Oxysterols
From LipidomicsWiki
Contents |
1. LIPID MAPS Subclasses ST0101
2. Basics
2.1. Structures
Oxysterols are 27-carbon oxidised cholesterol-related molecules with additional hydroxyl, carbonyl, epoxy, hydroperoxy or carboxyl moieties. Structures of some common oxysterols are depicted below. An extensive account of the oxysterol structures is found on the LIPID MAPS www pages [1]
2.2. Natural sources
The major oxysterols present in human circulation or tissues arise via enzymatic cholesterol oxidation processes (see 3.1.). Another important source of oxysterols is non-ezymatic autoxidation of cholesterol mediated mainly by lipid peroxides or free oxygen radicals. Moreover, cholesterol autoxidation products, mainly 7-ketocholesterol, 7alpha- and 7beta-hydroxycholesterol, and 5,6-epoxycholesterols, are obtained from nutrition. An estimated 1% of the sterol in Western diet is in oxidised form (van de Bovenkamp et al. 1988).
2.3. Nomenclature
For nomenclature of oxysterols see the LIPID MAPS www pages [2]
3. Biochemical pathways
3.1. Biochemical synthesis
The major oxysterols present in human plasma or tissues arise enzymatically via the activity of mitochondrial or microsomal cytochrome P450 family sterol hydroxylases (Russell 2000; Schroepfer, 2000; Björkhem and Diczfalusy, 2002). Additionally, 25-hydroxycholesterol is generated from cholesterol by a di-iron enzyme with a histidine cluster playing a key role in its hydoxylase activity (Lund et al. 1998), and 24(S),25-epoxycholesterol arises from a shunt of the mevalonic acid pathway of cholesterol biosynthesis catalyzed by the same enzymes that give rise to cholesterol (Nelson et al. 1981). The major sterol hydroxylases are:
CYP7A1 (liver) generating 7alpha-hydroxycholesterol, an intermediate in the so called neutral bile acid synthesis pathway.
CYP27A1 (liver and several peripheral cell types) generating 27-hydroxycholesterol (more accurate name 25(R),26-hydroxycholesterol), an intermediate in the alternative, acid pathway of bile acid synthesis; also plays a role in sterol efflux from cells. CYP27A1 further oxidises 27-hydroxycholesterol to 3β-hydroxy-5-cholestenoic acid.
CYP46A1 (CNS) generating 24(S)-hydroxycholesterol, which plays a central role in CNS sterol homeostasis.
CYP3A4 (liver), a drug metabolizing enzyme that generates 4beta-hydroxycholesterol, the levels of which are exceptionally high in patients on certain anti-epileptic drugs.
CYP7B1 (liver), oxysterol 7alpha-hydroxylase that hydoxylates 27-hydroxycholesterol in the 7-position thus routing it for bile acid synthesis.
CYP39 (liver), carries out efficient 7-alpha hydroxylation of 24(S)-hydroxycholesterol.
CH25H, a di-iron enzyme that catalyses 25-hydroxylation of cholesterol.
3.2. Metabolism
A majority of plasma oxysterols in are associated with lipoproteins and internalized by cells via cell surface lipoprotein receptors. The bulk of this internalization occurs in the liver, which excretes sterols as such or in the form of bile acids. In addition to CYP7A1, hepatocytes express CYP7B1, which carries out 7alpha-hydroxyl modification of both exogenous 27-OHC and that produced by hepatic CYP27A1, thus routing it to the bile acid synthetic pathway (Rose et al. 1997). Another cytochrome P450 species, CYP39, carries out efficient 7alpha-hydroxylation of 24(S)-hydroxycholesterol (Li-Hawkins et al. 2000). In the bile acid synthetic pathway, 3beta-hydroxy-delta5 steroids undergo oxidation and isomerization to produce the corresponding 3-oxo-Δ4 steroids, followed by saturation of the double bond formed in the A-ring and reduction of the oxo-group into a 3alpha-hydroxy group. Degradation of the steroid side chain is initiated by the 27-hydroxylation. Synthesis of the primary bile acid cholic acid requires a further 12alpha-hydroxylation carried out by another P450 enzyme CYP8B1 (for a review see Björkhem and Eggertsen 2001).
A major non-enzymatically formed oxysterol, 7-ketocholesterol, can be hydroxylated by CYP27A1 to form 27-hydroxy-7-ketocholesterol, which is further metabolized into aqueous-soluble products, apparently bile acids (Brown et al. 2000; Lyons and Brown 2001a). On the other hand, 7-KC is reduced to 7beta-hydroxycholesterol by hepatic 11beta-hydroxysteroid dehydrogenase type 1, HSD11B1 (Hult et al. 2004; Schweizer et al. 2004). The interconversion processes enable rapid hepatic catabolism of 7-KC into aqueous-soluble bile acids, and provide an obvious explanation to the findings that 7-KC added exogenously into the circulation is rapidly metabolized and does not accumulate in the vascular walls (Lyons and Brown 2001b; Lyons et al. 1999). On the other hand, oxysterols such as 7-KC and 25-OHC are substrates for the steroid/sterol sulfotransferase SULT2B1b, which modifies them at the 3-position and apparently has a detoxifying function (Fuda et al. 2007; Li et al. 2007).
3.3. New oxysterols
CYP27A1 is able to use 7- and 8-dehydrocholesterol as its substrates, generating their 27-hydroxymetabolites capable of liganding the LXRs (Wassif et al. 2003). CYP27A1 was also found to act in vitro on other intermediates of the cholesterol biosynthetic process, lanosterol, zymosterol, and desmosterol, bringing up the idea that in vivo there may be a large family of previously unstudied 27-hydroxylase products of cholesterol precursor forms, termed oxysteroids (Javitt 2004). Oxysterols can also arise from cholesterol through the action of ozone in human tissues. Of the products, 3beta-hydroxy-5-oxo-5,6-secocholestan-6-al (5,6-seco-sterol; secosterols have a modified sterol nucleus B-ring) has been suggested to play a role in initiating protein misfolding in certain neurological diseases (Zhang et al. 2004). Biologically active oxysterols are also formed during ozonolysis of cholesterol in atherosclerotic lesions (Takeuchi et al. 2006) and in lung surfactant (Pulfer et al. 2005).
4. Oxysterols in biological processes
Oxysterols are found in the circulation or in healthy tissues at very low concentrations, <1/10 000 of the concentration of cholesterol. In circulation most oxysterols are found in esterified form and are associated with HDL and LDL. Specific oxysterols are enriched in pathologic cells/tissues, such as in macrophage foam cells and atherosclerotic lesions (especially 27-hydroxycholesterol, 7-ketocholesterol, and 7beta-hydroxycholesterol). Several oxysterols display at least in vitro cytotoxic or proapoptotic activities. Moreover, certain oxysterols are reported to induce proinflammatory signals and to impact cellular differentiation, e.g. that of monocytes into macrophages. Therefore, oxysterols have been suggested to affect the development of atherosclerotic lesions. On the other hand, functions of endogenous oxysterols as signaling molecules that regulate gene expression in lipid metabolism have emerged during the past decade. Oxysterols have the capacity to act as ligands for liver X receptors (LXR; Zelcer and Tontonoz 2006), and as inhibitors of the maturation of sterol regulatory element binding proteins (SREBP) via interactions with the Insig proteins (Radhakrishnan et al. 2007). 24(S),25-epoxycholesterol apparently plays an important role in fine-tuning the cellular cholesterol homeostasis (Gill et al. 2008). Thus, the endogenous oxysterols are suggested to have a beneficial impact on cellular lipid metabolism considering atherogenesis. On the other hand, 27-hydroxycholesterol was reported to act as a selective estrogen receptor modulator (SERM), which may counteract the cardioprotective action of estrogen hormones (Umetani et al. 2007). New cellular oxysterols receptors have recently been identified - these include Niemann-Pick C 1 protein (Infante et al. 2008) and the cytoplasmic Oxysterol-binding_proteins. These proteins are implicated in several aspects of cellular lipid homeostasis, vesicle transport, and cell signaling. Furthermore, specific oxysterols have been found, via a yet unknown mechanism, to regulate the Hedgehog signaling pathways, thus affecting differentiation and developmental processes (Dwyer et al. 2007).
5. Technology
Oxysterols have traditionally been analyzed by isotope dilution gas chromatography - mass spectrometry (GC-MS)(Breuer and Björkhem 1990; Dzeletovic et al. 1995). On the other hand, also HPLC methods have been used in which oxysterols are converted into 3-oxo-delta4 derivatives detected using their UV chromofore (Ogishima and Okuda 1986; Zhang et al. 2001). More recently, Griffiths et al. (2006) developed methodology to analyze oxysterols that are converted to their 3-oxo-delta4 oxidation products by cholesterol oxidase treatment, followed by reaction with Girard P reagent to yield Girard hydrazones, which are analyzed by electrospray tandem mass spectrometry. In general, biases in the analysis of oxysterols readily arise through sterol autoxidation occurring during sample preparation. Therefore, it is extremely important to ensure well designed and efficient sample logistics and to protect the sterols from artefactual autoxidation by the use of anti-oxidants during the processing.
6. References
Björkhem I, Diczfalusy U (2002) Oxysterols: friends, foes, or just fellow passengers? Arterioscler Thromb Vasc Biol 22: 734-42.
Björkhem I, Eggertsen G (2001) Genes involved in initial steps of bile acid synthesis. Curr Opin Lipidol 12: 97-103.
Breuer O, Björkhem I (1990) Simultaneous quantification of several cholesterol autoxidation and monohydroxylation products by isotope-dilution mass spectrometry. Steroids 55: 185-92.
Brown AJ, Watts GF, Burnett JR, Dean RT, Jessup W (2000) Sterol 27-hydroxylase acts on 7-ketocholesterol in human atherosclerotic lesions and macrophages in culture. J Biol Chem 275: 27627-33.
Dwyer JR, Sever N, Carlson M, Nelson SF, Beachy PA, Parhami F (2007) Oxysterols are novel activators of the hedgehog signaling pathway in pluripotent mesenchymal cells. J Biol Chem 282: 8959-68.
Dzeletovic S, Breuer O, Lund E, Diczfalusy U (1995) Determination of cholesterol oxidation products in human plasma by isotope dilution-mass spectrometry. Anal Biochem 225: 73-80.
Fuda H, Javitt NB, Mitamura K, Ikegawa S, Strott CA (2007) Oxysterols are substrates for cholesterol sulfotransferase. J Lipid Res 48: 1343-52.
Gill S, Chow R, Brown AJ (2008) Sterol regulators of cholesterol homeostasis and beyond: The oxysterol hypothesis revisited and revised. Progr Lipid Res, May 6 [Epub ahead of print].
Griffiths WJ, Wang Y, Alvelius G, Liu S, Bodin K, Sjövall J (2006) Analysis of oxysterols by electrospray tandem mass spectrometry.
J Am Soc Mass Spectrom 17: 341-62.
Infante RE, Abi-Mosleh L, Radhakrishnan A, Dale JD, Brown MS, Goldstein JL (2008) Purified NPC1 protein. I. Binding of cholesterol and oxysterols to a 1278-amino acid membrane protein. J Biol Chem 283: 1052-63.
Javitt NB (2004) Oxysteroids: a new class of steroids with autocrine and paracrine functions. Trends Endocrinol Metab 15: 393-7.
Li X, Pandak WM, Erickson SK, Ma Y, Yin L, Hylemon P, Ren S (2007) Biosynthesis of the regulatory oxysterol, 5-cholesten-3beta,25-diol 3-sulfate, in hepatocytes. J Lipid Res 48: 2587-96.
Li-Hawkins J, Lund EG, Turley SD, Russell DW (2000) Disruption of the oxysterol 7alpha-hydroxylase gene in mice. J Biol Chem 275: 16536-42.
Lund EG, Kerr TA, Sakai J, Li WP, Russell DW (1998) cDNA cloning of mouse and human cholesterol 25-hydroxylases, polytopic membrane proteins that synthesize a potent oxysterol regulator of lipid metabolism. J Biol Chem 273: 34316-27.
Lyons MA, Brown AJ (2001a) 7-Ketocholesterol delivered to mice in chylomicron remnant-like particles is rapidly metabolised, excreted and does not accumulate in aorta. Biochim Biophys Acta 1530: 209-18.
Lyons MA, Brown AJ (2001b) Metabolism of an oxysterol, 7-ketocholesterol, by sterol 27-hydroxylase in HepG2 cells. Lipids 36: 701-11.
Lyons MA, Samman S, Gatto L, Brown AJ (1999) Rapid hepatic metabolism of 7-ketocholesterol in vivo: implications for dietary oxysterols. J Lipid Res 40: 1846-57.
Nelson JA, Steckbeck SR, Spencer TA (1981) Biosynthesis of 24,25-epoxycholesterol from squalene 2,3;22,23-dioxide. J Biol Chem 256: 1067-8.
Ogishima T, Okuda K (1986) An improved method for assay of cholesterol 7 alpha-hydroxylase activity. Anal Biochem 158: 228-32.
Pulfer MK, Taube C, Gelfand E, Murphy RC (2005) Ozone exposure in vivo and formation of biologically active oxysterols in the lung.
J Pharmacol Exp Ther 312: 256-64.
Radhakrishnan A, Ikeda Y, Kwon HJ, Brown MS, Goldstein JL (2007) Sterol-regulated transport of SREBPs from endoplasmic reticulum to Golgi: Oxysterols block transport by binding to Insig. Proc Natl Acad Sci U S A 104: 6511-8.
Rose KA, Stapleton G, Dott K, Kieny MP, Best R, Schwarz M, Russell DW, Björkhem 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 U S A 94: 4925-30.
Russell DW (2000) Oxysterol biosynthetic enzymes. Biochim Biophys Acta 1529: 126-35.
Schroepfer GJ, Jr. (2000) Oxysterols: modulators of cholesterol metabolism and other processes. Physiol Rev 80: 361-554.
Takeuchi C, Galvé R, Nieva J, Witter DP, Wentworth AD, Troseth RP, Lerner RA, Wentworth P Jr. (2006) Proatherogenic effects of the cholesterol ozonolysis products, atheronal-A and atheronal-B. Biochemistry 45: 7162-70.
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: 1185-92.
Wassif CA, Yu J, Cui J, Porter FD, Javitt NB (2003) 27-Hydroxylation of 7- and 8-dehydrocholesterol in Smith-Lemli-Opitz syndrome: a novel metabolic pathway. Steroids 68: 497-502.
Zelcer N, Tontonoz P (2006) Liver X receptors as integrators of metabolic and inflammatory signaling. J Clin Invest 116: 607-14.
Zhang Z, Li D, Blanchard DE, Lear SR, Erickson SK, Spencer TA (2001) Key regulatory oxysterols in liver: analysis as delta4-3-ketone derivatives by HPLC and response to physiological perturbations. J Lipid Res 42: 649-58.
Zhang Q, Powers ET, Nieva J, Huff ME, Dendle MA, Bieschke J, Glabe CG, Eschenmoser A, Wentworth P Jr, Lerner RA, Kelly JW (2004) Metabolite-initiated protein misfolding may trigger Alzheimer's disease. Proc Natl Acad Sci U S A 101: 4752-7.
