Cholesteryl ester transport protein

Cholesterol lowering drugs are indicated for the prevention and treatment of atherosclerosis. There are three families of these dmgs inhibitors of HMG-CoA reductase (statins), inhibitors of cholesterol transport protein, and inhibitors of cholesteryl ester transfer protein (CETP). They are important drugs from an economical point of view. Among them, several are fluorinated. 

Fig. 5.2.1 The major metabolic pathways of the lipoprotein metabolism are shown. Chylomicrons (Chylo) are secreted from the intestine and are metabolized by lipoprotein lipase (LPL) before the remnants are taken up by the liver. The liver secretes very-low-density lipoproteins (VLDL) to distribute lipids to the periphery. These VLDL are hydrolyzed by LPL and hepatic lipase (HL) to result in intermediate-density lipoproteins (IDL) and low-density lipoproteins (LDL), respectively, which then is cleared from the blood by the LDL receptor (LDLR). The liver and the intestine secrete apolipoprotein AI, which forms pre-jS-high-density lipoproteins (pre-jl-HDL) in blood. These pre-/ -HDL accept phospholipids and cholesterol from hepatic and peripheral cells through the activity of the ATP binding cassette transporter Al. Subsequent cholesterol esterification by lecithinxholesterol acyltransferase (LCAT) and transfer of phospholipids by phospholipid transfer protein (PLTP) transform the nascent discoidal high-density lipoproteins (HDL disc) into a spherical particle and increase the size to HDL2. For the elimination of cholesterol from HDL, two possible pathways exist (1) direct hepatic uptake of lipids through scavenger receptor B1 (SR-BI) and HL, and (2) cholesteryl ester transfer protein (CfiTP)-mediated transfer of cholesterol-esters from HDL2 to chylomicrons, and VLDL and hepatic uptake of the lipids via the LDLR pathway...

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. 

The apoproteins of HDL are secreted by the liver and intestine. Much of the lipid comes from the surface monolayers of chylomicrons and VLDL during lipolysis. HDL also acquire cholesterol from peripheral tissues in a pathway that protects the cholesterol homeostasis of cells. In this process, free cholesterol is transported from the cell membrane by a transporter protein, ABCA1, acquired by a small particle termed prebeta-1 HDL, and then esterified by lecithin cholesterol acyltransferase (LCAT), leading to the formation of larger HDL species. The cholesteryl esters are transferred to VLDL, IDL, LDL, and chylomicron remnants with the aid of cholesteryl ester transfer protein (CETP). Much of the cholesteryl ester thus transferred is ultimately delivered to the liver by endocytosis of the acceptor lipoproteins. HDL can also deliver cholesteryl esters directly to the liver via a docking receptor (scavenger receptor, SR-BI) that does not endocytose the lipoproteins. 

Tchoua, U., D Souza, W., Mukhamedova, N., Blum, D., Niesor, E., Mizrahi, J., Maugeais, C., Sviridov, D. (2008 The effect of cholesteryl ester transfer protein overexpression and inhibition on reverse cholesterol transport. Cardiovasc. Res. 77, 732-739. 

P Barter, K-A Rye. Cholesteryl ester transfer protein its role in plasma lipid transport. Clin Exp Pharmacol Physiol 21 663-672, 1994. 

C7. Chajek, T., Aron, L., and Fielding, C. J., Interaction of lecithin cholesterol acyltransfer-ase and cholesteryl ester transfer protein in the transport of cholesteryl ester into sphingomyelin liposomes. Biochemistry 19, 3673-3677 (1980). 

Figure 15-2. A simplified schematic of cholesterol transport. Cholesterol travels to non-hepatic cells, such as the macrophage, via VLDL and LDL particles, while excess cholesterol is shuttled to the liver via HDL particles. Note that AHCAl mediates nascent HDL formation by translocating cellular cholesterol and phospholipids to apolipoprotein A-I (apoA-I) in an active, energy-dependent reaction. CETP, cholesteryl ester transfer protein LCAT, lecithinxholesterol acyltransferase LDLR, low-density lipoprotein receptor SR-B1, scavenger receptor Bl.

Cholesteryl esters are transferred out of HDL by cholesteryl ester transfer protein (CETP). CETP promotes the transfer of cholesteryl esters to VLDL and LDL in exchange for triacylglycerol. In this way, CETP enables HDL to transport more cholesteryl esters derived from the LCAT reaction. 

Figure 26-21 Reverse cholesterol transport pathway. HDl High-density lipoproteins LDL, low-density lipoproteins tDL, intermediate-density lipoproteins HTL, hepatic lipoprotein lipase LCAT, lecithin cholesterol acyltransferase CETP, cholesteryl ester transfer protein apo E, apoiipoprotein E. Cholesterol is removed from macrophages and other arterial wall cells by an HDL-mediated process. The LCAT esterifies the cholesterol content of HDL to prevent it from reentering the ceils. Cholesterol esters are delivered to the liver by one of three pathways ( ) cholesterol esters are transferred from HDL to LDL by CETP and enter the liver through the specific LDL receptor pathway (2) cholesterol esters are selectively taken from HDL by HDL receptors and HDL particles are returned to circulation for further transport or (3) HDL have accumulated apo E and therefore the particles can enter the liver through remnant receptors, (From Gwynne JT. High density lipoprotein cholesterol levels as a marker of reverse cho/estero/ tronsport./ m j Cardiol I989 64 10G-I7G. Copyright 1989, with permission from Excerpta Medico Inc.)...

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. 

Barter, P, and K-A. Rye. 1994. Cholesteryl ester transfer protein Its role in plasma hpid transport. Clinical and Experimental Pharmacology and Physiology 21 663-672. 

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). 

Zhang Z, Yamashita S, Hirano K, et al. Expression of cholesteryl ester transfer protein in human atherosclerotic lesions and its implication in reverse cholesterol transport. Atherosclerosis 2001 159 67-75. 

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 . 

Fat absorbed from the diet and lipids synthesized by the liver and adipose tissue must be transported between the various tissues and organs for utilization and storage. Since lipids are insoluble in water, the problem of how to transport them in the aqueous blood plasma is solved by associating nonpolar lipids (triacylglycerol and cholesteryl esters) with amphipathic hpids (phospholipids and cholesterol) and proteins to make water-miscible hpoproteins. 

Figure 26-5. Factors affecting cholesterol balance at the cellular level. Reverse cholesterol transport may be initiated by pre 3 HDL binding to the ABC-1 transporter protein via apo A-l. Cholesterol is then moved out of the cell via the transporter, lipidating the HDL, and the larger particles then dissociate from the ABC-1 molecule. (C, cholesterol CE, cholesteryl ester PL, phospholipid ACAT, acyl-CoA cholesterol acyltransferase LCAT, lecithinicholesterol acyltransferase A-l, apolipoprotein A-l LDL, low-density lipoprotein VLDL, very low density lipoprotein.) LDL and HDL are not shown to scale.

Lipoproteins. A lipoprotein is an endogenous macromolecule consisting of an inner apolar core of cholesteryl esters and triglycerides surrounded by a monolayer of phospholipid embedded with cholesterol and apoproteins. The functions of lipoproteins are to transport lipids and to mediate lipid metabolism. There are four main types of lipoproteins (classified based on their flotation rates in salt solutions) chylomicrons, very-low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL). These differ in size, molecular weight, and density and have different lipid, protein, and apoprotein compositions (Table 11). The apoproteins are important determinants in the metabolism of lipoproteins—they serve as ligands for lipoprotein receptors and as mediators in lipoproteins interconversion by enzymes. 

Uittenbogaard, A, Everson, WV, Matveev, SV, and Smart, EJ, 2002. Cholesteryl ester is transported from caveolae to internal membranes as part of a caveolin-annexin II lipid-protein complex. J Biol Chem 277,4925—4-931. 

Most plasma cholesterol is in an esterified form (with a fatty add attached at C-3, see Figure 18.2), which makes the structure sen more hydrophobic than free cholesterol. Cholesteryl esters are rot found in membranes, and are normally present only in low levels in most cells. Because of their hydrophobicity, cholesterol and ils esters must be transported in association with protein as a compo nent of a lipoprotein particle (see p. 225) or be solubilized by phos pholipids and bile salts in the bile (see p. 223). 

The search for intestinal cholesterol transporters extended for many years, beginning with a debate about whether or not it was even a protein-facilitated process (4, 5). The pancreatic enzyme carboxyl ester lipase (CEL, also called cholesterol esterase) was believed to be important to this process (6,7) and several companies devoted considerable resources to the development and testing of compounds to inhibit CEL, with mixed results (8-10). These efforts were abandoned in the mid-1990s, however, after studies with gene-knockout mice demonstrated that the enzyme was important only for absorption of cholesteryl ester (11, 12), which is a minor component of dietary cholesterol and is present at very low levels in bile. Interestingly, CEL is also found in liver where it has been shown to affect HDL metabolism (13). Thus, it may ultimately play an important role in cholesterol metabolism and may yet prove to be a useful drug target for CVD treatment (Camarota and Howies, unpublished). 

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