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Keywords:

  • Antley–Bixler syndrome;
  • bile acid homeostasis;
  • cholesterol;
  • CYP;
  • cytochrome P450;
  • knockout;
  • lipid homeostasis;
  • mouse models;
  • P450 reductase

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

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


Abbreviations
ABS

Antley–Bixler syndrome

AD

Alzheimer’s disease

BA

bile acid

CA

cholic acid

CDCA

chenodeoxycholic acid

CTX

cerebrotendinous xanthomatosis

CYP

cytochrome P450

CYP27A1

sterol 27-hydroxylase

CYP39A1

oxysterol 7α-hydroxylase II

CYP46A1

cholesterol 24-hydroxylase

CYP51

lanosterol 14α-demethylase

CYP7A1

cholesterol 7α-hydroxylase

CYP7B1

oxysterol 7α-hydroxylase

CYP8B1

sterol 12α-hydroxylase

DHEA

dehydroepiandrosterone

HDL

high-density lipoprotein

LDL

low-density lipoprotein

OMIM

Online Mendelian Inheritance in Man

POR

cytochrome P450 reductase

SNP

single nucleotide polymorphism

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

Biochemically, cholesterol plays a crucial role in eukaryotic cell growth and development. Biosynthesis of cholesterol presents the foundation for normal membrane synthesis and the regulation of membrane fluidity. Cholesterol is also an important precursor for the synthesis of biomolecules necessary for normal cell function. Furthemore, cholesterol provides building blocks for the synthesis of vitamin D, bile acids (BAs) and steroid hormones, all of which play important roles in maintaining the integrity of the organism.

Cholesterol can be obtained either from the diet or it can be synthesized de novo from acetyl coenzyme A. These two pathways are interdependent, meaning that the dietary intake and cellular requirements influence cholesterol biosynthesis in a complex feedback manner. However, daily needs can be met equally well from one or the other [1]. Cholesterol homeostasis is also maintained through its elimination pathways. The major disposal route in mammals is conversion into BAs, which are ultimately excreted from the body. As a result of their physicochemical properties, BAs stimulate biliary lipid secretion and enable the solubilization and absorption of dietary lipids and lipid soluble vitamins in the intestine [2].

In these intertwined mechanisms of cholesterol homeostasis, cytochrome P450s (CYPs) play crucial roles. Lanosterol 14α-demethylase (CYP51) is the only CYP involved in cholesterol synthesis [3]. It removes the 14α-methyl group from lanosterol and 24,25-dihydrolanosterol to produce the intermediate follicular fluid-meiosis-activating sterol [4]. Additional CYPs take part in the catabolism of cholesterol to BAs in two major pathways: the classic (‘neutral’) and the alternative (‘acidic’). The classic pathway in humans leads to two primary BAs: cholic acid (CA) and chenodeoxycholic acid (CDCA). Synthesis begins with the formation of 7α-hydroxycholesterol by cholesterol 7α-hydroxylase (CYP7A1), the rate-limiting enzyme of the classic pathway. Several enzymatic steps follow, including sterol 12α-hydroxylation by sterol 12α-hydroxylase (CYP8B1) that directs the synthesis to CA, and sterol 27-hydroxylation by sterol 27-hydroxylase (CYP27A1). In the alternative pathway, the side-chain oxidation of cholesterol precedes the steroid ring modifications. The first step involves the oxidation of cholesterol to 27-hydroxycholesterol by CYP27A1, which is subsequently hydroxylated by oxysterol 7α-hydroxylase (CYP7B1). This pathway leads mainly to the formation of CDCA in humans [2]. Under normal conditions, the alternative pathway salvages ∼ 10% of daily BA losses [5], although it can become the major pathway when CYP7A1 activity is deficient [6,7].

The third possibility for BA synthesis is through oxidation of cholesterol to 24(S)- and 25-hydroxycholesterol derivatives. Cholesterol 24-hydroxylase (CYP46A1) is expressed mainly in the brain [8]. After 24(S)-hydroxycholesterol passes the blood–brain barrier, it is 7α-hydroxylated by oxysterol 7α-hydroxylase II (CYP39A1) in the liver. Although the contribution of this route to the overall BA synthesis is minor, 24(S)-hydroxycholesterol and other oxysterols are potent regulators of lipid homeostasis via activation of the liver X receptor [9,10].

The present review discusses CYPs that participate in cholesterol and BA synthesis. Figure 1 shows the classic and the alternative BA synthesis pathways, together with the chemical structures and names of the CYPs involved. A special focus of the present review is placed on transgenic mouse models as an invaluable in vivo system for understanding human diseases caused by deficiencies in these pathways. Table 1 shows the genotype–phenotype correlations of the cholesterol and BA synthesis CYP genes in humans and the corresponding phenotypes of the mouse knockout models.

image

Figure 1.  Schematic representation of cholesterol and BA biosynthesis pathways. Only steps involving CYPs are emphasized. There is a single CYP in cholesterol biosynthesis. In the classic pathway, cholic and chenodeoxycholic acid, two primary BAs in humans, are formed. In the alternative pathway, mainly chenodeoxycholic acid is formed. It usually accounts for 5–10% of daily BA losses, although it can become a major BA synthesis pathway when the classic pathway is disrupted. It also produces oxysterols, which are potent regulators of lipid homeostasis in vitro.

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Table 1.   A summary of the CYPs involved in cholesterol and BA synthesis showing their association with disease and mouse knockout models.
EnzymeGeneLocationDisease (OMIM database)PolymorphismPhenotypePhenotype of corresponding knockout mouse model
Lanosterol 14α-demethylaseCYP51A17q21.2 Exon 1 missense mutation (rs2229188)Positive correlation with HDL-cholesterol [39]Embrionally lethal (D15) with similarities to human ABS [23]
Cholesterol 7α-hydroxylaseCYP7A18q11–q12 1302–1303delTTHigh level of plasma LDL-cholesterol, increased hepatic cholesterol content and a deficient rate of BA excretion [55]Increased rate of postnatal death, fat malabsorption, wasting, skin abnormalities and vision problems [42] Hypercholesterolemia [46]
Promoter A-204C (rs3808607)Increased LDL-cholesterol level, gallstone disease, hypertriglyceridemia, hypercholesterolemia, and risk for arteriosclerosis, colorectal cancer, neuromyelitis optica [56–63]
Sterol 12α-hydroxylaseCYP8B13p22–p21.3   Lack of CA, fat malabsorption [64]
Sterol 27-hydroxylaseCYP27A12q35Cerebrotendinous xanthomatosis (213700) [77,90–93]  Reduced BA pool size [81] Hypertriglyceridemia and enlargement of the liver and adrenal glands [87] Accumulation of cholestanol in the brain and tendons [86]
Oxysterol 7α-hydroxylaseCYP7B18q21.3Congenital BA synthesis defect type 3 (613812) [99,109]  No gross phenotype, increased levels of 25- and 27-hydroxycholesterol in plasma, liver and kidneys [97]
Spastic paraplegia autosomal recessive type 5A (270800) [110,111]  
Cholesterol 24-hydroxylaseCYP46A114q32.2 Intron 2 SNP (T/C) (rs754203)Late onset sporadic AD, mild cognitive impairment [122–126]40% reduction in brain cholesterol excretion [115] Impaired learning abilities [113]
Intron 3 SNP (T/C) (rs3742376)AD [126,127]
Promoter region SNP (A/G) (rs7157609)AD [128]
Intron 3 SNP (C/T) (rs4900442)AD [128]
Oxysterol 7α-hydroxylase IICYP39A16p21.1–p11.2    

CYP51 from cholesterol synthesis

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

CYP51 is the most evolutionarily conserved member of the CYP superfamily [11]. It has been characterized in several species, including humans and mice. Even if it is expressed ubiquitously, it appears to have tissue-specific roles [12–16]. In the liver, the gene is regulated by the sterol regulatory element-binding protein and by cyclic adenosine monophosphate signaling pathways [17–19], as well as in a circadian manner [20,21]. CYP51 is a microsomal enzyme that requires CYP reductase (POR) as an ubiquitous redox partner. The enzymatic activity (i.e. removal of the methyl group at position C14 of the sterol ring) is demethylation of lanosterol or 24,25-dihydrolanosterol by three consecutive monooxygenation reactions [4,22].

The Cyp51 knockout is embryonically lethal

We have recently developed a Cyp51 mouse knockout model that exhibited several prenatal Antley–Bixler syndrome (ABS)-like symptoms, leading to lethality at embryonic day 15 [23]. Homozygous Cyp51−/− mice do not express functional Cyp51 mRNA or CYP51 protein, leading to an accumulation of the sterol precursors lanosterol and 24,25-dihydrolanosterol. Lethality was ascribed to heart failure as a result of a variety of heart defects. As noted previously, mutations of the human CYP51A1 gene are, at present, not linked to ABS. However, the developed Cyp51 mouse knockout model encourages the reconsideration of a possible functional link between CYP51A1 defects and malformations in humans. Several developmental defects of Cyp51 knockout mice, especially skeletal and heart abnormalities, resemble the ABS-like phenotype of humans. Porter and Herman [24] reported heart ventricular defects in some ABS patients, a similar phenotype as that observed in the Cyp51 knockout mice. Syndactyly is also observed in ABS patients, whereas polydactyly resulted from the teratogenic effects of azole antimicotics. We postulate that the hypoplastic heart development in Cyp51 knockout embryos might correspond to the inhibition of CYP51 activity in a manner similar to that reported for the azoles [21,25].

The mouse knockout of Por, the obligatory redox partner of CYP51 and other microsomal CYPs

CYP reductase is a ubiquitously expressed enzyme that serves as electron donor to all endoplasmatic reticulum CYPs. Because of its general role in supporting electron transfer in microsomal monooxygenation reactions, the phenotypes of Por knockout mice were expected to be severe and pleiotropic. The two described Por knockouts [26,27] are both embrionally lethal at embryonic days 9.5–10.5. In one case [26], the deletion of one allele (Por heterozygotes) also resulted in some embryonic lethality. Cardiac development and vascularization were majorly impaired and linked to defects in retinoic acid metabolism [27]. Heterozygous mice in a study by Otto et al. [27] were apparently normal, whereas Shen et al. [26] reported some embryonic lethality. This indicates a possible influence of the mouse genetic background on the observed phenotypes. Because POR mutations in humans associate with ABS and limb and skeletal abnormalities (see below), a conditional Por knockout has been developed specifically in mouse limb bud mesenchyme [28]. Transcriptional analysis of embryonic day 12.5 mouse forelimb buds demonstrated up-regulation of the entire cholesterol biosynthesis pathway, where cholesterol deficiency may explain many aspects of the phenotype. It was thus concluded that cellular POR-dependent cholesterol synthesis is essential during limb and skeletal development.

The importance of POR for cholesterol and BA homeostasis has been demonstrated by two independent liver conditional Por deletions [29,30]. Both models show hepatomegaly and modified cholesterol and BA metabolism. Changes in the expression of key enzymes from cholesterol and lipid homeostasis were observed, such as reduced expression of Cyp7b1, as well as elevated expression of Cyp7a1 and Cyp8b1. This provided important insights into the control of metabolic pathways by the CYP system [31].

POR mutations in humans lead to ABS

ABS is a rare complex syndrome with pleiotropic body malformations. Although a modified sterol profile in the fluconazole-exposed patient suggested a deficiency of CYP51, mutational analysis of the CYP51A1 gene showed no obvious pathogenic mutations in exons or exon–intron boundaries [32]. Subsequently, it was revealed that the cause of the abnormal sterol profile and genital malformations lies in abnormalities of the POR gene [33]. POR is located on human chromosome 7q11.2 and has 16 exons, where exons 2–16 encode a 677 amino acid POR protein [34]. There is a single copy of the 50-kb POR gene in humans.

Another gene associated with ABS is the fibroblast growth factor receptor 2 gene, although several cases of the disease remain unexplained. The current assumption is that ABS with abnormal genitalia and/or impaired steroidogenesis is a result of POR mutations, and that the variable clinical features can be explained primarily by impaired activities of POR-dependent CYPs [35]. Inhibition of CYP51 activity as a result of mutations of POR has important implications because sterols play a regulatory role in the embryonic activation of hedgehog proteins [36]. It is thus plausible that some of the ABS patients might have functional mutations in CYP51A1. Similar to POR, the CYP51A1 gene also lies on chromosome 7 (7q21.2). It spans 22 kb, and contains ten exons and a housekeeping promoter with several transcription start sites [37]. Two polymorphisms of CYP51A1 have been described so far in unrelated studies. A C/T transition in intron 2 of CYP51A1 was discovered in a study of the association of leptin with blood pressure, although the rs6 appears to be uninformative [38]. On the other hand, a C/T change in exon 1 causes a missense mutation transforming Val to Ala [39]. Here, CYP51A1 has been placed among 14 genes where the genotypes are known to be important for the high-density lipoprotein (HDL)-cholesterol phenotype.

CYP7A1

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

CYP7A1 is a microsomal enzyme that catalyzes the first step in the neutral pathway of BA synthesis and is expressed only in the liver [40]. Conversion of cholesterol to 7α-hydroxycholesterol is highly regulated and a rate-limiting step in the synthesis of CA and CDCA. In general, expression of the CYP7A1 gene is increased when the BA pool size is reduced and decreased when excess BAs are present in the diet [41].

Cyp7a1 knockout mice

Disruption of the Cyp7a1 gene in mice reveals a complex phenotype, emphasizing the importance of adequate BA synthesis in early murine life [42]. Cyp7a1-null mice are subjected to an increased rate of postnatal death, fat malabsorption, wasting, skin abnormalities and vision problems. An absence of BAs in newborn animals combined with fat-soluble vitamin deficiency is considered to be the most likely explanation for this phenotype [43]. However, a small proportion of mice that do survive this early postnatal period experience a total regression of the abnormalities and are essentially indistinguishable from their wild-type littermates in later life. It was shown that, at ∼ 3 weeks of age, the alternative pathway of BA synthesis becomes active in mice and is able to compensate for the loss of Cyp7a1 [43]. Unexpectedly, serum levels of cholesterol and triglycerides are normal in Cyp7a1 knockout mice, suggesting that other regulatory mechanisms are able to effectively maintain lipid homeostasis. Further research on Cyp7a1-deficient mice [44] showed that cholesterol absorption is reduced to undetectable levels as a result of a reduced BA pool size; however, the source of sterols is replaced by an increased de novo synthesis in the liver and intestine. Interestingly, alternative pathways of BA synthesis are not up-regulated in Cyp7a1-null mice, and they remain unresponsive to marked increases in the enterohepatic flux of cholesterol or BAs [45].

By contrast to the studies described above, another study on Cyp7a1-deficient mice showed a milder phenotype with a much lower mortality rate but with elevated levels of serum cholesterol or hypercholesterolemia [46]. The observed increase in the expression of Cyp27a1 is suggestive of a secondary induction of the acidic pathway of BA synthesis. There is also no increase in hepatic de novo cholesterol synthesis in these mice.

The discrepancies in the studies noted above were ultimately ascribed to differences in the genetic background or the environment of the mice [46]. Of note, the latter Cyp7a1 knockout mouse model is more representative of the human CYP7A1 deficiency, as will be described subsequently.

Overexpression of CYP7A1 in mouse models

To further elucidate the role of Cyp7a1 in vivo, mice that overexpress human CYP7A1 were generated. These mouse models provide valuable information on cholesterol and BA homeostasis, as well as an informative approach for understanding human diseases.

One of the first studies investigating the overexpression of CYP7A1 was performed in LDLr-deficient mice [47], which suffer from overtly elevated serum low-density lipoprotein (LDL) concentrations [48]. Interventions that accelerate the conversion of cholesterol to BAs have been widely used to reduce plasma LDL concentrations mainly through the mechanism of increased expression of the LDL receptor pathway [49]. Interestingly, even in the absence of LDL receptors, expression of CYP7A1 in mice results in a lowering of plasma LDL concentrations [47], an effect that has been ascribed to a decrease in the rate of LDL entry into the plasma space. Use of high-fat cholesterol-enriched diet in this type of mice showed a marked resistance to diet-induced hypercholesterolemia [50].

In studies on normal mice with a stable expression of CYP7A1 [51], it was shown that the production of apolipoprotein B-containing lipoproteins is increased. However, plasma levels of triglycerides and cholesterol remain unchanged as a result of the counter-effect of an increased expression of LDL receptors. This type of transgenic mice is also protected from atherosclerosis and gallstone formation when fed an atherogenic diet rich in BAs because there is no possibility of dietary suppression of CYP7A1. In this case, cholesterol homeostasis is effectively maintained, which makes CYP7A1-overexpressing mice resistant to diet-induced dyslipidemia [52]. Moreover, in a recent study, the overexpression of CYP7A1 in mice prevented high-fat diet-induced obesity, fatty liver and insulin resistance [53]. Most likely, enhanced enzyme activity and the activation of cellular BA signaling as a result of an expanded BA pool size both contribute to this phenomenon [54].

CYP7A1 defects in humans

In humans, CYP7A1 lies on chromosome 8q11–q12 and spans almost 10 kb, containing five introns and six exons, which encode a 504 amino acid protein. Despite the importance of CYP7A1, its absence is not lethal in humans; however, deficiencies are linked to a hyper-cholesterolemic phenotype.

A 2-bp deletion in exon 6 (1302–1303delTT; numbering of nucleotides in the cDNA from the transcription start site) in the CYP7A1 gene results in a frameshift: L413fsX414. This mutation causes a Leu[RIGHTWARDS ARROW]Arg substitution at codon 413 followed immediately by a premature stop codon, resulting in a truncated protein lacking the C-terminal 91 residues with a loss of the heme-binding domain, which is essential for activity. In this case, the loss of activity resulted in high level of plasma LDL-cholesterol, doubled hepatic cholesterol content and a markedly deficient rate of BA excretion. In addition, the alternative BA pathway was up-regulated in this patient [55].

Genetic variations in the CYP7A1 gene associated with disorders of cholesterol and BA metabolism have been studied extensively in different laboratories. Most studies have focused on a single nucleotide polymorphism (SNP) in the promoter region of the CYP7A1 gene (rs3808607). The association of an A/C transversion polymorphism (−278 from the translation initiation or −204 from the transcriptional start site) with plasma lipid levels, hypertriglyceridemia, hypercholesterolemia, the risk of arteriosclerosis, gallstone disease and colorectal cancer has been studied in adults and children in Caucasian and Asian populations, with conflicting results being reported [56]. This polymorphism was initially associated with LDL-cholesterol levels; namely, plasma LDL-cholesterol concentrations were significantly higher in –278C homozygotes than in –278A homozygotes [57].

Further studies confirmed that plasma total cholesterol and LDL-cholesterol were lower in patients with an A containing-allele (AA homozygote or AC heterozygote) than in CC homozygotes, although the A allele might be considered as a risk factor for gallstone disease [58]. A allele carriers have shown a better response to atorvastatin treatment in terms of total cholesterol and a LDL-cholesterol-lowering effect [59]. Studies investigating the response to plant sterols in humans suggest that the promoter A-204C variant is associated with enhanced CYP7A1 enzyme activity. Increased intestinal BA levels, which lead to more efficient cholesterol absorption, may explain why C allele carriers (AC heterozygotes and CC homozygotes) show enhanced cholesterol-lowering and increased feedback cholesterol synthesis in response to an intervention by plant sterols [60].

The CC variant of the CYP7A1 promoter polymorphism increases the progression of atherosclerosis and possibly the risk of a new clinical event [61]. On the other hand, a colon cancer study reported a lower risk for the CYP7A1 CC genotype with respect to proximal colon cancer, although not for distal colon or rectal cancer [62]. The CC genotype of this common CYP7A1 promoter polymorphism was also associated with a higher protective effect regarding the risk of neuromyelitis optica, which is a severe idiopathic inflammatory disease of the central nervous system primarily affecting the optic nerves and spinal cord [63].

CYP8B1

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

CYP8B1 is a microsomal CYP that catalyzes the addition of hydroxyl group at position 12 of various sterol intermediates destined to become CA in the BA synthesis pathways [64]. Activity of CYP8B1 thus effectively determines the ratio of CDCA to CA formed in the neutral pathway [2].

Cyp8b1 knockout mice

As expected, the disruption of the Cyp8b1 gene in mice results in a complete lack of CA, which is replaced mostly by muricholates (muricholates are considered to be metabolites of CDCA) [65] and CDCA [64]. In mice, β-muricholic acid and CA are the dominating BAs as opposed to CDCA and CA in humans. Cyp8b1-null mice also have an enlarged BA pool size as a consequence of the up-regulation of Cyp7a1. However, the absorption of dietary cholesterol and lipids is reduced (steatorrhea), suggesting that the composition of BA pool is more important than its size. Loss in cholesterol absorption is compensated with increased de novo synthesis by the liver, resulting in unchanged serum levels of cholesterol and triglycerides. There is also no change in expression of Cyp7b1, Cyp27a1 and Cyp39a1. The addition of CA to the diet suppresses Cyp7a1 and several cholesterogenic genes in Cyp8b1-null mice. At the same time, the intestinal cholesterol absorption is normalized, suggesting an important regulative function of this BA in lipid homeostasis [66,67].

The absence of CA synthesis was also studied in apolipoprotein E knockout mice, a common animal model for atherosclerosis [68]. Given the fact that CA is involved in multiple sites of cholesterol homeostasis, such as in promoting cholesterol absorption and inhibiting BA synthesis, the lack of CA might prove beneficial against cholesterol-driven diseases. In general, the phenotype of Cyp8b1 gene ablation is preserved in ApoE and Cyp8b1 double knockout mice, resulting in a 50% reduction of atherosclerotic lesions compared to ApoE knockout mice [69].

Cholic acid as a key molecule for effective cholesterol absorption

The aforementioned studies established that CA is a key factor in the formation of micelles (together with phospholipids) as a means to achieve an efficient absorption of cholesterol. In mice, CA also functions as a signaling molecule that down-regulates the expression of Cyp7a1, as well as Cyp8b1 [66]. The combined effects of CA have a great impact on maintaining optimal cholesterol balance, which is probably one of the reasons why the expression of Cyp8b1 is so highly regulated [70,71]. The therapeutic approach of decreasing CA synthesis appears to represent a promising strategy for atherosclerosis susceptible patients. Unfortunately, it is difficult to anticipate all the effects of such therapeutic intervention because, in humans, CDCA instead of CA appears to be the primary ligand for farnesoid X receptor, through which BAs act as signaling molecules [72]. Nevertheless, the inhibition of Cyp8b1 might also be considered for treating gallstone disease because gallstone formation can be averted with depletion of CA [67,69].

Defects of the CYP8B1 gene in humans

The CYP8B1 gene was mapped to chromosome 3p22–p21.3; it spans 3949 bp encoding 504 amino acids protein and lacks introns. The human CYP8B1 has a broad specificity towards steroids; however, its main activity is the conversion of 7α-hydroxy-4-cholesten-3-one into 7α,12α-dihydroxy-4-cholesten-3-one. Thus, CYP8B1 activity determines the primary BA composition [73]. In human bile, CA and CDCA, representing two major primary BAs, occur in a molar ratio of ∼ 2 : 1 [74].

The loss of activity of CYP8B1 leads to the production of CDCA over CA, which results in the modification of BA biosynthesis and changes in the excretion of cholesterol [75]. Polymorphism in the coding part of the CYP8B1 gene does not explain the marked differences in the ratio of CA and CDCA in human bile [76]. To the best of our knowledge, polymorphisms in CYP8B1 have not yet been associated with any disease.

CYP27A1

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

CYP27A1 is located on the inner membranes of the mitochondria and is expressed in almost all cells of the body [77]. It catalyzes the first step in the alternative BA synthesis pathway, and also plays a significant role in the classic synthesis pathway, where it catalyzes the 27-hydroxylation of BA intermediates. Approximately 25–30% of all synthesized BAs come from the alternative pathway in mice and rats; however, in humans, it accounts for only 5–10% of the daily losses [40]. Because of the universal expression of CYP27A1 and the general presence of cholesterol throughout the body, minor amounts of 27-hydroxycholesterol and cholestenoic acid are formed in the extrahepatic tissues, and their flux to the liver could be regarded as an alternative mechanism to the common HDL-dependent reversed cholesterol transport [78]. 27-Hydroxycholesterol was also found to be a strong inhibitor of cholesterol synthesis in vitro [79]. A deficiency of CYP27A1 in humans is associated with a disease called cerebrotendinous xanthomatosis (CTX), which is characterized by dementia, ataxia, cataracts and xanthomas of the tendons and the nervous system [80].

Cyp27a1 knockout in mice

Mice with a disrupted Cyp27a1 gene are normal in appearance with no gross histological abnormalities. As expected, the total amount of BAs is severely reduced, with CA predominating in the remaining BA pool size [81]. Interestingly, BA synthesis is not completely disrupted, as might be expected, because Cyp27a1 is involved in both the neutral and acidic pathways of the BA synthesis. Low levels of BA synthesis are maintained through the induction of microsomal 25-hydroxylase pathway [82]; however, compensatory synthesis via bile alcohols as intermediates is unable to fully compensate for the absence of Cyp27a1. Because of the decreased BA pool size, there is a compensatory up-regulation of Cyp7a1 as a result of a lack of negative feedback suppression. Low levels of BAs also result in a decreased cholesterol absorption, which leads to a secondary increase of de novo synthesis to maintain normal serum cholesterol levels. As opposed to humans, in Cyp27-null mice, there is no hepatic or bloodstream accumulation of cholestanol, a substrate formed mainly from the neutral pathway intermediate 7α-hydroxy-4-cholesten-3-one [83]. Although the intermediates in the early steps of the classic BA synthesis and 25-hydroxylated bile alcohols are increased in Cyp27 knockout mice compared to wild-type mice, the levels are much lower than in CTX patients [83]. The explanation of these differences is probably a result of the Cyp7a1 pathway being much less up-regulated in conjunction with the marked increase of CYP3A11 enzyme in mice [83,84]. CYP3A was shown to be capable of carrying out 25- and other side chain hydroxylations on various BA intermediates in mice [83], as well as in humans [85]. In CTX patients, however, CYP3A4 is not induced. A more recent study identified a marked accumulation of cholestanol in the brain and tendons of Cyp27a1 knockout mice [86]. Despite the fact that the contents of cholestanol can reach as much as 10% of the sterol fraction in the brain, no xanthomas (i.e. the most specific feature of CTX) are formed as a consequence.

Further investigation of CYP27A1 deficiency in mice identified hypertriglyceridemia and a significant enlargement of the liver and adrenal glands [87]. Hepatic fatty acid synthesis was also found to be increased, indicating a more global role of Cyp27a1 in the whole body lipid metabolism than previously assumed. Because of a decrease in cholesterol absorption, a compensatory up-regulation of cholesterol synthesis can be seen in various organs, especially the liver and adrenal glands. Histological observations indicated a cortical cell hypertrophy and, presumably, the increase in adrenal cholesterol synthesis and content is necessary to maintain normal levels of corticosterone [87].

Several differences can also be observed between homozygous and heterozygous Cyp27a1 knockout mice. The phenotype of the heterozygotes is generally milder compared to the homozygotes, and there are two distinct features between them. First, heterozygous mice have increased plasma cholesterol compared to homozygotes, indicating that a partial deficiency of Cyp27a1 may be a greater risk factor for atherosclerosis than its complete absence. Second, heterozygotes show no increase in Cyp7a1 expression and could thus represent a model of Cyp27a1 deficiency that is uncomplicated by the effects of a secondary up-regulation of the CYP7A1 pathway [88].

Overexpression of CYP27A1 in mice

To gain further insights into the physiological role of CYP27A1 in regulating cholesterol metabolism, transgenic mice overexpressing the human CYP27A1 gene were generated [89]. The expected increased output from the acidic pathway of BA synthesis does not manifest in these mice. Also, there are no changes in cholesterol metabolism; an observation that does not favor the hypothesis of 27-hydroxycholesterol being a potent signaling molecule in vivo.

Defects of CYP27A1 in humans

In humans, CYP27A1 was mapped to the long arm of chromosome locus 2q35 and spans almost 33.5 kb, containing nine exons and eight introns that encode for 531 amino acids of the protein, which consists of a 498 amino acid mature enzyme and a 33 amino acid mitochondrial signal sequence. Subsequent to 1991, when CYP27A1 was reported to be associated with CTX [Online Mendelian Inheritance in Man (OMIM) database: 606530] [90], the gene was (and still is) under investigation. To date, over 50 mutations have been reported to be associated with CTX, which is a rare autosomal recessive disease characterized by an accumulation of cholesterol and cholestanol in the brain and tendons. Although the disorder is rare, its incidence is substantially greater than previously recognized. A greater awareness of CTX is important because specific treatments are available.

The first patient with CTX was reported in 1937 [91]. The patient suffered from dementia, ataxia, cataracts and xanthomas of the tendons and the brain. Subsequently, several hundred patients have been diagnosed, with clusters reported from Japan, Israel and the Netherlands [92]. In general, CTX patients have normal levels of cholesterol in the blood but increased levels in tissues. By contrast to the xanthomas in patients with familiar hypercholesterolemia, xanthomas from CTX patients contain up to 30% of cholestanol, the 5α-saturated analogue of cholesterol. The typical onset of the disease consists of early bilateral cataracts and diarrhea, followed by cerebellar and pyramidal signs, mental retardation and xanthomas. The most serious consequence of the disease is the development of xanthomas in the brain and consequent neurological symptoms. The preferential site of brain xanthomas is the white matter of the cerebellum [77].

Various mutations in all nine exons and introns 2, 4, 6 and 7 have been associated with CTX. Approximately 50% of mutations in the CYP27A1 gene were found in the region of exons 6–8 and 16% and 14% in regions of exons 2 and 4, respectively. Deletion, insertion and nonsense mutations cause a truncation of CYP27A1 mRNA, resulting in the lack of heme-binding and adrenodoxin-binding sites. Splice site mutations lead to aberrant splicing and exon skipping. Missense mutations could influence the stability and catalytic activity of the enzyme. These mutations are inferred to be pathogenic when they disrupt heme-binding and adrenodoxin-binding domains, or when they identify a potential substrate binding or other protein contact site. Mutation analysis showed that 45% of variations are missense mutations; 20% are reported as nonsense mutations; and splice site mutations, deletions and insertions constitute 18%, 14% and 2% of reported mutations, respectively [93].

CYP7B1

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

CYP7B1 is presumably located in the endoplasmic reticulum and is present in many organs, with the highest expression in the brain and kidneys. In mice, the expression of Cyp7b1 is restricted mainly to the liver and lungs, although it can also be found in the kidneys, brain and reproductive tract [94]. It acts as an inactivating enzyme for 25- and 27-hydroxycholesterol, producing 7α-hydroxylated oxysterols, which are substrates for BA synthesis. CYP7B1 is also active towards steroid hormones and two studies using knockout mouse models have been reported [95,96].

Cyp7b1 knockout mice

Interestingly, knocking out Cyp7b1 in mice does not have an effect on the levels of plasma cholesterol and triglycerides, tissue cholesterol, composition and BA pool size, and intestinal cholesterol absorption [97]. The loss of the alternative BA synthetic pathway is presumably fully compensated by the CYP7A1 pathway, as suggested by the elevated CYP7A1 protein levels in knockout mice. On the other hand, levels of 25- and 27-hydroxycholesterol in the plasma, liver and kidneys are markedly increased. Unexpectedly, the accumulation of these oxysterols does not affect cholesterol synthesis in contrast to their described regulatory effects on cholesterol homeostasis in vitro [9,10]. Apparently, the major physiological role of CYP7B1 is to inactivate oxysterols as a means to produce intermediates in the BA synthesis pathway [94]. It is likely to have an important role in steroid metabolism and the regulation of lipid metabolism [98].

Physiological functions of CYP7B1 in humans

The human CYP7B1 gene spans ∼ 220 kb on chromosome 8q21.3 and encompasses six exons separated by five introns, which encode a 506 amino acid protein. CYP7B1 is involved not only in an alternative BA synthesis pathway, but also in steroid hormone metabolism, metabolism of estrogen receptor ligands and immunoglobulin production in humans. As a result of expression in different tissues (liver, brain and reproductive tract) and a wide spectrum of substrates, CYP7B1 performs different physiological functions in each tissue [94].

In the liver, CYP7B1 catalyzes the 7α-hydroxylation of two major oxysterols; 25-hydroxycholesterol and 27-hydroxycholesterol [45,99,100].

In the brain, CYP7B1 is involved in steroid hormone metabolism, with activity toward two neurosteroids: pregnenolone and dehydroepiandrosterone (DHEA) [95,101].

The 7α-hydroxydehydroepiandrosterone, a product of DHEA hydroxylation by CYP7B1, was proposed to be an activator of estrogen β subtype in human prostatic cells, although conflicting results have been reported [102,103]. The other CYP7B1 substrate, 27-hydroxycholesterol, is known to be a selective estrogen receptor modulator, antagonizing estrogen-mediated activation in the vascular wall [104,105] and activating the receptor in breast cancer and other cell lines [106]. Another steroid substrate of CYP7B1 is 5α-androstane-3β,17β-diol, an agonist for the estrogen receptor, which, unlike the others, is inactivated by 6α-hydroxylation catalyzed by CYP7B1 [94].

Activation of macrophages via Toll-like receptors induces the production of 25-hydroxycholesterol, comprising an oxysterol that suppresses rearrangement of the Ig heavy chain locus through class switch recombination in naive B cells, which consequently suppresses the production of immunoglobulin A. The 25-hydroxycholesterol is one of the CYP7B1 substrates and a loss of function of CYP7B1 leads to the accumulation of 25-hydroxycholesterol and significantly reduced IgA levels in the serum [107].

CYP7B1 expression in the heart was also reported; however, its main function remains unclear [108].

CYP7B1 defects in humans

Because of the wide spectrum of substrates and multiple physiological functions of CYP7B1 in humans, defects in the CYP7B1 gene give rise to two different diseases: liver failure in children as a result of congenital BA synthesis defect type 3 (OMIM database: 613812) and neuropathy in adults of a spastic paraplegia autosomal recessive type 5A (OMIM database: 270800) [94].

Both in human and mouse livers, CYP7B1 catalyzes hydroxylation of two substrates: 25-hydroxycholesterol and 27-hydroxycholesterol. Loss of function of CYP7B1 leads to the accumulation of total oxysterols both in the serum and urine, including 24-hydroxycholesterol, 25-hydroxycholesterol, 27-hydroxycholesterol and poly-hydroxy cholesterols. Liver damage in children was caused by a high concentration of abnormal BA 3β-hydroxy-5-cholen-24-oic acid detected both in serum and urine. In these patients, normal BAs were detected in small amounts or were not detected at all. Such a clinical presentation can result from two different mutations in the CYP7B1 gene: a homozygous nonsense mutation at codon 388 of exon 5 [99] or a homozygous nonsense mutation at codon 112 in exon 3 [109].

The spastic paraplegias represent a clinically heterogeneous group of disorders characterized by lower limb spasticity and weakness that is associated with the degradation of motor neuron axons in the spinal cord [110], and sometimes with ataxia, mental retardation and other neurological symptoms. To date, mutations in more than 17 different genes have been associated with spastic paraplegia, including the CYP7B1 gene. Eighteen different mutations in the six exons of CYP7B1, including missense, nonsense and frame-shift mutations, have been linked to spastic paraplegia [111].

CYP46A1

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

The major organ of cholesterol metabolism is the liver. Because all cells depend on adequate levels of cholesterol to function normally, an efficient transport system has evolved by which peripheral tissues turn over cholesterol. The exchange of cholesterol-rich and cholesterol-poor lipoprotein particles takes place on the surface of the cells. An exception to this rule is the brain. The protective blood–brain barrier prevents the transfer of cholesterol in and out of brain cells. To overcome this problem, cholesterol in the brain is utilized via an alternative pathway involving 24-hydroxylation of cholesterol by CYP46A1. 24(S)-hydroxycholesterol is then able to pass the blood–brain barrier and is cleared by the liver via the BA synthesis pathway [112]. In addition to the brain, CYP46A1 is also expressed in the testes and liver, although at much lower levels [8].

Cyp46a1 knockout and CYP46A1 knock-in mouse models

An important role of CYP46A1 in the brain cholesterol turnover was established by a study of Cyp46a1 knockout mice [112]. The disruption of the gene causes a 40% reduction in brain cholesterol excretion in mice; however, this decrease is compensated for by a reduction of de novo synthesis, such that cholesterol turnover in the brain is effectively maintained. Unexpectedly, cholesterol synthesis is not completely abolished, indicating the existence of other routes of cholesterol removal from the brain. Behavioral studies on mice lacking Cyp46a1 showed that their learning abilities are impaired [113]. The cholesterol synthesis pathway provides the cells not only with its final product cholesterol, but also with metabolites that lead to the synthesis of other important molecules, such as dolichol, ubiquinine or heme A [114]. One important aspect involves the supply of different isoprenoids. An observed reduction in the brain de novo cholesterol synthesis was shown to limit the supply of the polyisoprenoid geranylgeraniol, which is apparently required for normal learning processes [113].

The effect of the reduced cholesterol synthesis in the brain was also investigated in transgenic mice that develop AD-like pathology. The 24-hydroxylase was found to be active in the hippocampal and cortical neurons that are affected by AD [115,116]. Disruption of the Cyp46a1 gene in these mice does not affect the rate or the extent of amyloid plaque formation, which is the most prominent feature of the disease. Interestingly, as a result of the ablation of the 24-hydroxylase gene, longevity is prolonged in this strain of mice [117].

24(S)-hydroxycholesterol was established as the most potent ligand for liver X receptor among the oxysterols in vitro [118]. To investigate whether it has the same effect on lipid homeostasis in vivo, a mouse model with a stable expression of human CYP46A1 was engineered. The expression of CYP46A1 mRNA in several organs and systemic levels of 24(S)-hydroxycholesterol were found to be significantly elevated in these mice [119]. Similar to the case of overexpression of CYP27A1 [89], there is no significant impact on cholesterol metabolism in the mouse liver, again discarding the notion of oxysterols being potent regulators of cholesterol homeostasis in vivo.

Defects in CYP46A1 in humans

CYP46A1 is located on chromosome 14q32.2 and spans over 42.8 kbp, containing 15 exons and 14 introns that encode for a 500 amino acid protein.

Some studies have provided evidence showing that cholesterol metabolism has an important role in the pathogenesis of AD. A high serum cholesterol concentration increases the risk of AD [120,121]. CYP46 is a key protein in brain cholesterol metabolism and might represent a genetic risk factor of AD.

Recently, all reported polymorphisms in CYP46A1 were summarized [122]. A single nucleotide polymorphism (rs754203) in intron 2 of the CYP46A1 gene has been studied, with very conflicting results. Different reports associate both the T and C variant with an increased risk for late onset sporadic AD. The frequency of the CYP46A1 T allele and TT genotype was significantly higher in AD patients from Switzerland, Greece and Italy compared to controls [123]. The same association was observed in patients from Spain [124], although it was absent in Swedish and Scottish patients [125]. Studies in Chinese patients have shown no association with AD [126], although the T allele was associated with mild cognitive impairment. On the other hand, seven independent studies of SNP rs754203 report an association of the C allele and CC genotype with AD in patients from Italy, Spain, Finland, China and Poland and, in two studies, no significant association with AD was found in German and French patients carrying the C allele [122]. A single nucleotide polymorphism (T/C) (rs3742376) in intron 3 of CYP46A1 was also reported, and the C allele variant has been associated with AD in patients from China [126,127].

A recent study in a German population investigated 16 SNPs in CYP46A1 and found two of them to be associated with AD risk. Both the G allele of rs7157609 SNP in the promoter region (and the C allele of rs4900442 SNP in intron 3) of the CYP46A1 gene are associated with an increased risk of AD and decreased cerebrospinal fluid levels of cholesterol and 24S-hydroxycholesterol [128].

CYP39A1

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

CYP39A1 exists in humans and mice and is specific for 24-hydroxycholesterol. It is a poorly characterized enzyme, expressed only in the endoplasmic reticulum in the liver. Its physiological role is considered to be restricted to BA synthesis. Similar to the case of Cyp7b1, the expression of Cyp39a1 is sexually dimorphic, with higher expression in females [100]. To the best of our knowledge, no results of mouse models of either disrupting or overexpressing the Cyp39a1 gene have been published to date. In a study on Cyp7b1 knockout mice [97], the expression of Cyp39a1 was found to be essentially unchanged, indicating that this pathway of BA synthesis is constitutively active, although probably of minor importance.

Li-Hawkins et al. [100] showed that the CYP39A1 gene contains 12 exons and spans ∼ 150 kb of genomic DNA. They mapped the gene to chromosome 6p21.1–p11.2, whereas the mouse Cyp39a1 gene maps to a region of syntenic homology on chromosome 17.

CYP39 is poorly characterized also in humans. The enzyme was identified in the liver and was initially assumed to be expressed exclusively in this organ [100]. However, subsequent reports reported expression in the prostate and heart [108]. To the best of our knowledge, CYP39 polymorphisms have yet to be be associated with diseases or other defects in humans.

Concluding remarks

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

Despite the fact that the best model for humans is a human, our understanding of complex human diseases (including those associated with CYP genes of cholesterol and BA synthesis) would be much slower and less complete without animal models. This is especially true in cases where specific human mutations are created in mice, providing an informative approach for understanding human diseases. In the personalized medicine era, we are aware of important subtle differences in individual human genomes, as well as how these differences potentially affect disease progression and phenotypes. Here, mouse knockout models can be very helpful for understanding the different pathologies associated with a disease. The examples included in the present review comprise Cyp7a1 knockout mice, where the milder phenotype is similar to the observed phenotypes in humans. However, different genetic backgrounds resulted in Cyp7a1 knockout mice with a much more severe phenotype and an increased rate of postnatal death. Accordingly, it might be reasonable to screen for the Cyp7a1 genotype in a more general manner to avoid some of the unexplained infant deaths. Another example of the importance of animal models is the recent Cyp51 mouse knockout, which led us to revisit the possibility that human CYP51A1 mutations are linked to ABS. It is important to note that, so far, mutations in only two CYPs described in the present review (CYP7B1 and CYP27A1) are linked to human disorders listed in the OMIM database. One possible explanation lies in the difference between cholesterol and BA homeostasis in humans and rodents. Consequently, the disease phenotypes of transgenic mice might differ from diseases in humans. For example, cholic and chenodeoxycholic acid are the primary human BAs, whereas, in rodents, alternative hydroxylations lead to muricholic acids [129]. This chemical diversity is further increased by the action of human and rodent intestinal bacteria (which differ between the species), leading to a different spectrum of secondary BAs. BAs represent important natural ligands of nuclear farnesoid X receptor [129] and are sensed by pregnane X receptor and constitutive androstane receptor [130,131], which regulate BA homeostasis. The different spectrum of BAs in humans and rodents, combined with the different specificities of human versus rodent nuclear receptors, might result in the observed phenotype variations. Interestingly, only mice can compensate for secondary BA metabolism, showing less severe phenotypes in Cyp27a1 and Cyp7b1 knockouts compared to mutations of homologous genes in humans. Nevertheless, it is expected that further progress with respect to providing and characterizing mouse models, as well as high-throughput sequencing of individual genes/genomes, will soon reveal additional links between mutations of CYP genes in cholesterol and BA synthesis and disease phenotypes. From the 57 genes for human CYP, most are reasonably well characterized. CYP39 appears to remain an ‘orphan’ CYP where the availability of a mouse model could play a crucial role in revealing its physiological roles in vivo.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References

This work was supported by the Slovenian Research Agency program grants P1-0104 and J7-4053. G. Lorbek is funded as a young researcher by the Slovenian Research Agency. M. Lewinska is a FP7 ITN Marie Curie student paid by the ‘FightingDrugFailure’ grant #238132.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. CYP51 from cholesterol synthesis
  5. CYP7A1
  6. CYP8B1
  7. CYP27A1
  8. CYP7B1
  9. CYP46A1
  10. CYP39A1
  11. Concluding remarks
  12. Acknowledgements
  13. References
  • 1
    Russell DW (1992) Cholesterol biosynthesis and metabolism. Cardiovasc Drugs Ther 6, 103110.
  • 2
    Monte MJ, Marin JJ, Antelo A & Vazquez-Tato J (2009) Bile acids: chemistry, physiology, and pathophysiology. World J Gastroenterol 15, 804816.
  • 3
    Hafner M, Rezen T & Rozman D (2011) Regulation of hepatic cytochromes p450 by lipids and cholesterol. Curr Drug Metab 12, 173185.
  • 4
    Stromstedt M, Rozman D & Waterman MR (1996) The ubiquitously expressed human CYP51 encodes lanosterol 14 alpha-demethylase, a cytochrome P450 whose expression is regulated by oxysterols. Arch Biochem Biophys 329, 7381.
  • 5
    Duane WC & Javitt NB (1999) 27-hydroxycholesterol: production rates in normal human subjects. J Lipid Res 40, 11941199.
  • 6
    Axelson M & Sjovall J (1990) Potential bile acid precursors in plasma – possible indicators of biosynthetic pathways to cholic and chenodeoxycholic acids in man. J Steroid Biochem 36, 631640.
  • 7
    Beigneux A, Hofmann AF & Young SG (2002) Human CYP7A1 deficiency: progress and enigmas. J Clin Invest 110, 2931.
  • 8
    Lund EG, Guileyardo JM & Russell DW (1999) cDNA cloning of cholesterol 24-hydroxylase, a mediator of cholesterol homeostasis in the brain. Proc Natl Acad Sci USA 96, 72387243.
  • 9
    Edwards PA & Ericsson J (1998) Signaling molecules derived from the cholesterol biosynthetic pathway: mechanisms of action and possible roles in human disease. Curr Opin Lipidol 9, 433440.
  • 10
    Edwards PA, Kennedy MA & Mak PA (2002) LXRs; oxysterol-activated nuclear receptors that regulate genes controlling lipid homeostasis. Vascul Pharmacol 38, 249256.
  • 11
    Rezen T, Debeljak N, Kordis D & Rozman D (2004) New aspects on lanosterol 14alpha-demethylase and cytochrome P450 evolution: lanosterol/cycloartenol diversification and lateral transfer. J Mol Evol 59, 5158.
  • 12
    Rozman D & Waterman MR (1998) Lanosterol 14alpha-demethylase (CYP51) and spermatogenesis. Drug Metab Dispos 26, 11991201.
  • 13
    Tacer KF, Haugen TB, Baltsen M, Debeljak N & Rozman D (2002) Tissue-specific transcriptional regulation of the cholesterol biosynthetic pathway leads to accumulation of testis meiosis-activating sterol (T-MAS). J Lipid Res 43, 8289.
  • 14
    Rozman D, Seliskar M, Cotman M & Fink M (2005) Pre-cholesterol precursors in gametogenesis. Mol Cell Endocrinol 234, 4756.
  • 15
    Seliskar M & Rozman D (2007) Mammalian cytochromes P450 – importance of tissue specificity. Biochim Biophys Acta 1770, 458466.
  • 16
    Fink M, Spaninger K, Prosenc U & Rozman D (2008) High-fat medium and circadian transcription factors (cryptochrome and clock) contribute to the regulation of cholesterogenic Cyp51 and Hmgcr genes in mouse embryonic fibroblasts. Acta Chim Slov 55, 8592.
  • 17
    Rozman D, Fink M, Fimia GM, Sassone-Corsi P & Waterman MR (1999) Cyclic adenosine 3′,5′-monophosphate(cAMP)/cAMP-responsive element modulator (CREM)-dependent regulation of cholesterogenic lanosterol 14alpha-demethylase (CYP51) in spermatids. Mol Endocrinol 13, 19511962.
  • 18
    Halder SK, Fink M, Waterman MR & Rozman D (2002) A cAMP-responsive element binding site is essential for sterol regulation of the human lanosterol 14 alpha-demethylase gene (CYP51). Mol Endocrinol 16, 18531863.
  • 19
    Fink M, Acimovic J, Rezen T, Tansek N & Rozman D (2005) Cholesterogenic lanosterol 14alpha-demethylase (CYP51) is an immediate early response gene. Endocrinology 146, 53215331.
  • 20
    Acimovic J, Fink M, Pompon D, Bjorkhem I, Hirayama J, Sassone-Corsi P, Golicnik M & Rozman D (2008) CREM modulates the circadian expression of CYP51, HMGCR and cholesterogenesis in the liver. Biochem Biophys Res Commun 376, 206210.
  • 21
    Horvat S, McWhir J & Rozman D (2011) Defects in cholesterol synthesis genes in mouse and in humans: lessons for drug development and safer treatments. Drug Metab Rev 43, 6990.
  • 22
    Debeljak N, Fink M & Rozman D (2003) Many facets of mammalian lanosterol 14alpha-demethylase from the evolutionarily conserved cytochrome P450 family CYP51. Arch Biochem Biophys 409, 159171.
  • 23
    Keber R, Motaln H, Wagner KD, Debeljak N, Rassoulzadegan M, Acimovic J, Rozman D & Horvat S (2011) Mouse knockout of the cholesterogenic cytochrome P450 Lanosterol 14{alpha}-demethylase (Cyp51) resembles Antley–Bixler syndrome. J Biol Chem 286, 2908629097.
  • 24
    Porter FD & Herman GE (2011) Malformation syndromes caused by disorders of cholesterol synthesis. J Lipid Res 52, 634.
  • 25
    Rozman D & Monostory K (2010) Perspectives of the non-statin hypolipidemic agents. Pharmacol Ther 127, 1940.
  • 26
    Shen AL, O’Leary KA & Kasper CB (2002) Association of multiple developmental defects and embryonic lethality with loss of microsomal NADPH-cytochrome P450 oxidoreductase. J Biol Chem 277, 65366541.
  • 27
    Otto DM, Henderson CJ, Carrie D, Davey M, Gundersen TE, Blomhoff R, Adams RH, Tickle C & Wolf CR (2003) Identification of novel roles of the cytochrome p450 system in early embryogenesis: effects on vasculogenesis and retinoic acid homeostasis. Mol Cell Biol 23, 61036116.
  • 28
    Schmidt K, Hughes C, Chudek JA, Goodyear SR, Aspden RM, Talbot R, Gundersen TE, Blomhoff R, Henderson C, Wolf CR et al. (2009) Cholesterol metabolism: the main pathway acting downstream of cytochrome P450 oxidoreductase in skeletal development of the limb. Mol Cell Biol 29, 27162729.
  • 29
    Henderson CJ, Otto DM, Carrie D, Magnuson MA, McLaren AW, Rosewell I & Wolf CR (2003) Inactivation of the hepatic cytochrome P450 system by conditional deletion of hepatic cytochrome P450 reductase. J Biol Chem 278, 1348013486.
  • 30
    Gu J, Weng Y, Zhang QY, Cui H, Behr M, Wu L, Yang W, Zhang L & Ding X (2003) Liver-specific deletion of the NADPH-cytochrome P450 reductase gene: impact on plasma cholesterol homeostasis and the function and regulation of microsomal cytochrome P450 and heme oxygenase. J Biol Chem 278, 2589525901.
  • 31
    Wang XJ, Chamberlain M, Vassieva O, Henderson CJ & Wolf CR (2005) Relationship between hepatic phenotype and changes in gene expression in cytochrome P450 reductase (POR) null mice. Biochem J 388, 857867.
  • 32
    Kelley RI, Kratz LE, Glaser RL, Netzloff ML, Wolf LM & Jabs EW (2002) Abnormal sterol metabolism in a patient with Antley–Bixler syndrome and ambiguous genitalia. Am J Med Genet 110, 95102.
  • 33
    Fluck CE, Tajima T, Pandey AV, Arlt W, Okuhara K, Verge CF, Jabs EW, Mendonca BB, Fujieda K & Miller WL (2004) Mutant P450 oxidoreductase causes disordered steroidogenesis with and without Antley–Bixler syndrome. Nat Genet 36, 228230.
  • 34
    Haniu M, McManus ME, Birkett DJ, Lee TD & Shively JE (1989) Structural and functional analysis of NADPH-cytochrome P-450 reductase from human liver: complete sequence of human enzyme and NADPH-binding sites. Biochemistry 28, 86398645.
  • 35
    Fukami M, Horikawa R, Nagai T, Tanaka T, Naiki Y, Sato N, Okuyama T, Nakai H, Soneda S, Tachibana K et al. (2005) Cytochrome P450 oxidoreductase gene mutations and Antley–Bixler syndrome with abnormal genitalia and/or impaired steroidogenesis: molecular and clinical studies in 10 patients. J Clin Endocrinol Metab 90, 414426.
  • 36
    Arlt W (2007) P450 oxidoreductase deficiency and Antley–Bixler syndrome. Rev Endocr Metab Disord 8, 301307.
  • 37
    Rozman D, Stromstedt M, Tsui LC, Scherer SW & Waterman MR (1996) Structure and mapping of the human lanosterol 14alpha-demethylase gene (CYP51) encoding the cytochrome P450 involved in cholesterol biosynthesis; comparison of exon/intron organization with other mammalian and fungal CYP genes. Genomics 38, 371381.
  • 38
    Ma D, Feitosa MF, Wilk JB, Laramie JM, Yu K, Leiendecker-Foster C, Myers RH, Province MA & Borecki IB (2009) Leptin is associated with blood pressure and hypertension in women from the National Heart, Lung, and Blood Institute Family Heart Study. Hypertension 53, 473479.
  • 39
    Charlesworth JC, Peralta JM, Drigalenko E, Goring HH, Almasy L, Dyer TD & Blangero J (2009) Toward the identification of causal genes in complex diseases: a gene-centric joint test of significance combining genomic and transcriptomic data. BMC Proc 3(Suppl 7), S92.
  • 40
    Russell DW (2003) The enzymes, regulation, and genetics of bile acid synthesis. Annu Rev Biochem 72, 137174.
  • 41
    Russell DW & Setchell KD (1992) Bile acid biosynthesis. Biochemistry 31, 47374749.
  • 42
    Ishibashi S, Schwarz M, Frykman PK, Herz J & Russell DW (1996) Disruption of cholesterol 7alpha-hydroxylase gene in mice. I. Postnatal lethality reversed by bile acid and vitamin supplementation. J Biol Chem 271, 1801718023.
  • 43
    Schwarz M, Lund EG, Setchell KD, Kayden HJ, Zerwekh JE, Bjorkhem I, Herz J & Russell DW (1996) Disruption of cholesterol 7alpha-hydroxylase gene in mice. II. Bile acid deficiency is overcome by induction of oxysterol 7alpha-hydroxylase. J Biol Chem 271, 1802418031.
  • 44
    Schwarz M, Russell DW, Dietschy JM & Turley SD (1998) Marked reduction in bile acid synthesis in cholesterol 7alpha-hydroxylase-deficient mice does not lead to diminished tissue cholesterol turnover or to hypercholesterolemia. J Lipid Res 39, 18331843.
  • 45
    Schwarz M, Russell DW, Dietschy JM & Turley SD (2001) Alternate pathways of bile acid synthesis in the cholesterol 7alpha-hydroxylase knockout mouse are not upregulated by either cholesterol or cholestyramine feeding. J Lipid Res 42, 15941603.
  • 46
    Erickson SK, Lear SR, Deane S, Dubrac S, Huling SL, Nguyen L, Bollineni JS, Shefer S, Hyogo H, Cohen DE et al. (2003) Hypercholesterolemia and changes in lipid and bile acid metabolism in male and female cyp7A1-deficient mice. J Lipid Res 44, 10011009.
  • 47
    Spady DK, Cuthbert JA, Willard MN & Meidell RS (1998) Overexpression of cholesterol 7alpha-hydroxylase (CYP7A) in mice lacking the low density lipoprotein (LDL) receptor gene. LDL transport and plasma LDL concentrations are reduced. J Biol Chem 273, 126132.
  • 48
    Osono Y, Woollett LA, Herz J & Dietschy JM (1995) Role of the low density lipoprotein receptor in the flux of cholesterol through the plasma and across the tissues of the mouse. J Clin Invest 95, 11241132.
  • 49
    Scaldaferri F, Pizzoferrato M, Ponziani FR, Gasbarrini G & Gasbarrini A (2011) Use and indications of cholestyramine and bile acid sequestrants. Intern Emerg Med doi:10.1007/s11739-011-0653-0.
  • 50
    Ratliff EP, Gutierrez A & Davis RA (2006) Transgenic expression of CYP7A1 in LDL receptor-deficient mice blocks diet-induced hypercholesterolemia. J Lipid Res 47, 15131520.
  • 51
    Miyake JH, Doung XD, Strauss W, Moore GL, Castellani LW, Curtiss LK, Taylor JM & Davis RA (2001) Increased production of apolipoprotein B-containing lipoproteins in the absence of hyperlipidemia in transgenic mice expressing cholesterol 7alpha-hydroxylase. J Biol Chem 276, 2330423311.
  • 52
    Miyake JH, Duong-Polk XT, Taylor JM, Du EZ, Castellani LW, Lusis AJ & Davis RA (2002) Transgenic expression of cholesterol-7-alpha-hydroxylase prevents atherosclerosis in C57BL/6J mice. Arterioscler Thromb Vasc Biol 22, 121126.
  • 53
    Li T, Owsley E, Matozel M, Hsu P, Novak CM & Chiang JY (2010) Transgenic expression of cholesterol 7alpha-hydroxylase in the liver prevents high-fat diet-induced obesity and insulin resistance in mice. Hepatology 52, 678690.
  • 54
    Li T, Matozel M, Boehme S, Kong B, Nilsson LM, Guo G, Ellis E & Chiang JY (2011) Overexpression of cholesterol 7alpha-hydroxylase promotes hepatic bile acid synthesis and secretion and maintains cholesterol homeostasis. Hepatology 53, 9961006.
  • 55
    Pullinger CR, Eng C, Salen G, Shefer S, Batta AK, Erickson SK, Verhagen A, Rivera CR, Mulvihill SJ, Malloy MJ et al. (2002) Human cholesterol 7alpha-hydroxylase (CYP7A1) deficiency has a hypercholesterolemic phenotype. J Clin Invest 110, 109117.
  • 56
    Nakamoto K, Wang S, Jenison RD, Guo GL, Klaassen CD, Wan YJ & Zhong XB (2006) Linkage disequilibrium blocks, haplotype structure, and htSNPs of human CYP7A1 gene. BMC Genet 7, 29.
  • 57
    Wang J, Freeman DJ, Grundy SM, Levine DM, Guerra R & Cohen JC (1998) Linkage between cholesterol 7alpha-hydroxylase and high plasma low-density lipoprotein cholesterol concentrations. J Clin Invest 101, 12831291.
  • 58
    Jiang ZY, Han TQ, Suo GJ, Feng DX, Chen S, Cai XX, Jiang ZH, Shang J, Zhang Y, Jiang Y et al. (2004) Polymorphisms at cholesterol 7alpha-hydroxylase, apolipoproteins B and E and low density lipoprotein receptor genes in patients with gallbladder stone disease. World J Gastroenterol 10, 15081512.
  • 59
    Kajinami K, Brousseau ME, Ordovas JM & Schaefer EJ (2004) Interactions between common genetic polymorphisms in ABCG5/G8 and CYP7A1 on LDL cholesterol-lowering response to atorvastatin. Atherosclerosis 175, 287293.
  • 60
    De Castro-Oros I, Pampin S, Cofan M, Mozas P, Pinto X, Salas-Salvado J, Rodriguez-Rey JC, Ros E, Civeira F & Pocovi M (2011) Promoter variant -204A > C of the cholesterol 7alpha-hydroxylase gene: association with response to plant sterols in humans and increased transcriptional activity in transfected HepG2 cells. Clin Nutr 30, 239246.
  • 61
    Hofman MK, Princen HM, Zwinderman AH & Jukema JW (2005) Genetic variation in the rate-limiting enzyme in cholesterol catabolism (cholesterol 7alpha-hydroxylase) influences the progression of atherosclerosis and risk of new clinical events. Clin Sci (Lond) 108, 539545.
  • 62
    Hagiwara T, Kono S, Yin G, Toyomura K, Nagano J, Mizoue T, Mibu R, Tanaka M, Kakeji Y, Maehara Y et al. (2005) Genetic polymorphism in cytochrome P450 7A1 and risk of colorectal cancer: the Fukuoka Colorectal Cancer Study. Cancer Res 65, 29792982.
  • 63
    Kim HJ, Park HY, Kim E, Lee KS, Kim KK, Choi BO, Kim SM, Bae JS, Lee SO, Chun JY et al. (2010) Common CYP7A1 promoter polymorphism associated with risk of neuromyelitis optica. Neurobiol Dis 37, 349355.
  • 64
    Li-Hawkins J, Gafvels M, Olin M, Lund EG, Andersson U, Schuster G, Bjorkhem I, Russell DW & Eggertsen G (2002) Cholic acid mediates negative feedback regulation of bile acid synthesis in mice. J Clin Invest 110, 11911200.
  • 65
    Mahowald TA, Matschiner JT, Hsia SL, Doisy EA Jr, Elliott WH & Doisy EA (1957) Bile acids. III. Acid I; the principal bile acid in urine of surgically jaundiced rats. J Biol Chem 225, 795802.
  • 66
    Murphy C, Parini P, Wang J, Bjorkhem I, Eggertsen G & Gafvels M (2005) Cholic acid as key regulator of cholesterol synthesis, intestinal absorption and hepatic storage in mice. Biochim Biophys Acta 1735, 167175.
  • 67
    Wang J, Einarsson C, Murphy C, Parini P, Bjorkhem I, Gafvels M & Eggertsen G (2006) Studies on LXR- and FXR-mediated effects on cholesterol homeostasis in normal and cholic acid-depleted mice. J Lipid Res 47, 421430.
  • 68
    Meir KS & Leitersdorf E (2004) Atherosclerosis in the apolipoprotein-E-deficient mouse: a decade of progress. Arterioscler Thromb Vasc Biol 24, 10061014.
  • 69
    Slatis K, Gafvels M, Kannisto K, Ovchinnikova O, Paulsson-Berne G, Parini P, Jiang ZY & Eggertsen G (2010) Abolished synthesis of cholic acid reduces atherosclerotic development in apolipoprotein E knockout mice. J Lipid Res 51, 32893298.
  • 70
    Kerr TA, Saeki S, Schneider M, Schaefer K, Berdy S, Redder T, Shan B, Russell DW & Schwarz M (2002) Loss of nuclear receptor SHP impairs but does not eliminate negative feedback regulation of bile acid synthesis. Dev Cell 2, 713720.
  • 71
    Wang L, Lee YK, Bundman D, Han Y, Thevananther S, Kim CS, Chua SS, Wei P, Heyman RA, Karin M et al. (2002) Redundant pathways for negative feedback regulation of bile acid production. Dev Cell 2, 721731.
  • 72
    Ellis E, Axelson M, Abrahamsson A, Eggertsen G, Thorne A, Nowak G, Ericzon BG, Bjorkhem I & Einarsson C (2003) Feedback regulation of bile acid synthesis in primary human hepatocytes: evidence that CDCA is the strongest inhibitor. Hepatology 38, 930938.
  • 73
    Gafvels M, Olin M, Chowdhary BP, Raudsepp T, Andersson U, Persson B, Jansson M, Bjorkhem I & Eggertsen G (1999) Structure and chromosomal assignment of the sterol 12alpha-hydroxylase gene (CYP8B1) in human and mouse: eukaryotic cytochrome P-450 gene devoid of introns. Genomics 56, 184196.
  • 74
    Wang J, Greene S, Eriksson LC, Rozell B, Reihner E, Einarsson C, Eggertsen G & Gafvels M (2005) Human sterol 12a-hydroxylase (CYP8B1) is mainly expressed in hepatocytes in a homogenous pattern. Histochem Cell Biol 123, 441446.
  • 75
    Hebanowska A (2010) [Bile acid biosynthesis and its regulation]. Postepy Hig Med Dosw (Online) 64, 544554.
  • 76
    Abrahamsson A, Gafvels M, Reihner E, Bjorkhem I, Einarsson C & Eggertsen G (2005) Polymorphism in the coding part of the sterol 12alpha-hydroxylase gene does not explain the marked differences in the ratio of cholic acid and chenodeoxycholic acid in human bile. Scand J Clin Lab Invest 65, 595600.
  • 77
    Bjorkhem I & Hansson M (2010) Cerebrotendinous xanthomatosis: an inborn error in bile acid synthesis with defined mutations but still a challenge. Biochem Biophys Res Commun 396, 4649.
  • 78
    Babiker A, Andersson O, Lund E, Xiu RJ, Deeb S, Reshef A, Leitersdorf E, Diczfalusy U & Bjorkhem I (1997) Elimination of cholesterol in macrophages and endothelial cells by the sterol 27-hydroxylase mechanism. Comparison with high density lipoprotein-mediated reverse cholesterol transport. J Biol Chem 272, 2625326261.
  • 79
    Fu X, Menke JG, Chen Y, Zhou G, MacNaul KL, Wright SD, Sparrow CP & Lund EG (2001) 27-hydroxycholesterol is an endogenous ligand for liver X receptor in cholesterol-loaded cells. J Biol Chem 276, 3837838387.
  • 80
    Bjorkhem I & Leitersdorf E (2000) Sterol 27-hydroxylase deficiency: a rare cause of xanthomas in normocholesterolemic humans. Trends Endocrinol Metab 11, 180183.
  • 81
    Rosen H, Reshef A, Maeda N, Lippoldt A, Shpizen S, Triger L, Eggertsen G, Bjorkhem I & Leitersdorf E (1998) Markedly reduced bile acid synthesis but maintained levels of cholesterol and vitamin D metabolites in mice with disrupted sterol 27-hydroxylase gene. J Biol Chem 273, 1480514812.
  • 82
    Honda A, Salen G, Matsuzaki Y, Batta AK, Xu G, Leitersdorf E, Tint GS, Erickson SK, Tanaka N & Shefer S (2001) Differences in hepatic levels of intermediates in bile acid biosynthesis between Cyp27(–/–) mice and CTX. J Lipid Res 42, 291300.
  • 83
    Honda A, Salen G, Matsuzaki Y, Batta AK, Xu G, Leitersdorf E, Tint GS, Erickson SK, Tanaka N & Shefer S (2001) Side chain hydroxylations in bile acid biosynthesis catalyzed by CYP3A are markedly up-regulated in Cyp27–/– mice but not in cerebrotendinous xanthomatosis. J Biol Chem 276, 3457934585.
  • 84
    Lyons MA, Maeda N & Brown AJ (2002) Paradoxical enhancement of hepatic metabolism of 7-ketocholesterol in sterol 27-hydroxylase-deficient mice. Biochim Biophys Acta 1581, 119126.
  • 85
    Furster C & Wikvall K (1999) Identification of CYP3A4 as the major enzyme responsible for 25-hydroxylation of 5beta-cholestane-3alpha,7alpha,12alpha-triol in human liver microsomes. Biochim Biophys Acta 1437, 4652.
  • 86
    Bavner A, Shafaati M, Hansson M, Olin M, Shpitzen S, Meiner V, Leitersdorf E & Bjorkhem I (2010) On the mechanism of accumulation of cholestanol in the brain of mice with a disruption of sterol 27-hydroxylase. J Lipid Res 51, 27222730.
  • 87
    Repa JJ, Lund EG, Horton JD, Leitersdorf E, Russell DW, Dietschy JM & Turley SD (2000) Disruption of the sterol 27-hydroxylase gene in mice results in hepatomegaly and hypertriglyceridemia. Reversal by cholic acid feeding. J Biol Chem 275, 3968539692.
  • 88
    Dubrac S, Lear SR, Ananthanarayanan M, Balasubramaniyan N, Bollineni J, Shefer S, Hyogo H, Cohen DE, Blanche PJ, Krauss RM et al. (2005) Role of CYP27A in cholesterol and bile acid metabolism. J Lipid Res 46, 7685.
  • 89
    Meir K, Kitsberg D, Alkalay I, Szafer F, Rosen H, Shpitzen S, Avi LB, Staels B, Fievet C, Meiner V et al. (2002) Human sterol 27-hydroxylase (CYP27) overexpressor transgenic mouse model. Evidence against 27-hydroxycholesterol as a critical regulator of cholesterol homeostasis. J Biol Chem 277, 3403634041.
  • 90
    Cali JJ, Hsieh CL, Francke U & Russell DW (1991) Mutations in the bile acid biosynthetic enzyme sterol 27-hydroxylase underlie cerebrotendinous xanthomatosis. J Biol Chem 266, 77797783.
  • 91
    van Bogaert L, Scherer HJ, Froelich A & Epstein E (1937) Une deuxieme observation de cholesterinose tendineuse symetrique avec symptomes cerebraux. Ann Med Interne 42, 69101.
  • 92
    Bjorkhem I, Boberg KM & Leitersdorf E (2001) Inborn errors in bile acid biosynthesis and storage of sterols other than cholesterol. In The Metabolic Bases of Inherited Diseases (Valle D, ed.), pp. 29612988. McGraw Hill Publishing Co, New York, NY.
  • 93
    Gallus GN, Dotti MT & Federico A (2006) Clinical and molecular diagnosis of cerebrotendinous xanthomatosis with a review of the mutations in the CYP27A1 gene. Neurol Sci 27, 143149.
  • 94
    Stiles AR, McDonald JG, Bauman DR & Russell DW (2009) CYP7B1: one cytochrome P450, two human genetic diseases, and multiple physiological functions. J Biol Chem 284, 2848528489.
  • 95
    Rose K, Allan A, Gauldie S, Stapleton G, Dobbie L, Dott K, Martin C, Wang L, Hedlund E, Seckl JR et al. (2001) Neurosteroid hydroxylase CYP7B: vivid reporter activity in dentate gyrus of gene-targeted mice and abolition of a widespread pathway of steroid and oxysterol hydroxylation. J Biol Chem 276, 2393723944.
  • 96
    Omoto Y, Lathe R, Warner M & Gustafsson JA (2005) Early onset of puberty and early ovarian failure in CYP7B1 knockout mice. Proc Natl Acad Sci USA 102, 28142819.
  • 97
    Li-Hawkins J, Lund EG, Turley SD & Russell DW (2000) Disruption of the oxysterol 7alpha-hydroxylase gene in mice. J Biol Chem 275, 1653616542.
  • 98
    Chiang JY (2004) Regulation of bile acid synthesis: pathways, nuclear receptors, and mechanisms. J Hepatol 40, 539551.
  • 99
    Setchell KD, Schwarz M, O’Connell NC, Lund EG, Davis DL, Lathe R, Thompson HR, Weslie Tyson R, Sokol RJ & Russell DW (1998) Identification of a new inborn error in bile acid synthesis: mutation of the oxysterol 7alpha-hydroxylase gene causes severe neonatal liver disease. J Clin Invest 102, 16901703.
  • 100
    Li-Hawkins J, Lund EG, Bronson AD & Russell DW (2000) Expression cloning of an oxysterol 7alpha-hydroxylase selective for 24-hydroxycholesterol. J Biol Chem 275, 1654316549.
  • 101
    Rose KA, Stapleton G, Dott K, Kieny MP, Best R, Schwarz M, Russell DW, Bjorkhem I, Seckl J & Lathe R (1997) Cyp7b, a novel brain cytochrome P450, catalyzes the synthesis of neurosteroids 7alpha-hydroxy dehydroepiandrosterone and 7alpha-hydroxy pregnenolone. Proc Natl Acad Sci USA 94, 49254930.
  • 102
    Martin C, Ross M, Chapman KE, Andrew R, Bollina P, Seckl JR & Habib FK (2004) CYP7B generates a selective estrogen receptor beta agonist in human prostate. J Clin Endocrinol Metab 89, 29282935.
  • 103
    Pettersson H, Holmberg L, Axelson M & Norlin M (2008) CYP7B1-mediated metabolism of dehydroepiandrosterone and 5alpha-androstane-3beta,17beta-diol – potential role(s) for estrogen signaling. Febs J 275, 17781789.
  • 104
    Umetani M, Domoto H, Gormley AK, Yuhanna IS, Cummins CL, Javitt NB, Korach KS, Shaul PW & Mangelsdorf DJ (2007) 27-Hydroxycholesterol is an endogenous SERM that inhibits the cardiovascular effects of estrogen. Nat Med 13, 11851192.
  • 105
    Umetani M & Shaul PW (2011) 27-Hydroxycholesterol: the first identified endogenous SERM. Trends Endocrinol Metab 22, 130135.
  • 106
    DuSell CD, Umetani M, Shaul PW, Mangelsdorf DJ & McDonnell DP (2008) 27-hydroxycholesterol is an endogenous selective estrogen receptor modulator. Mol Endocrinol 22, 6577.
  • 107
    Bauman DR, Bitmansour AD, McDonald JG, Thompson BM, Liang G & Russell DW (2009) 25-Hydroxycholesterol secreted by macrophages in response to Toll-like receptor activation suppresses immunoglobulin A production. Proc Natl Acad Sci USA 106, 1676416769.
  • 108
    Steckelbroeck S, Watzka M, Lutjohann D, Makiola P, Nassen A, Hans VH, Clusmann H, Reissinger A, Ludwig M, Siekmann L et al. (2002) Characterization of the dehydroepiandrosterone (DHEA) metabolism via oxysterol 7alpha-hydroxylase and 17-ketosteroid reductase activity in the human brain. J Neurochem 83, 713726.
  • 109
    Ueki I, Kimura A, Nishiyori A, Chen HL, Takei H, Nittono H & Kurosawa T (2008) Neonatal cholestatic liver disease in an Asian patient with a homozygous mutation in the oxysterol 7alpha-hydroxylase gene. J Pediatr Gastroenterol Nutr 46, 465469.
  • 110
    Salinas S, Proukakis C, Crosby A & Warner TT (2008) Hereditary spastic paraplegia: clinical features and pathogenetic mechanisms. Lancet Neurol 7, 11271138.
  • 111
    Cao L, Fei QZ, Tang WG, Liu JR, Zheng L, Xiao Q, He SB, Fu Y & Chen SD (2011) Novel mutations in the CYP7B1 gene cause hereditary spastic paraplegia. Mov Disord 26, 13541356.
  • 112
    Lund EG, Xie C, Kotti T, Turley SD, Dietschy JM & Russell DW (2003) Knockout of the cholesterol 24-hydroxylase gene in mice reveals a brain-specific mechanism of cholesterol turnover. J Biol Chem 278, 2298022988.
  • 113
    Kotti TJ, Ramirez DM, Pfeiffer BE, Huber KM & Russell DW (2006) Brain cholesterol turnover required for geranylgeraniol production and learning in mice. Proc Natl Acad Sci USA 103, 38693874.
  • 114
    Goldstein JL & Brown MS (1990) Regulation of the mevalonate pathway. Nature 343, 425430.
  • 115
    Xie C, Lund EG, Turley SD, Russell DW & Dietschy JM (2003) Quantitation of two pathways for cholesterol excretion from the brain in normal mice and mice with neurodegeneration. J Lipid Res 44, 17801789.
  • 116
    Ramirez DM, Andersson S & Russell DW (2008) Neuronal expression and subcellular localization of cholesterol 24-hydroxylase in the mouse brain. J Comp Neurol 507, 16761693.
  • 117
    Halford RW & Russell DW (2009) Reduction of cholesterol synthesis in the mouse brain does not affect amyloid formation in Alzheimer’s disease, but does extend lifespan. Proc Natl Acad Sci USA 106, 35023506.
  • 118
    Janowski BA, Grogan MJ, Jones SA, Wisely GB, Kliewer SA, Corey EJ & Mangelsdorf DJ (1999) Structural requirements of ligands for the oxysterol liver X receptors LXRalpha and LXRbeta. Proc Natl Acad Sci USA 96, 266271.
  • 119
    Shafaati M, Olin M, Bavner A, Pettersson H, Rozell B, Meaney S, Parini P & Bjorkhem I (2011) Enhanced production of 24S-hydroxycholesterol is not sufficient to drive liver X receptor target genes in vivo. J Intern Med 270, 377387.
  • 120
    Notkola IL, Sulkava R, Pekkanen J, Erkinjuntti T, Ehnholm C, Kivinen P, Tuomilehto J & Nissinen A (1998) Serum total cholesterol, apolipoprotein E epsilon 4 allele, and Alzheimer’s disease. Neuroepidemiology 17, 1420.
  • 121
    Kivipelto M, Helkala EL, Laakso MP, Hanninen T, Hallikainen M, Alhainen K, Soininen H, Tuomilehto J & Nissinen A (2001) Midlife vascular risk factors and Alzheimer’s disease in later life: longitudinal, population based study. BMJ 322, 14471451.
  • 122
    Garcia AN, Muniz MT, Souza e Silva HR, da Silva HA & Athayde-Junior L (2009) Cyp46 polymorphisms in Alzheimer’s disease: a review. J Mol Neurosci 39, 342345.
  • 123
    Papassotiropoulos A, Streffer JR, Tsolaki M, Schmid S, Thal D, Nicosia F, Iakovidou V, Maddalena A, Lutjohann D, Ghebremedhin E et al. (2003) Increased brain beta-amyloid load, phosphorylated tau, and risk of Alzheimer disease associated with an intronic CYP46 polymorphism. Arch Neurol 60, 2935.
  • 124
    Fernandez Del Pozo V, Alvarez Alvarez M, Fernandez Martinez M, Galdos Alcelay L, Gomez Busto F, Pena JA, Alfonso-Sanchez MA, Zarranz Imirizaldu JJ & de Pancorbo MM (2006) Polymorphism in the cholesterol 24S-hydroxylase gene (CYP46A1) associated with the APOEpsilon3 allele increases the risk of Alzheimer’s disease and of mild cognitive impairment progressing to Alzheimer’s disease. Dement Geriatr Cogn Disord 21, 8187.
  • 125
    Johansson A, Katzov H, Zetterberg H, Feuk L, Johansson B, Bogdanovic N, Andreasen N, Lenhard B, Brookes AJ, Pedersen NL et al. (2004) Variants of CYP46A1 may interact with age and APOE to influence CSF Abeta42 levels in Alzheimer’s disease. Hum Genet 114, 581587.
  • 126
    Ma SL, Tang NL, Lam LC & Chiu HF (2006) Polymorphisms of the cholesterol 24-hydroxylase (CYP46A1) gene and the risk of Alzheimer’s disease in a Chinese population. Int Psychogeriatr 18, 3745.
  • 127
    Fu BY, Ma SL, Tang NL, Tam CW, Lui VW, Chiu HF & Lam LC (2009) Cholesterol 24-hydroxylase (CYP46A1) polymorphisms are associated with faster cognitive deterioration in Chinese older persons: a two-year follow up study. Int J Geriatr Psychiatry 24, 921926.
  • 128
    Kolsch H, Lutjohann D, Jessen F, Popp J, Hentschel F, Kelemen P, Schmitz S, Maier W & Heun R (2009) CYP46A1 variants influence Alzheimer’s disease risk and brain cholesterol metabolism. Eur Psychiatry 24, 183190.
  • 129
    Lefebvre P, Cariou B, Lien F, Kuipers F & Staels B (2009) Role of bile acids and bile acid receptors in metabolic regulation. Physiol Rev 89, 147191.
  • 130
    Staudinger JL, Goodwin B, Jones SA, Hawkins-Brown D, MacKenzie KI, LaTour A, Liu Y, Klaassen CD, Brown KK, Reinhard J et al. (2001) The nuclear receptor PXR is a lithocholic acid sensor that protects against liver toxicity. Proc Natl Acad Sci USA 98, 33693374.
  • 131
    Kakizaki S, Takizawa D, Tojima H, Horiguchi N, Yamazaki Y & Mori M (2011) Nuclear receptors CAR and PXR; therapeutic targets for cholestatic liver disease. Front Biosci 17, 29883005.