Lipoproteins oxysterols

Mammalian cells acquire cholesterol either by de novo synthesis from acetyl-coen-zyme A (CoA) or via the low-density lipoprotein (LDL)-receptor-mediated uptake of LDL particles that contain cholesterol esterified with long-chain fatty acids. These LDL cholesterol esters are subsequently hydrolyzed in lysosomes, after which free cholesterol molecules become available for synthesis of membranes, steroid hormones, bile acids, or oxysterols [1]. 

The initial steps in BA synthesis are characterised by the introduction of a hy-droxylic group in the la position, or in position 27, followed by another in the la position into the cholesterol nucleus. Both synthetic pathways (the neutral and the acidic pathways) possess a distinct microsomal 7-oxysterol hydroxylase, which is regulated by different genes. The most recently described disorder of BA synthesis is cholesterol 7a-hydroxylase deficiency, in which their decreased production through the classical pathway is partially balanced by activation of the alternative pathway. Cholesterol levels increase in the liver, with a consequent low-density lipoprotein hypercholesterolemia, and cholesterol gallstones may result, although there is no liver disease. In contrast, a defect in the conversion of 27-hydroxy-cholesterol to la,27-dihydroxy-cholesterol due to deficiency of the oxysterol 7a-hydroxylase specific for the alternate pathway, causes severe neonatal liver disease [8]. 

Brown, A.J., Jessup, W. 1999. Oxysterols and atherosclerosis. Atherosclerosis 142, 1-28. Brown, A. J., Dean, R.T., Jessup, W. 1996. Free and esterified oxysterol formation during copper-oxidation of low-density lipoprotein and uptake of macrophage. J. Lipid Res. 37, 320-335. 

Dzeletovic, S., Babiker, A., Lund, E., Diczfalusy, U. 1995. Time course of oxysterol formation during in vitro oxidation of low density lipoprotein. Chem. Phys. Lipids 78, 119-128. 

Mattsson-Hulten, L., Lindmark, H., Diczfalusy, U., Bjorkhem, I., Ottosson, M., Liu, Y., Bondgers, G., Wilklund, O. 1996. Oxysterols present in atherosclerotic tissue decrease the expression of lipoprotein lipase messenger RNA in human monocyte-derived macrophages. J. Clin. Invest. 97, 461-468. 

Patel, R.P., Diazfalusy, U., Dzeletovic, S., Wilson, M.T., Darley-Usmar, V.M. 1996. Formation of oxysterols during oxidation of low-density lipoprotein by peroxynitrite, myoglobin and copper. J. Lipid Res. 37, 2361-2371. 

Vine, D.F., Mamo, J.C.L., Beilin, L.J., Mori, T.A., Croft, K.D. 1998. Dietary oxysterols are incorporated in plasma triglyceride-rich lipoproteins, increase their susceptibility to oxidation and increase aortic cholesterol concentration in rabbits. J. Lipid Res. 39, 1995-2004. 

Zarev, S., Therond, P., Bonnefont-Rousselot, D. Beaudeux, J.-L., Gardes-Albert, M., Legrand, A. 1999. Major differences in oxysterol formation in human low density lipoproteins (LDLs) oxidized by OH/O free radicals or by copper. FEBS Letts. 451, 103-108. 

For example, hepatocytes express LXR (liver X receptor), a nuclear receptor that senses the levels of oxysterols. When cellular cholesterol Increases in the liver, oxysterols are generated and activate LXR. Activated LXR stimulates the expression of cholesterol 7ct-hydroxylase, the key rate-limiting enzyme In the hepatic conversion of cholesterol Into bile acids, a major pathway for disposing of excess cholesterol from the body. LXR also stimulates the expression of the ABC proteins that export cholesterol Into the bile (ABCG5/8) or onto lipoproteins in the blood (ABCAl). In... 

Essential non-steroidal isoprenoids, such as dolichol, prenylated proteins, heme A, and isopentenyl adenosine-containing tRNAs, are also synthesized by this pathway. In extrahepatic tissues, most cellular cholesterol is derived from de novo synthesis [3], whereas hepatocytes obtain most of their cholesterol via the receptor-mediated uptake of plasma lipoproteins, such as low-density lipoprotein (LDL). LDL is bound and internalized by the LDL receptor and delivered to lysosomes via the endocytic pathway, where hydrolysis of the core cholesteryl esters (CE) occurs (Chapter 20). The cholesterol that is released is transported throughout the cell. Normal mammalian cells tightly regulate cholesterol synthesis and LDL uptake to maintain cellular cholesterol levels within narrow limits and supply sufficient isoprenoids to satisfy metabolic requirements of the cell. Regulation of cholesterol biosynthetic enzymes takes place at the level of gene transcription, mRNA stability, translation, enzyme phosphorylation, and enzyme degradation. Cellular cholesterol levels are also modulated by a cycle of cholesterol esterification mediated by acyl-CoA cholesterol acyltransferase (ACAT) and hydrolysis of the CE, by cholesterol metabolism to bile acids and oxysterols, and by cholesterol efflux. 

Excess cholesterol can also be metabolized to CE. ACAT is the ER enzyme that catalyzes the esterification of cellular sterols with fatty acids. In vivo, ACAT plays an important physiological role in intestinal absorption of dietary cholesterol, in intestinal and hepatic lipoprotein assembly, in transformation of macrophages into CE laden foam cells, and in control of the cellular free cholesterol pool that serves as substrate for bile acid and steroid hormone formation. ACAT is an allosteric enzyme, thought to be regulated by an ER cholesterol pool that is in equilibrium with the pool that regulates cholesterol biosynthesis. ACAT is activated more effectively by oxysterols than by cholesterol itself, likely due to differences in their solubility. As the fatty acyl donor, ACAT prefers endogenously synthesized, monounsaturated fatty acyl-CoA. 

In our above proposed hypothesis for the in vivo formation of oxysterols, we attribute their occuiTence to the various phenomena that make it possible, (i) for various almost water-insoluble (at 25°C cholesterol solubility is approximately 0.2 mg/dl or 4.7 xM) cholesterol species to occur in serum, an aqueous medium [15], and (ii) to allow these species as lipoprotein entities to circulate in blood throughout our bodies. 

Yasunobu, Y Hayashi, K. Shingu, T. Yamagata, T. Kajiyama, G. Kambe, M., Coronary atherosclerosis and oxidative stress as reflected by autoantibodies against oxidized low-density lipoprotein and oxysterols. Athewsclew 2001, 155(2), 445-53. 

In this connection, the type and concentration of oxysterols that are actually detectable in oxidized low-density lipoproteins (oxLDL) and, above all, those detectable in atherosclerotic plaques, must first be considered. 

Vaya, J., M. Aviram, S. Mahmood et al. 2001. Selective distribution of oxysterols in atherosclerotic lesions and human plasma lipoproteins. Q e adic 34 (5) 485-97. 

Table 2 Content of oxysterols in isolated lipoprotein fractions VLDL, LDL, HDL), lipoprotein-free plasma (LFP), and unfractionated plasma (plasma)...

Sevanian et al. (1994) applied GLC and LC/TS/MS for the analysis of plasma cholesterol-7-hydroperoxides and 7-ketocholesterol. Analysis of human and rabbit plasma identified the commonly occurring oxidation products, yet dramatic increases in 7-ketocholesterol and cholesterol-5p, 6P-epoxide were observed. The study failed to reveal the presence of choles-terol-7-hydroperoxides, which were either too unstable for isolation, metabolized or further decomposed. The principal ions of cholesterol oxides monitored by LC/TS/MS were m/z 438 (cholestane triol) m/z 401 (cholesterol-7-hydroperoxide) m/z 401 (7-ketocholesterol) m/z 367 (7a-hydroxycholesterol) m/z 399 (cholesta-3,5-dien-7-one) and m/z 385 (choles-terol-5a,6a-epoxide). The major ions were supported by minor ions consistent with the steroid structure. Kamido et al. (1992a, b) synthesized the cholesteryl 5-oxovaleroyl and 9-oxononanoyl esters as stable secondary oxidation products of cholesteryl arachidonate and linoleate, respectively. These compounds were identified as the 3,5-dinitrophenylhydrazone (DNPH) derivatives by reversed-phase LC/NICI/MS. These standards were used to identify cholesteryl and 7-ketocholesteryl 5-oxovaleroyl and 9-oxononanoyl esters as major components of the cholesteryl ester core aldehydes generated by copper-catalysed peroxidation of low-density lipoprotein (LDL). In addition to 9-oxoalkanoate (major product), minor amounts of the 8, 9, 10, 11 and 12 oxo-alkanoates were also identified among the peroxidation products of cholesteryl linoleate. Peroxidation of cholesteryl arachidonate yielded the 4, 6, 7, 8, 9 and 10 oxo-alkanoates of cholesterol as minor products. The oxysterols resulting from the peroxidation of the steroid ring were mainly 7-keto, 7a-hydroxy and 7P-... 

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