Acyl-CoA cholesterol acyltransferase ACAT

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.

ACAT Acyl-CoA cholesterol acyltransferase cAPK Protein kinase A (or cyclic AMP-... 

Figure 7 Process of lipid droplet formation in macrophages and inhibition sites of inhibitors. ACAT (acyl-CoA cholesterol acyltransferase) and ACS (acyl-CoA synthetase).

ACAT acyl-CoA cholesterol acyltransferase EC Enzyme Commission... 

Fig. 1. Overview of the metabolic and transport pathways that control cholesterol levels in mammalian cells. Cholesterol is synthesized from acetyl-CoA and the four key enzymes that regulate cholesterol synthesis are indicated. Cells also obtain cholesterol by uptake and hydrolysis of LDL s cholesteryi esters (CE). End products derived from cholesterol or intermediates in the pathway include bile acids, oxysterols, cholesteryi esters, and non-steroidal isoprenoids. ACAT, acyl-CoA cholesterol acyltransferase.

Although lanosterol may appear similar to cholesterol in structure, another 20 steps are required to convert lanosterol to cholesterol (Figure 25.35). The enzymes responsible for this are all associated with the endoplasmic reticulum. The primary pathway involves 7-dehydroeholesterol as the penultimate intermediate. An alternative pathway, also composed of many steps, produces the intermediate desmosterol. Reduction of the double bond at C-24 yields cholesterol. Cholesterol esters—a principal form of circulating cholesterol—are synthesized by acyl-CoA cholesterol acyltransferases (ACAT) on the cytoplasmic face of the endoplasmic reticulum. 

FIGURE 25.39 Endocytosis and degradation of lipoprotein particles. (ACAT is acyl-CoA cholesterol acyltransferase.)... 

Regulation of the LDL receptor gene involves a hormone-response element (HRE, see p. 238).] Third, if the cholesterol is not required immediately for some structural or synthetic purpose, it is esterified by acyl CoA cholesterol acyltransferase (ACAT, AC AT transfers a fatty acid from a fatty acyl CoA derivative to cholesterol, producing a cholesteryl ester that can be stored in the cell (Figure 18.21). The activity of ACAT is enhanced in the presence of increased intracellular cholesterol. 

Of the various lipid components of the lipoproteins, only the biosynthesis of cholesteryl esters has not yet been mentioned. Cholesteryl ester is the storage form of cholesterol in cells. It is synthesized from cholesterol and acyl-CoA by acyl-CoA cholesterol acyltransferase (ACAT) (fig. 20.13), which is located on the cytosolic surface of hepatic endoplasmic reticulum. Acylation of the 3 hydroxyl group of cholesterol eliminates the polarity of cholesterol and facilitates the packing of cholesterol as its ester in the core of the lipoprotein or for storage in lipid droplets within cells. 

Intestinal acyl-CoA cholesterol acyltransferase (ACAT-2, also present in liver), which esterifies free cholesterol with palmitic or oleic acid, is another enzyme that was identified early on as a potential target to inhibit cholesterol absorption because most cholesterol in chylomicrons is esterified before being secreted by enterocytes (6, 14). As for CEL, various inhibitors of this enzyme were also developed and tested with mixed results (10, 15-17). However, the importance of ACAT-2 was later confirmed by studies of gene-knockout mice, which exhibit markedly reduced cholesterol absorption and atherosclerosis when fed Western diet (18). Nonetheless, progress in developing effective ACAT inhibitors has been slow, in part because of concerns about the potential for deleterious systemic effects resulting from inhibition of the more widely expressed ACAT-1 (19). Despite these... 

Third, acyl-CoA cholesterol acyltransferase (ACAT) [EC 2.3.1.26], an enzyme that works after the formation of cholesterol, was considered a unique target of inhibition [32], ACAT catalyzes the synthesis of cholesteiyl esters from cholesterol and long-chain fatty acyl-CoA. ACAT plays important roles in the body, for example, in the absorption of dietary cholesterol from the intestines, production of lipoprotein in liver and formation of foam cells from macrophages in arterial walls. Therefore, ACAT inhibition is expected not only to lower plasma cholesterol levels but also to have a direct effect at the arterial wall. A number of synthetic ACAT inhibitors such as ureas, imidazoles, and acyl amides have been developed [33], Several groups have searched for novel ACAT inhibitors... 

LDLs are then taken up by target cells through receptor-mediated endocytosis (see Topic E4). The LDL receptor, a transmembrane glycoprotein on the surface of the target cells, specifically binds apoB-100 in the LDL coat. The receptors then cluster into clathrin-coated pits and are internalized (see Topic E4, Fig. 3). Once in the lysosomes, the LDLs are digested by lysosomal enzymes, with the cholesterol esters being hydrolyzed by a lysosomal lipase to release the cholesterol (Fig. 1). This is then incorporated into the cell membrane and any excess is re-esterified for storage by acyl CoA cholesterol acyltransferase (ACAT). 

Both IDL and LDL can be removed from the circulation by the liver, which contains receptors for ApoE (IDL) and ApoB-100 (IDL and LDL). After IDL or LDL interacts with these receptors, they are internalized by the process of receptor-mediated endocytosis. Receptors for ApoB-100 are also present in peripheral tissues, so that clearance of LDL occurs one-half by the liver and one-half by other tissues. In the liver or other cells, LDL is degraded to cholesterol esters and its other component parts. Cholesterol esters are hydrolyzed by an acid lipase and may be used for cellular needs, such as the building of plasma membranes or bile salt synthesis, or they may be stored as such. Esterification of intracellular cholesterol by fatty acids is carried out by acyl-CoA-cholesterol acyltransferase (ACAT). Free cholesterol derived from LDL inhibits the biosynthesis of endogenous cholesterol. B-100 receptors are regulated by endogenous cholesterol levels. The higher the latter, the fewer ApoB-100 receptors are on the cell surface, and the less LDL uptake by cells takes place. 

The released unesterified cholesterol can then be usedfor membrane biosynthesis. Alternatively, it can be reesterified for storage inside the cell. In fact, free cholesterol activates acyl CoA cholesterol acyltransferase (ACAT), the enzyme catalyzing this reaction. Reesterified cholesterol contains mainly oleate and palmitoleate, which are monounsaturated fatty acids, in contrast with the cholesterol esters in LDL, which are rich in linoleate, a polyunsaturated fatty acid (see Table 24.1). It is imperative that the cholesterol be reesterified. High concentrations of unesterified cholesterol disrupt the integrity of cell membranes. 

An increase in 18 1 in liver cells results in an increased availability of this fatty acid to acyl-CoA cholesterol acyltransferase (ACAT). ACAT is the enzyme that catalyzes the attachment of free fatty acids to cholesterol, creating cholesteryl esters. 

Enniatins (Fig. 44) are produced by Fusarium species. Enniatin B exhibits binding to alkali ions, as well as valinomycin. While valinomycin shows preferential binding to K, Rb, and Cs over Na", enniatin B is considerably less specific Enniatins D, E and F were isolated from Fusarium sp. FO-1305 as inhibitors of acyl-CoA cholesterol acyltransferase (ACAT), by Tomoda et al. in 1992. The IC50 values of enniatins D, E and F against ACAT in an enzyme assay using rat liver microsomes are 87, 57 and 40 pM, respectively ... 

Acyl-CoA cholesterol acyltransferase (ACAT) esterifies free cholesterol by linking it to a fatty acid. ACAT is an intracellular enzyme that prepares cholesterol for storage as in liver parenchymal cells. The roles of these enzymes in lipoprotein metabolism will become more apparent in the discussion of the origin and fate of each class of lipoprotein. 

Enzymes and proteins that synthesize, transport, and hydrolyze CE are found both inside and outside of cells. In most cases, the intracellular and extracellular enzymes use entirely different cofactors, and have different pH optima. The enzymes found within cells typically include both a CE-synthesizing enzyme, acyl-CoA cholesterol acyltransferase (ACAT), and 2 or 3 CE-degrading enzymes acid CE hydrolase (CEH), neutral CEH, and possibly a mitochondrial CEH. Blood plasma and the extracellular fluid, on the other hand, contain only a CE-synthesizing enzyme, lecithin cholesterol acyltransferase (LCAT), and a CE transfer protein (CETP). Finally, pancreatic juice contains a CE-degrading enzyme, pancreatic CEH. Each of these very different proteins is discussed below, with the exception of pancreatic CEH, which is discussed in Chapter 5. 

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. 

Cholesteryl esters are quantitatively minor constituents (5-15% of total lipids) of VLDLs but the amount of cholesteryl esters relative to TG in VLDLs increases when rats are fed a high cholesterol diet. The esterification of cholesterol is mediated by two distinct acyl-CoA cholesterol acyltransferases (ACATs) [11]. Inhibition of cholesterol esterification with an ACAT inhibitor in hepatocytes decreased apo B secretion in some studies but not in others. For example, severe reduction in cholesteryl ester content of hepatoma cells decreased apo B secretion, whereas increased cholesteryl ester content did not stimulate apo B secretion. In mouse liver and intestine, the majority of cholesteryl esters are made by ACAT2. Nevertheless, normal quantities of apo B-containing lipoproteins are produced in mice lacking ACAT2 despite the absence of essentially all hepatic ACAT activity. However, ACAT2-deficient mice exhibit reduced intestinal absorption of cholesterol and are resistant to diet-induced hypercholesterolemia (R.V. Farese, 2(X)0). Thus, the observed reduction of plasma cholesterol in response to ACAT inhibitors is probably due to decreased cholesterol absorption rather than decreased VLDL secretion. 

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