Acyltransferases acyl-CoA/cholesterol

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.)... 

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-... 

Cholesterol metabolism. Hydrogenated oil, administered orally to hamsters at a dose of 20% of diet for 4 weeks, induced hypercholesterolemia. Oil feeding had no effect on cholesterol synthesis but markedly inhibited cholesterol esterification in both the liver and the intestine. The diet-induced hypercholesterolemia was strongly correlated with an increase in acyl-CoA/cholesterol acyltransferase activity. The hypercholesterolemia increased aortic uptake of cholesterol and hence acyl-CoA/cholesterol acyltransferase activity " Coconut fat, administered orally to rabbits with partial ileal bypass, produced a significant increase of serum total cholesterol and phospholipids concentrations. The effect on semm lipids of the type of fat was similar in control and partial ileal bypass rabbits A Coconut—a main source of energy for two... 

Cellular uptake and degradation of LDL. AC AT 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. 

Isochromophilones VII (190) and VIII (191) from a Penicillium sp. inhibit diacylglycerol acytransferase activity with IC5o values of 20.0 and 127 pM and acyl-CoA cholesterol acyltransferase activity with IC50 values of 24.5 and 47.0, respectively. They also show moderate antimicrobial activity [153]. 

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. 

Biosynthesis of cholesteryl esters. The acyl-CoA cholesterol acyltransferase involved in cholesteryl ester synthesis is located on the cytosolic surface of liver endoplasmic reticulum. 

Arai N, Shiomi K, Tomoda H, Tabata N, Yang DJ, Masuma R, Kawakubo T, Omura S (1995) Isochromophilones III VI, Inhibitors of Acyl-CoA Cholesterol Acyltransferase Produced by Penicillium multicolor FO-3216. J Antibiot 48 696... 

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... 

Tomoda, H. Kim, Y. K. Nishida, H. Musuma, R. Omura, S. 1994. Pyripy-ropenes, novel inhibitors of acyl-CoA Cholesterol acyltransferase produced by Aspergillus fumigatus. I. Production, isolation, and biological properties. /. Antibiotics, 47,148-153. 

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... 

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

H Tomoda, H Nishida, YK Kim, R Obata, T Sunazuka, S Omura, J Bordner, M Guadllana, PG Dormer, AB Smith III. Relative and absolute stereochemistry of pyripyropene A, a potent, bioavailable inhibitor of acyl-CoA cholesterol acyltransferase. J Am Chem Soc 116 12097-12098, 1994. 

H Tomoda, H Nishida, R Masuma, J Cao, S Okuda, S Omura. Purpactins, new inhibitors of acyl-CoA cholesterol acyltransferase produced by Penicilliumpurpur-ogenum. I. Production, isolation and physico-chemical and biological properties. J Antibiot 44 136-143, 1991. 

S Omura, H Tomoda, YK Kim, H Nishida. Pyripyropenes, highly potent inhibitors of acyl-CoA cholesterol acyltransferase produced by Aspergillus fumigatus. J Antibiot 46 1168-1169, 1993. 

JF Reindel, MA Dominick, TM Bocan, AW Gough, EJ McGuire. Toxicologic effects of a novel acyl-CoA cholesterol acyltransferase inhibitor in cynomolgus monkeys. Toxicol Pathol 22 510-518, 1994. 

H Tomoda, XH Huang, J Cao, H Nishida, R Nagoa, S Okuda, H Tanaka, S Omura. Inhibition of acyl-CoA cholesterol acyltransferase activity by cyclodepsipeptide antibiotics. J Antibiot 45 1626-1632, 1992. 

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). 

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

Wilcox, L.J., Borradaile, N., Kurowska, E., Telford, D.E., and Huff, M.W. 1998. Naringenin, a citrus flavonoid, markedly decreases apoB secretion in HepG2 cells and inhibits acyl CoA cholesterol acyltransferase. Circulation 98, 531—537. 

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. 

Shand, J.H., West, D.W. 1991. Acyl-CoA Cholesterol acyltransferase activity in the rat mammary gland Variation during pregnancy and lactation. Lipids. 26, 150-154. 

---