Oxysterols

CYP7A1 catalyzes the 7a-hydroxylation of cholesterol, the first and rate limiting step of bile acid synthesis. This is also the principal way to eliminate cholesterol. CYP7B1 is primarily expressed in brain and catalyzes the synthesis of various neurosteroids and also the 7a-hydroxylation of oxysterols. 

Peroxidation is also catalyzed in vivo by heme compounds and by lipoxygenases found in platelets and leukocytes. Other products of auto-oxidation or enzymic oxidation of physiologic significance include oxysterols (formed from cholesterol) and isoprostanes (prostanoids). 

Studies conducted by Barenghi eta.1. (1990) and Lodge etal. (1993) independently have demonstrated the facile, multicomponent analysis of a wide range of PUFA-derived peroxidation products (e.g. conjugated dienes, epoxides and oxysterols) in samples of oxidized LDL by high-field H-NMR spectroscopy. Figure 1.9 shows the applications of this technique to the detection of cholesterol oxidation products (7-ketocholesterol and the 5a, 6a and 5/3,60-epoxides) in isolated samples of plasma LDL pretreated with added coppcr(Il) or an admixture of this metal ion with H2O2, an experiment conducted in the authors laboratories. 

Extensive studies in vitro from many groups have confirmed that exposure of LDL to a variety of pro-oxidant systems, both cell-free and cell-mediated, results in the formation of lipid hydroperoxides and peroxidation products, fragmentation of apoprotein Bioo, hydrolysis of phospholipids, oxidation of cholesterol and cholesterylesters, formation of oxysterols, preceded by consumption of a-tocopherol and accompanied by consumption of 8-carotene, the minor carotenoids and 7-tocopherol. 

Turchetto, E., Lercker, G., Bortolomeazzi, R., Oxysterol determination in selected coffees, Toxicol. Ind. Health, 9(3), 519, 1993. (CA121 81456v)... 

Joffre, C, Leclere, L, Buteau, B, Martine, L, Cabaret, S, Malvitte, L, Acar, N, Lizard, G, Bron, A, and Creuzot-Garcher, C, 2007. Oxysterols induced inflammation and oxidation in primary porcine retinal pigment epithelial cells. Curr Eye Res 32, 271-280. 

Venkateswaran, A, Laffitte, BA, Joseph, SB, Mak, PA, Wilpitz, DC, Edwards, PA, and Tontonoz, P, 2000. Control of cellular cholesterol efflux by the nuclear oxysterol receptor LXR alpha. Proc Natl Acad Sci USA91, 12097-12102. 

A recent observation shows that P-carotene was able to counteract the dangerous effect of 7-ketocholesterol in human macrophages by limiting the apoptotic processes reducing the intracellular ROS production and inhibiting the phosphorylation of p38, JNK, and ERK1/2 induced by the oxysterol (Palozza et al., 2007b). 

As mentioned earlier, oxidation of LDL is initiated by free radical attack at the diallylic positions of unsaturated fatty acids. For example, copper- or endothelial cell-initiated LDL oxidation resulted in a large formation of monohydroxy derivatives of linoleic and arachi-donic acids at the early stage of the reaction [175], During the reaction, the amount of these products is diminished, and monohydroxy derivatives of oleic acid appeared. Thus, monohydroxy derivatives of unsaturated acids are the major products of the oxidation of human LDL. Breuer et al. [176] measured cholesterol oxidation products (oxysterols) formed during copper- or soybean lipoxygenase-initiated LDL oxidation. They identified chlolcst-5-cnc-3(3, 4a-diol, cholest-5-ene-3(3, 4(3-diol, and cholestane-3 3, 5a, 6a-triol, which are present in human atherosclerotic plaques. 

Janowski, B.A., Grogan, M.J., Jones, S.A., Wisely, G.B., Kliewer, S.A., Corey, E.J. and Mangelsdorf, D.J. (1999) Structural requirements of ligands for the oxysterol liver X receptors LXRa and LXRp. Proceedings of the National Academy of Sciences of the United States of America, 96, 266-271. 

M. H. and Willson, T.M. (2001) Pharmacophore analysis of the nuclear oxysterol receptor LXRa. Journal of Medicinal Chemistry, 44, 886-897. 

Agassandian M, Mathur SN, Zhou J, Field FJ, Mallampalli RK (2004) Oxysterols trigger ABCAl-mediated basolateral surfactant efflux. Am J Respir Cell Mol Biol 31(2) 227-233... 

The toxic effect of 7-oxysterols, 25- and 27-hydroxycholesterols and their involvement in LDL cytotoxicity have been extensively studied on the different vascular cell types (Lizard et al, 1999 Aupeix et al, 1995 Clare et al, 1995 Ramasamy et al, 1992). 7a and 7p-hydroxycholesterols, 7-ketocholesterol, 25 and 27-hydroxycholesterol induce apoptosis (Brown and Jessup, 1999 Lizard et al, 1999, 1998 Zhang et al, 1997 Hughes et al, 1994). 7p-hydroperoxycholesterol is one of the most toxic Oxysterols present in oxidized LDL (Brown and Jessup, 1999 Colles et al, 1996). 25-hydroxycholesterol, though less active (Aupeix et al 1995), is able to trigger a cytochrome c release and subsequent caspase activation in CHO cells, but also calcium inaease in relation with apoptosis (Rusinol et al, 2000). 

Oxysterol-mediated cytoxicity occurs by neaosis or apoptosis (Lizard et al, 1999 Yin et al, 2000), and can be partially prevented by Bcl2 overexpression (which inhibits caspase-3 activation and subsequent apoptosis), and by glutathione or antioxidants (Lizard et al, 1998). The mechanism of cytotoxicity is unclear but could involve an oxidative stress (Lizard et al, 1998), a direct stimulation of apoptotic cascade (Harada et al, 1997 Christ et al, 1993), a calcium rise (Spyridopoulos et al, 2001), or metabolic dysfunctions such as inhibition of HMGCoA reductase or incorporation of oxysterols into membranes (Brown and Jessup, 1999). 

The contribution of oxysterols to the cytotoxicity of oxidized LDL is likely, but it is to be noted that oxysterol concentrations necessary to trigger apoptosis in cultured cells are much higher than that reported in toxic concentrations of oxidized LDL and in atherosclerotic plaques (Brown and Jessup, 1999). This could be due to a synergistic effect of the different toxic molecules brought by the oxLDL. 

Aupeix, K, Weltin, D, Mejia, JE, Christ, M, Marchal, J, Freyssinet, JM, Bischoff, P., 1995, Oxysterol-induced apoptosis in human monocytic cell lines. Immunobiology 194 15-28. 

Brown, A.J., Dean, R.T., and Jessup, W., 1996, Free and esteiified oxysterol formation during copper-oxidation of low density hpoprotein and uptake by macrophages, /. Lipid Res. 37 320-335. 

Christ, M., Luu, B., Mejia, J.E., Moosbmgger, I., and Bischoff, P., 1993, Apoptosis induced by oxysterols in murine lymphoma cells and in normal thymocytes. Immunology 78 455-460. 

Harada, K., Ishibashi, S., Miyashita, T, Osuga, J., Yagyu, H., Ohashi, K., Yazaki, Y., and Yamada, N., 1997, Bcl-2 inhibits oxysterol-induced apoptosis through suppressing CPP32-mediated pathway, FEBS Lett. 411 63-66. 

Hughes, H., Mathews, B., Lenz, M.L., and Guyton, J.R., 1994, Cytotoxicity of oxidized LDL to porcine aortic smooth muscle cells is associated with the oxysterols 7-ketocholesterol and 7-hydroxycholesterol, Arteriosc/er. Thromb. 14 1177- 1185. 

Ramasamy, S., Boissonneault, G.A., and Hennig, B., 1992, Oxysterol-induced endothehal cell dysfunction in culture, J. Am. Coll. Nutr. 11 532-538. 

Spyridopoulos, I., Wischhusen, J., Rabenstein, B., Mayer, P., Axel, D.L, Frohheh, K.U., and Karsch, K.R., 2001, Alcohol Errhances Oxysterol-lnduced Apoptosis in Human Endothelial Cells by a Caldum-Dependent Mechanism, Arterioscler. Thromb. Vase. Biol. 21 439 144. 

This isoform was found as the result of a mouse deficient in the oxysterol 7a-hydroxylase gene (CYP7B1 in humans), which nonetheless did not accumulate 24-hydroxy cholesterol from the CYP46pathway, suggesting the existence of another 7ot-hydroxylase (Li-Hawkins et al., 2000). However, very httle evidence exists for its expression in human brain. Nishimura et al. quantified CYP39A1 mRNA by RT-RT-PCR and reported some 100-fold less than they reported for CYP51 mRNA (Nishimura et al., 2003). There is no other evidence for the expression of this isoform in the brain as of this writing and this is beheved to be the first report of its kind. The isoform has been shown to be sexually dimorphic in the liver (Li-Hawkins et al., 2000). 

Ikeda H, Ueda M, Ikeda M, Kobayashi H, Honda Y. 2003. Oxysterol 7alpha-hydroxylase CYP39A1) in the ciliary nonpigmented epithelium of bovine eye. I b Invest 83 ... 

Li-Hawkins J, Lund EG, Bronson AD, RusseU DW. 2000. Expression cloning of an oxysterol 7alpha-hydroxylase selective for 24-hydroxycholesterol. J Biol Chem 275 16543-16549. 

Steckelbroeck S, Watzka M, Lutjohann D, Makiola P, Nassen A, et al. 2002. Characterization of the dehydroepi-androsterone (DHEA) metabolism via oxysterol 7alpha-hydroxylase and 17-ketosteroid reductase activity in the human brain. J Neurochem 83 713-726. 

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 73-81. 

Wu Z, Martin KO, Javitt NB, Chiang JY. 1999. Structure and functions of human oxysterol 7alpha-hydroxylase cDNAs and gene CYP7BL J Lipid Res 40 2195-2203. 

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