Plasma cholesterol ester transfer protein

CETP is a plasma protein of unknown origin that transfers CE from one lipoprotein or artificially prepared bilayer to another (Fig. 2A). In addition to CE, the protein may also transfer other lipids such as TG or PC. It has been referred to as lipid transfer complex (ETC) [68,69], esterified cholesterol transfer/exchange protein (ECTEP) [70], CE transfer protein (CETP) [71], and lipid transfer fraction (LTP-1) [72]. The original observations regarding plasma CE transfer activity were made nearly 20 years ago [73], but even today the literature is confusing and incomplete and no generally accepted recent review is available. 

CETP activity can be measured in vitro by a variety of methods, conditions, and substrates, but a common method is to monitor the transfer of [ C]CE between HDL and LDL. The donor and receptor lipoproteins are separated by heparin-MnCl2 precipitation and aliquots are counted [66,72]. CETP activity is determined by the difference between CE transfer with and without CETP. Fielding and coworkers [74] have devised an equally useful method, measuring CE transfer from cholesterol-lecithin liposomes to sphingomyelin-cholesterol liposomes. 

Several investigators have studied the physiological relation between these activities under a variety of in vitro conditions. Zilversmit and colleagues [68] demonstrated that transfer of CE from reconstituted lipoproteins to small, unilamellar vesicles proceeded on an equimolar, reciprocal basis with TG transfer. However, Fielding and coworkers [74] clearly showed that CETP can mediate the net transfer of CE in the complete absence of TG. Generating CE by co-incubating LCAT, A-1 and UC-PC liposomes, they found that CE accumulated up to a finite limit that was 

Several years ago it was suggested that CE exchange activity is associated with apo D [63,76]. CETP is found largely with HDL2 and HDL3 fractions [77], and apo D does appear to be associated with these components [71,76,77]. However, CETP activity and apo D have been separated by immunoabsorption chromatography [78], electrophoresis, and molecular sieve chromatography [71]. 

The plasma of several animal species has been found to contain a protein that can inhibit the CETP-mediated exchange of both CE and TG [79]. The presence of this protein may account for the apparent lack of CETP activity in some species. In particular, the plasma of animals such as the rat appears to contain a large amount of the inhibitor, which has an apparent molecular weight of 35 000 and a p7 of 4 [79]. Addition of plasma from rat or pig, both of which exhibit little detectable CETP activity, to partially purified rabbit CETP inhibited its activity [79]. 

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Jimenez F. Low-fat and high-monounsaturated fatty acid diets decrease plasma cholesterol ester transfer protein concentrations in young, healthy, normolipemic men. Am. J. Clin. Nutr. 

Terpstra, A. H. M., Lapre, J. A., de Vries, H. T., and Beynen, A. C. (1998). Dietary pectin with high viscosity lowers plasma and liver cholesterol concentration and plasma cholesteryl ester transfer protein activity in hamsters. /. Nutr. 128,1944-1949. 

The nascent HDL particles change shape and composition as they acquire additional free cholesterol by passive cellular diffusion of free cholesterol from cell membranes or from other plasma lipoproteins. HDL surface-localized LCAT progressively converts the free cholesterol on the surface of the particles to cholesterol ester, which occupies the core of the lipoprotein particle. This process converts the shape of the HDL particles from discoidal to spherical. The lipid unloading of HDL in the liver follows at least two pathways. In the first route, the cholesterol ester transfer protein (CETP) mediates cholesterol ester transfer from HDL to VLDL and LDL in exchange for triglyceride LDL in turn are taken up by the liver via the LDL receptor. In the second route, HDL binds to the scavenger receptor Bl, and cholesterol esters are selectively taken into the liver cells without internalization of HDL proteins (Fig. 15-2). 

Wallace, A.J., Mann, J.L, Sutherland, W.H., Williams, S., Chisholm, A., Skeaff, C.M., Gudnason, V., Talmud, P.J., and Humphries, S.E. (2000) Variants in the Cholesterol Ester Transfer Protein and Lipoprotein Lipase Genes Are Predictors of Plasma Cholesterol Response to Dietary Change, Ar/iero-sclerosis 152, 327-336. 

Anthocyanins, as a supplement, have also shown beneficial results on HDL- and LDL-cholesterol concentrations. Increased HDL-cholesterol and decreased LDL-cholesterol were reported in a recent study on dyslipidemic patients who were given 160 mg anthocyanins twice daily throughout a 12-week trial. Furthermore, the anthocyanin supplementation led to decreased mass and activity of plasma cholesteryl ester transfer protein and increased cellular cholesterol efflux to serum [48]. 

Figure 21-1 Movement of triacylglycerols from liver and intestine to body cells and lipid carriers of blood. VLDL very low density lipoprotein which contains triacylglycerols, phospholipids, cholesterol, and apolipoproteins B, and C. IDL intermediate density lipoproteins found in human plasma. LDL low density lipoproteins which have lost most of their triacylglycerols. ApoB-100, etc., are apolipoproteins listed in Table 21-2. LCAT, lecithin cholesterol acyltransferase CETP, cholesteryl ester transfer protein (see Chapter 22).

Unlike fatty acids, cholesterol is not degraded to yield energy. Instead excess cholesterol is removed from tissues by HDL for delivery to the liver from which it is excreted in the form of bile salts into the intestine. The transfer of cholesterol from extrahepatic tissues to the liver is called reverse cholesterol transport. When HDL is secreted into the plasma from the liver, it has a discoidal shape and is almost devoid of cholesteryl ester. These newly formed HDL particles are good acceptors for cholesterol in the plasma membranes of cells and are converted into spherical particles by the accumulation of cholesteryl ester. The cholesteryl ester is derived from a reaction between cholesterol and phosphatidylcholine on the surface of the HDL particle catalyzed by lecithimcholesterol acyltransferase (LCAT) (fig. 20.17). LCAT is associated with FIDL in plasma and is activated by apoprotein A-I, a component of HDL (see table 20.3). Associated with the LCAT-HDL complex is cholesteryl ester transfer protein, which catalyzes the transfer of cholesteryl esters from HDL to VLDL or LDL. In the steady state, cholesteryl esters that are synthesized by LCAT are transferred to LDL and VLDL and are catabolized as noted earlier. The HDL particles themselves turn over, but how they are degraded is not firmly established. 

HI Nishida, H Arai, T Nishida. Cholesterol ester transfer mediated by lipid transfer protein as influenced by changes in the charge characteristics of plasma lipoproteins. J Biol Chem 268 16352-16360, 1993. 

ApoD is found in association with LCAT and with apoA-I in the HDL fraction. Albers et al. used a specific antibody to apoD to remove all apoD by immunoadsorption chromatography from plasma about 64% of LCAT activity and 11% of apoA-I were also removed from plasma (A14). Purified apoD has an apparent Mr of 32,500, and appears as three isoforms on isoelectric focusing (pi 5.20, 5.08, and 5.00) (A14). An HDL apolipoprotein, Mr 35,000, has been thought to be apoD, and to be a cholesteryl ester transfer protein (i.e., to transfer newly synthesized esterified cholesterol from HDL to LDL) (C8). Cholesteryl ester transfer activity in plasma was removed by polyclonal immunoglobulin to apoD (C8, F10). However, Morton and Zilversmit (M41) were able to separate apoD and lipid transfer protein (i.e., the cholesteryl ester transfer protein, or lipid transfer protein I) by chromatography, and they showed that the removal of apoD from plasma by precipitation with specific antisera did not remove any lipid transfer activity. Albers et al. (A14) also showed that immunoadsorption with antibody specific for apoD removed all the apoD from plasma without removing any cholesteryl ester transfer activity. 

FIGURE 6.22 Shuttling of cholesterol from one type of cell to another. An HDL may pick up cholesterol from a macrophage, a white blood cell that phagocytizes debris in the bloodstream (e.g., dead red blood cells). A dead red blood cell contains cholesterol, since it contains a plasma membrane. (1) The macrophage can donate the cholesterol (that it has "eaten") to a passing HDL. (2) The cholesteryl ester that is formed is then transferred, in the circulation, to a VLDL. This transfer is catalyzed by an enzyme in the bloodstream called cholesteryl ester transfer protein (CETP). (3) Eventually, the cholesteryl ester can be delivered to the liver and excreted as a bile salt or (4) delivered to a cholesterol-needy cell. This ceU may be a premature red blood cell that is engaging in membrane synthesis and mitosis. 

HDLs are secreted in nascent form by hepatocytes and en-terocytes (Figure 20-7). Loss of surface components, including phospholipids, free cholesterol, and protein from chylomicrons and VLDL as they are acted on by lipoprotein lipase, may also contribute to formation of HDL in plasma. Discoidal, nascent HDL is converted to spherical, mature HDL by acquiring free cholesterol from cell membranes or other lipoproteins. This function of HDL in peripheral cholesterol removal may underlie the strong inverse relationship between plasma HDL levels and incidence of coronary heart disease. After esterification of HDL surface cholesterol by LCAT, which is activated by apo A-I, HDL sequesters the cholesteryl ester in its hydrophobic core. This action increases the gradient of free cholesterol between the cellular plasma membrane and HDL particles. Cholesteryl esters are also transferred from HDL to VLDL and LDL via apo D, the cholesteryl ester transfer protein (Figure 20-8). 

Another factor that regulates HDL cholesterol levels is the plasma level of cholesteryl ester transfer protein (CETP). CETP, a hydrophobic glycoprotein (M.W. 741,000), facilitates the transfer of cholesteryl esters in HDL and triacylglycerols in LDL and VLDL (see above). In CETP deficiency due to a point mutation (G A) in a splice donor site that prevents normal processing of mRNA, the plasma HDL cholesterol levels of affected individuals are markedly high, with decreased LDL cholesterol. In the affected families, there was no evidence of premature atherosclerosis and, in fact, there was a trend toward longevity. These observations support the role of CETP and the antiatherogenic property of HDL. However, not all factors that elevate HDL levels may be... 

As far as HDL levels and metabolism are concerned, one result of the LCAT- and transfer protein-catalyzed reactions is the production of a dynamic spectrum of particles with a wide range of sizes and lipid compositions (Chapter 19). Nascent HDL particles contain mostly apo A1 and phospholipids, and undergo modulation and maturation in the circulation. For instance, the unesterified cholesterol incorporated into plasma HDL is converted to cholesteryl esters by LCAT, creating a concentration gradient of cholesterol between HDL and cell membranes, which is required for efficient cholesterol efflux from cells to HDL. In addition, cholesteryl ester transfer protein transfers a significant amount of HDL cholesteryl ester to VLDL, IDL, and LDL for further transport, primarily to the liver. Thus, a substantial fraction of cell-derived cholesterol is delivered as part of HDL indirectly to the liver via hepatic endocytic receptors for IDL and LDL this process is termed reverse cholesterol transport . However, receptor-mediated delivery of HDL cholesterol to cells is fundamentally different from the classic LDL receptor-mediated endocytic pathway, as described in Section 7.3.2. 

The esterification of cholesterol in animals has attracted considerable research because of the possible involvement of cholesterol and its ester in various disease states (cf. Glomset and Norum, 1973, and Sections 12.1, 12.3 and 12.6). Cholesterol esters are formed by the action of lecithin cholesterol acyltransferase (LCAT, EC 2.3.1.43) which is particularly active in plasma (cf. Sabine, 1977, for a review of cholesterol metabolism). The reaction involves transfer of a fatty acid from position 2 of lecithin (phosphatidylcholine) to the 3-hydroxyl group of cholesterol with the formation of monoacyl-phosphatidylcholine. Although LCAT esterifies plasma cholesterol solely at the interface of high-density lipoprotein and very-low-density lipoprotein, the cholesterol esters are transferred to other lipoproteins by a particular transport protein (CETP cholesteryl ester transfer protein). Cholesteryl esters, in contrast to free cholesterol, are taken up by cells mostly via specific receptor pathways (Brown et aL, 1981), are hydrolysed by lysosomal enzymes and eventually re-esterified and stored within cells. LCAT may also participate in the movement of cholesterol out of cells by esterifying excess cholesterol in the intravascular circulation (cf. Marcel, 1982). 

The metabolism of HDL involves several different enzymes and transfer proteins but is not completely understood [7]. The major apolipoprotein of HDL is apoA-I. The liver and intestine are the sources of apoA-I, which interacts with peripheral cells to remove excess cellular cholesterol via the ATP-binding cassette protein A1 (ABCAl). Unesterified cholesterol associated with nascent HDL is a substrate for the plasma enzyme lecithin cholesterol acyltransferase (LCAT), resulting in the formation of cholesteryl ester and enlargement of the HDL particle. Genetic defects in apoA-I, ABCAl and LCAT can cause low levels of HDL, termed hypoalphalipopro-teinemia. HDL cholesteryl ester is transferred to apoB-containing lipoproteins (such as LDL) by the cholesteryl ester transfer protein (CETP) and can be returned to the liver via the LDL receptor. HDL may also deliver some cholesterol directly to the liver via the scavenger receptor class BI (SR-BI). The removal of excess cholesterol from peripheral cells and delivery to the liver for excretion in the bile is a process that has been termed reverse cholesterol transport . 

When most lipids circulate in the body, they do so in the form of lipoprotein complexes. Simple, unesterified fatty acids are merely bound to serum albumin and other proteins in blood plasma, but phospholipids, triacylglycerols, cholesterol, and cholesterol esters are all transported in the form of lipoproteins. At various sites in the body, lipoproteins interact with specific receptors and enzymes that transfer or modify their lipid cargoes. It is now customary to classify lipoproteins according to their densities (Table 25.1). The densities are... 

The intestinal absorption of dietary cholesterol esters occurs only after hydrolysis by sterol esterase steryl-ester acylhydrolase (cholesterol esterase, EC 3.1.1.13) in the presence of taurocholate [113][114], This enzyme is synthesized and secreted by the pancreas. The free cholesterol so produced then diffuses through the lumen to the plasma membrane of the intestinal epithelial cells, where it is re-esterified. The resulting cholesterol esters are then transported into the intestinal lymph [115]. The mechanism of cholesterol reesterification remained unclear until it was shown that cholesterol esterase EC 3.1.1.13 has both bile-salt-independent and bile-salt-dependent cholesterol ester synthetic activities, and that it may catalyze the net synthesis of cholesterol esters under physiological conditions [116-118], It seems that cholesterol esterase can switch between hydrolytic and synthetic activities, controlled by the bile salt and/or proton concentration in the enzyme s microenvironment. Cholesterol esterase is also found in other tissues, e.g., in the liver and testis [119][120], The enzyme is able to catalyze the hydrolysis of acylglycerols and phospholipids at the micellar interface, but also to act as a cholesterol transfer protein in phospholipid vesicles independently of esterase activity [121],... 

T2. Tall, A. R, Abreu, E., and Shuman, J., Separation of a plasma phospholipid transfer protein from cholesterol ester/phospholipid exchange protein. J. Biol. Chem. 258, 2174-2180 (1983). 

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