pathophysiology, which is incompletely understood. Creation of human cellular models, especially models of brain cells, would be useful, and in vivo human studies are also essential. Biomarker development will be crucial in facilitating clinical trials in this rare condition, because the clinical phenotype can change over many years. Additional research in these and other areas is critical if we are to make headway towards ameliorating the effects of this devastating condition.

Malformation syndromes caused by disorders of cholesterol synthesis

Cholesterol homeostasis is critical for normal growth and development. In addition to being a major membrane lipid, cholesterol has multiple biological functions. These roles include being a precursor molecule for the synthesis of steroid hormones, neuroactive steroids, oxysterols, and bile acids. Cholesterol is also essential for the proper maturation and signaling of hedgehog proteins, and thus cholesterol is critical for embryonic development. After birth, most tissues can obtain cholesterol from either endogenous synthesis or exogenous dietary sources, but prior to birth, the human fetal tissues are dependent on endogenous synthesis. Due to the blood-brain barrier, brain tissue cannot utilize dietary or peripherally produced cholesterol. Generally, inborn errors of cholesterol synthesis lead to both a deficiency of cholesterol and increased levels of potentially bioactive or toxic precursor sterols. Over the past couple of decades, a number of human malformation syndromes have been shown to be due to inborn errors of cholesterol synthesis. Herein, we will review clinical and basic science aspects of Smith-Lemli-Opitz syndrome, desmosterolosis, lathosterolosis, HEM dysplasia, X-linked dominant chondrodysplasia punctata, Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects Syndrome, sterol-C-4 methyloxidase-like deficiency, and Antley-Bixler syndrome.

NPC2, the Protein Deficient in Niemann-Pick C2 Disease, Consists of Multiple Glycoforms That Bind a Variety of Sterols

Niemann-Pick C disease is a fatal neurodegenerative disorder characterized by an endolysosomal accumulation of cholesterol and other lipids. One form of the disease is caused by a deficiency in NPC2, a soluble lysosomal glycoprotein that binds cholesterol. To better understand the biological function of NPC2 and how its deficiency results in disease, we have characterized the structural and functional properties of recombinant human protein. Highly purified NPC2 consists of a complex mixture of glycosylated isoforms, similar to that observed in human brain autopsy specimens. Mass spectrometric analysis revealed that of the three potential N-linked glycosylation sites present in the mature protein, Asn-19 is not utilized; Asn-39 is linked to an endoglycosidase H (Endo H)-sensitive oligosaccharide, and Asn-116 is variably utilized, either being unmodified or linked to Endo H-sensitive or Endo H-resistant oligosaccharides. All glycoforms are endocytosed and ameliorate the cholesterol storage phenotype of NPC2-deficient fibroblasts. In addition, the purified preparation contains a mixture of both free and lipid-bound protein. All glycoforms bind cholesterol, and sterol binding to NPC2 significantly alters its behavior upon cation-exchange chromatography. Based on this observation, we developed chromatography-based binding assays and determined that NPC2 forms an equimolar complex with the fluorescent cholesterol analog dehydroergosterol. In addition, we find that NPC2 binds a range of cholesterol-related molecules (cholesterol precursors, plant sterols, some oxysterols, cholesterol sulfate, cholesterol acetate, and 5-alpha-cholestan-3-one) and that 27-hydroxysterol accumulates in NPC2-deficient mouse liver. Binding was not detected for various glycolipids, phospholipids, or fatty acids. These biochemical properties support a direct and specialized function of NPC2 in lysosomal sterol transport.

Hypercholesterolemia in rats with hepatomas: increased oxysterols accelerate efflux but do not inhibit biosynthesis of cholesterol

Hypercholesterolemia is an important paraneoplastic syndrome in patients with hepatoma, but the nature of this defect has not yet been identified. We investigated the molecular mechanisms of hypercholesterolemia in a hepatoma-bearing rat model. Buffalo rats were implanted in both flanks with Morris hepatoma 7777 (McA-RH7777) cells. After 4 weeks, tumor weight was 5.5+/-1.7 g, and serum cholesterol level increased from 60+/-2 to 90+/-2 mg/dL. Protein and mRNA expression of the ATP-binding cassette transporters A1 and G1 (ABCA1 and ABCG1) was markedly higher in tumors than in livers. These increases were associated with activation of liver X receptor alpha (LXRalpha) as a result of the increased tissue oxysterol concentrations. The accumulation of oxysterols in the hepatomas appeared to be caused mainly by the upregulation of cholesterol biosynthesis, despite the increased tissue sterol concentrations. Overexpression of the sterol regulatory element-binding protein (SREBP) processing system relative to sterol concentration contributed to the resistance to sterols in this tumor. In addition, bile acid biosynthesis was inhibited despite the reduced expression of the small heterodimer partner (SHP) and activated LXRalpha, which also appeared to contribute to the accumulation of oxysterols followed by the acceleration of cholesterol efflux. In conclusion, hypercholesterolemia in McA-RH7777 hepatoma-bearing rats was caused by increased cholesterol efflux from tumors as a result of activation of LXRalpha. Overexpression of the SREBP processing system contributed to the activation of LXRalpha by maintaining high oxysterol levels in tissue.

Stigmasterol reduces plasma cholesterol levels and inhibits hepatic synthesis and intestinal absorption in the rat

Plant sterols compete with cholesterol (cholest-5-en-3beta-ol) for intestinal absorption to limit absorption and lower plasma concentrations of cholesterol. Stigmasterol (24-ethyl-cholesta-5,22-dien-3beta-ol; Delta(22) derivative of sitosterol [24-ethyl-cholest-5-en-3beta-ol]), but not campesterol (24-methyl-cholest-5-en-3beta-ol) and sitosterol, is reported to inhibit cholesterol biosynthesis via inhibition of sterol Delta(24)-reductase in human Caco-2 and HL-60 cell lines. We studied the effect of feeding 0.5% stigmasterol on plasma and liver sterols and intestinal cholesterol and sitosterol absorption in 12 wild-type Kyoto (WKY) and 12 Wistar rats. After 3 weeks of feeding, cholesterol and sitosterol absorption was determined in 6 rats from each group by plasma dual-isotope ratio method. After 3 more weeks, plasma and hepatic sterols and hepatic enzyme activities were determined in all rats. After feeding stigmasterol, baseline plasma cholesterol was 1.3 times and plant sterols 3 times greater in WKY compared with Wistar rats. Stigmasterol feeding lowered plasma cholesterol by approximately 11%, whereas plasma campesterol and sitosterol levels were virtually unchanged in both rat strains, and stigmasterol constituted 3.2% of plasma sterols in WKY rats and 1% in Wistar rats. After 6 weeks of feeding, cholesterol and sitosterol absorption decreased 23% and 30%, respectively, in WKY, and 22% and 16%, respectively, in the Wistar rats as compared with untreated rats. The intestinal bacteria in both rat strains metabolized stigmasterol to mainly the 5beta-H stanol (>40%), with only small amounts of 5alpha-H derivative (approximately 1.5%), whereas the C-22 double bond was resistant to bacterial metabolism. Hepatic stigmasterol levels increased from 11 microg/g liver tissue to 104 mug/g in WKY rats and from 5 microg/g liver tissue to 21 microg/g in Wistar rats. 3-Hydroxy-3-methylglutaryl coenzyme A reductase activity was suppressed 4-fold in the WKY and almost 1.8-fold in Wistar rats, cholesterol 7alpha-hydroxylase activity was suppressed 1.6-fold in the WKY and 3.5-fold in Wistar rats, whereas cholesterol 27-hydroxylase activity was unchanged after feeding. In conclusion, stigmasterol, when fed, lowers plasma cholesterol levels, inhibits intestinal cholesterol and plant sterol absorption, and suppresses hepatic cholesterol and classic bile acid synthesis in Wistar as well as WKY rats. However, plasma and hepatic incorporation of stigmasterol is low.

Residual cholesterol synthesis and simvastatin induction of cholesterol synthesis in Smith-Lemli-Opitz syndrome fibroblasts

Smith-Lemli-Opitz syndrome (RSH/SLOS) is an autosomal recessive, malformation syndrome caused by mutations in the 3beta-hydroxysterol delta7-reductase gene (DHCR7). DHCR7 catalyzes the reduction of 7-dehydrocholesterol (7DHC) to cholesterol. We report the mutation analysis and determination of residual cholesterol synthesis in 47 SLOS patients, and the effects of treatment of SLOS skin fibroblasts with simvastatin. Using deuterium labeling we have quantified the amount of synthesized cholesterol and 7DHC in homozygote, heterozygote, and control fibroblast cell lines. In SLOS fibroblasts, the fraction of synthesized cholesterol to total sterol synthesis ranged from undetectable to over 50%. This establishes that different mutant alleles encode enzymes with varying degrees of residual activity. There was a correlation between increased phenotypic severity and decreased residual cholesterol synthesis (r(2)=0.45, p<0.0001). Simvastatin treatment of SLOS fibroblasts with residual DHCR7 enzymatic activity decreased 7DHC levels and increased cholesterol synthesis. This increase in cholesterol synthesis is due to increased expression of a mutant allele with residual function. Determination of residual enzymatic activity for specific DHCR7 mutant alleles will help in understanding the processes underlying the broad phenotypic spectrum found in this disorder and will be useful in identifying patients who may benefit from simvastatin therapy.

Characterization of liver involvement in defects of cholesterol biosynthesis: long-term follow-up and review

Inborn defects of cholesterol biosynthesis are a group of metabolic disorders presenting with mental retardation and multiple congenital anomalies (MCA/MR syndromes). Functional and structural liver involvement has been reported as a rare (2.5-6%) complication of the Smith-Lemli-Opitz syndrome (SLOS) and it has not been fully characterized. Here, we report on a long-term follow-up study of four patients with SLOS, and one case with lathosterolosis who presented with liver disease and underwent an extensive diagnostic work-up. Reports of liver involvement in cholesterol biosynthesis defects are reviewed. Two main different patterns of liver involvement emerged: progressive cholestasis, and stable isolated hypertransaminasemia. In our series, the first pattern was found in two patients with SLOS and one with lathosterolosis, and the second in two SLOS cases. Cholestasis was associated with early lethality and normal serum gamma-glutamyl-transferase (GGT) levels in SLOS, while possible prolonged survival and high GGT levels were seen in lathosterolosis. Hepatic fibrosis was present in both conditions. Liver biopsy performed in one of our SLOS patients with isolated hypertransaminasemia, showed only mild hydropic degeneration of the hepatocytes. The presence of liver involvement in 16% of the SLOS patients diagnosed at our Center suggests that this complication might have been underestimated in previously reported cases, possibly overshadowed by the severity of multiple malformations. Fetal hepatopathy, cholestasis, and isolated hypertransaminasemia can occur also in other disorders of cholesterol biosynthesis, such as mevalonic aciduria, desmosterolosis, Conradi-Hunermann syndrome, Greenberg dysplasia, and Pelger-Huet homozygosity syndrome. This group of inherited disorders should be considered in the differential diagnosis of patients presenting with liver disease associated with developmental delay and/or multiple malformations. Periodic liver function evaluations are recommended in these patients.

Bile acid synthetic defects and liver disease: a comprehensive review

Bile acid synthetic defects (BASD), uncommon genetic disorders that are responsible for approximately 2% of persistent cholestasis in infants, are reviewed with emphasis on morphology of associated liver disease. The associated liver diseases may be life threatening, and are treatable, usually by replacement of deficient primary bile acids. Specific diagnosis is made by analysis of body fluids (bile, blood, and urine) using fast atom bombardment-mass spectroscopy (FAB-MS) and gas chromatography-mass spectroscopy (GC-MS). Inborn errors have been demonstrated for four single enzymes involved in modification of the sterol nucleus and in five steps in modification of the side-chain to form cholic and chenodeoxycholic acids, the primary bile acids. With few exceptions, BASD cause liver diseases that vary from severe to mild depending on the defect. In three of four known defects of sterol nucleus modification, liver disease is progressive. Progression of liver disease is most rapid when the defect results in accumulation of toxic monohydroxy and unsaturated oxo-bile acids. Liver disease may be transient, delayed in onset and mild. Reduced bile flow caused by atypical bile acids contributes to cholestasis and may be the dominant factor in defects of side-chain synthesis, peroxisomal abiogenesis and S-L-O syndrome. Pathological findings may include intralobular cholestasis with giant cell transformation, prevalence of necrotic hepatocytes including giant cell forms, and hepatitic injury confined to the portal limiting plate where the smallest bile ductules may be injured and where fibrosis typically develops. Interlobular bile ducts are usually spared. Ultrastructure of liver reveals nonspecific changes with the possible exception of unusual canalicular morphology in some defects. The course of BASD may be modified by replacement of deficient primary bile acids, which produces beneficial feedback inhibition of abnormal bile acid production and enhances choluresis. Giant cell transformation is present in all symptomatic infants with BASD and seems to have a more consistent association with BASD than with the many other liver diseases in infants where it occurs. We hypothesize that immature hepatocytes of infants may fuse to form multinucleate hepatocytes whenever atypical or toxic bile acids are present and the pool of normal bile acids is critically reduced.

Significance of plasma 7alpha-hydroxy-4-cholesten-3-one and 27-hydroxycholesterol concentrations as markers for hepatic bile acid synthesis in cholesterol-fed rabbits

Plasma 7alpha-hydroxy-4-cholesten-3-one has been used as an index of hepatic bile acid synthesis. The aim of the current study was to ascertain whether the level of this oxysterol reflects hepatic cholesterol 7alpha-hydroxylase activity when plasma cholesterol concentrations are markedly changed. In addition, the relationship of hepatic sterol 27-hydroxylase activity with plasma concentrations of 27-hydroxycholesterol and 3beta-hydroxy-5-cholestenoic acid was studied. We used New Zealand white rabbits fed 2% cholesterol for 5 or 10 days and/or constructed bile fistula. Feeding cholesterol markedly increased and bile drainage reduced plasma cholesterol concentrations. Initially, in these models there was no correlation between plasma 7alpha-hydroxy-4-cholesten-3-one concentrations and hepatic cholesterol 7alpha-hydroxylase activities (r = -0.24, n = 10). Cholesterol feeding was associated with downregulated 7alpha-hydroxylase activities, while plasma 7alpha-hydroxy-4-cholesten-3-one concentrations were elevated in the presence of increased plasma cholesterol levels. However, this discrepancy was overcome and significant correlation was observed (r = 0.73, P <.05, n = 10) by expressing 7alpha-hydroxy-4-cholesten-3-one levels relative to cholesterol. In contrast, hepatic sterol 27-hydroxylase activities were not significantly correlated with plasma absolute (r = 0.23, difference not significant [NS], n = 10) nor cholesterol-related levels of 27-hydroxycholesterol (r = -0.13, NS, n = 10), or 3beta-hydroxy-5-cholestenoic acid concentrations (r = 0.30, NS, n = 10). In conclusion, plasma 7alpha-hydroxy-4-cholesten-3-one concentrations reflected hepatic cholesterol 7alpha-hydroxylase activities when the sterol levels were adjusted to plasma cholesterol concentrations in rabbits with hypercholesterolemia. The results suggest that plasma 7alpha-hydroxy-4-cholesten-3-one relative to cholesterol is a better marker for hepatic cholesterol 7alpha-hydroxylase activity than the absolute concentration when hypercholesterolemia is present. In contrast, 27-hydroxycholesterol and 3beta-hydroxy-5-cholestenoic acid levels in plasma did not reflect hepatic sterol 27-hydroxylase activities even if the levels were adjusted to plasma cholesterol concentrations.

Novel sterols synthesized via the CYP27A1 metabolic pathway

A major biologic role of the ubiquitous mitochondrial P450 enzyme CYP27A1 is the generation of ligands such as 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid, which regulate the expression of nuclear receptors that govern many aspects of cholesterol homeostasis. We now report that sterol intermediates in cholesterol synthesis, beginning with the initial post-cyclization sterol, lanosterol, continuing with zymosterol, and ending with desmosterol are also substrates for the enzyme. Using the human enzyme expressed in Escherichia coli, we characterized the retention times and major mass fragments of these novel metabolites. Although sequestration of the enzyme in the inner mitochondrial membrane and normal subcellular organization probably greatly restrict the proportion of these and other intermediates in cholesterol synthesis that undergo side chain oxidation, disruption of compartmentalization can bypass cholesterol as the end product and give rise to potent ligands that further modify gene expression.

Human malformation syndromes due to inborn errors of cholesterol synthesis

PURPOSE OF REVIEW

This review covers a group of human malformation syndromes, which are caused by inborn errors of cholesterol synthesis. The Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive, multiple malformation, and mental retardation syndrome that is the prototypical example of this group of disorders. In the 10 years since the biochemical cause of SLOS was identified, other malformation syndromes have been shown to result from defects in this pathway. These include desmosterolosis, lathosterolosis, X-linked dominant chondrodysplasia punctata type 2 (CDPX2), congenital hemidysplasia with ichthyosiform erythroderma and limb defects (CHILD syndrome), hydrops-ectopic calcification-moth-eaten skeletal dysplasia (HEM dysplasia), and some cases of Antley-Bixler syndrome. These disorders represent the first true merging of dysmorphology with biochemical genetics.

RECENT FINDINGS

Recent studies report the identification of human lathosterolosis patients, indicate that SLOS is a relatively common genetic disorder that may be a major unrecognized cause of fetal loss, suggest that correction of the biochemical defect can improve central nervous system function, and show that perturbed sonic hedgehog signaling due to decreased sterol levels likely underlies some of the malformations in SLOS and lathosterolosis.

SUMMARY

Recognition of the biochemical defect in these syndromes has given insight into the role that cholesterol plays during normal development, into understanding the pathophysiological processes that underlie the clinical problems found in these disorders, and into developing therapeutic interventions.

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.

27-Hydroxylation of 7- and 8-dehydrocholesterol in Smith-Lemli-Opitz syndrome: a novel metabolic pathway

Smith-Lemli-Opitz syndrome (SLOS) is attributable to mutations in the gene coding for 7-dehydrocholesterol reductase. Low to absent enzyme activity accounts for the accumulation of both 7-dehydrocholesterol and 8-dehydrocholesterol in plasma and other tissues. Since oxysterols can participate in the regulation of cholesterol homeostasis, we examined the possibility that they are formed from these dehydrocholesterol intermediates. In patients with SLOS, we found serum levels of 27-hydroxy-7-dehydrocholesterol ranging from 0.1 to 0.25micro M and evidence for circulating levels of 27-hydroxy-8-dehydrocholesterol (0.04-0.51 micro M). Picomolar quantities of 27-hydroxy-7-dehydrocholesterol were identified in normal individuals. Biologic activities of 27-hydroxy-7-dehydrocholesterol were found to include inhibition of sterol synthesis and the activation of nuclear receptor LXRalpha but not that of LXRbeta. These activities occurred at concentrations found in plasma and presumably at those existing in tissues. Thus, patients with SLOS have increased levels of metabolites derived from intermediates in cholesterol synthesis that are biologically active and may contribute to the regulation of cholesterol synthesis in vivo.

Selective inhibition of CYP27A1 and of chenodeoxycholic acid synthesis in cholestatic hamster liver

The aim of this study was to explore the regulation of serum cholic acid (CA)/chenodeoxycholic acid (CDCA) ratio in cholestatic hamster induced by ligation of the common bile duct for 48 h. The serum concentration of total bile acids and CA/CDCA ratio were significantly elevated, and the serum proportion of unconjugated bile acids to total bile acids was reduced in the cholestatic hamster similar to that in patients with obstructive jaundice. The hepatic CA/CDCA ratio increased from 3.6 to 11.0 (P<0.05) along with a 2.9-fold elevation in CA concentration (P<0.05) while the CDCA level remained unchanged. The hepatic mRNA and protein level as well as microsomal activity of the cholesterol 7alpha-hydroxylase, 7alpha-hydroxy-4-cholesten-3-one 12alpha-hydroxylase and 5beta-cholestane-3alpha,7alpha,12alpha-triol 25-hydroxylase were not significantly affected in cholestatic hamsters. In contrast, the mitochondrial activity and enzyme mass of the sterol 27-hydroxylase were significantly reduced, while its mRNA levels remained normal in bile duct-ligated hamster. In conclusion, bile acid biosynthetic pathway via mitochondrial sterol 27-hydroxylase was preferentially inhibited in bile duct-ligated hamsters. The suppression of CYP27A1 is, at least in part, responsible for the relative decreased production of CDCA and increased CA/CDCA ratio in the liver, bile and serum of cholestatic hamsters.

Human cholesterol 7alpha-hydroxylase (CYP7A1) deficiency has a hypercholesterolemic phenotype

Bile acid synthesis plays a critical role in the maintenance of mammalian cholesterol homeostasis. The CYP7A1 gene encodes the enzyme cholesterol 7alpha-hydroxylase, which catalyzes the initial step in cholesterol catabolism and bile acid synthesis. We report here a new metabolic disorder presenting with hyperlipidemia caused by a homozygous deletion mutation in CYP7A1. The mutation leads to a frameshift (L413fsX414) that results in loss of the active site and enzyme function. High levels of LDL cholesterol were seen in three homozygous subjects. Analysis of a liver biopsy and stool from one of these subjects revealed double the normal hepatic cholesterol content, a markedly deficient rate of bile acid excretion, and evidence for upregulation of the alternative bile acid pathway. Two male subjects studied had hypertriglyceridemia and premature gallstone disease, and their LDL cholesterol levels were noticeably resistant to 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors. One subject also had premature coronary and peripheral vascular disease. Study of the kindred, which is of English and Celtic background, revealed that individuals heterozygous for the mutation are also hyperlipidemic, indicating that this is a codominant disorder.

Side chain hydroxylations in bile acid biosynthesis catalyzed by CYP3A are markedly up-regulated in Cyp27-/- mice but not in cerebrotendinous xanthomatosis

The accumulation of various 25-hydroxylated C(27)-bile alcohols in blood and their excretion in urine are characteristic features of cerebrotendinous xanthomatosis (CTX) a recessively inherited inborn error of bile acid synthesis caused by mutations in the mitochondrial sterol 27-hydroxylase (CYP27) gene. These bile alcohols may be intermediates in the alternative cholic acid side chain cleavage pathway. The present study was undertaken to identify enzymes and reactions responsible for the formation of these bile alcohols and to explain why Cyp27(-/-) mice do not show CTX-related abnormalities. Microsomal activities of 5beta-cholestane-3alpha,7alpha,12alpha-triol 25- and 26-hydroxylases, 5beta-cholestane-3alpha,7alpha,12alpha,25-tetrol 23R-, 24S-, and 27-hydroxylases and testosterone 6beta-hydroxylase, a marker enzyme for CYP3A, in Cyp27(-/-) mice livers were markedly up-regulated (5.5-, 3.5-, 6.5-, 7.5-, 2.9-, and 5.4-fold, respectively). In contrast, these enzyme activities were not increased in CTX. The activities of 5beta-cholestane-3alpha,7alpha,12alpha-triol 25- and 26-hydroxylases and 5beta-cholestane-3alpha,7alpha,12alpha,25-tetrol 23R-, 24R-, 24S-, and 27-hydroxylases were strongly correlated with the activities of testosterone 6beta-hydroxylase in control human liver microsomes from eight unrelated donors. Troleandomycin, a specific inhibitor of CYP3A, markedly suppressed these microsomal side chain hydroxylations in both mouse and human livers in a dose-dependent manner. In addition, experiments using recombinant overexpressed human CYP3A4 confirmed that these microsomal side chain hydroxylations were catalyzed by a single enzyme, CYP3A4. The results demonstrate that microsomal 25- and 26-hydroxylations of 5beta-cholestane-3alpha,7alpha,12alpha-triol and microsomal 23R-, 24R-, 24S-, and 27-hydroxylations of 5beta-cholestane-3alpha,7alpha,12alpha,25-tetrol are mainly catalyzed by CYP3A in both mice and humans. Unlike Cyp27(-/-) mice, CYP3A activity was not up-regulated despite marked accumulation of 5beta-cholestane-3alpha,7alpha,12alpha-triol in CTX.

Liver disease caused by disorders of bile acid synthesis

Bile acid synthetic defects are uncommon disorders that cause progressive cholestatic liver disease that is often lethal in infancy or early childhood. Five specific primary defects have been described. Diagnosis is based on mass spectrometry of urine and serum. Pathogenesis of liver injury is related to persistent reduction in levels of normal bile acids and accumulation of abnormal, potentially hepatotoxic, intermediaries. Sites of injury are the liver cell, the bile canaliculus, and the smallest bile ductules. The interlobular bile ducts are normal. The liver lesion is progressive chronic hepatitis with an especially high incidence of GCT in patients who present in infancy. Bile acid replacement therapy is usually effective in arresting the liver injury. Regression of liver damage has been documented during treatment of patients who were diagnosed early in life. Because bile acid synthetic disorders are the only cholestatic diseases of infancy in which GCT of hepatocytes is consistently present, the author suggest that the injury responsible for GCT may be specific for toxic bile acids. Accordingly, immaturity of the bile acid synthetic pathway may render many otherwise normal infants vulnerable to transient "neonatal hepatitis" with GCT in a broad range of cholestatic disorders.