Liver cholesterol esterification

Simon, J. B., and Scheig, R., Serum cholesterol esterification in liver disease. Importance of lecithin-cholesterol acyltransferase. New Engl. J. Med. 283,841-846 (1970). 

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... 

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...

P23. Poorthuis, B. J. H. M., and Wirtz, K. W. A., Increased cholesterol esterification in rat liver microsomes by purified non-specific phospholipid transfer protein. Biochim. Biophys. Acta 710, 99-105 (1982). 

Triglycerides, along with other lipids and cholesterol, are derived mainly from the diet and transported from the intestine in the form of chylomicrons. Triglycerides are also formed in the liver by esterification of fatty acyl-CoAs with glycerol-3-phosphate. They act as a source of energy and a source of transporting energy from the intestine and liver to the peripheries. 

The CoA-fortlfled microsomal enzyme system was inhibited by SKF 525-A (B-diethylaminoethyl-diphenylpropyl acetate) and by 2,4-dlchloro-6-phenylphenoxyethylamine HBr (DPEA) in a dose-dependent fashion (28). Both are known inhibitors of the classical hepatic microsomal mixed-function oxidase system. This inhibition has been suggested to Involve a nonspecific binding to liver microsomal proteins and/or phospholipids (18.19). Binding to phospholipids would support the hypothesis proposed by Swell and co-workers (10) that fatty acyl CoA derivatives were not necessarily the only immediate source for the fatty acid moieties of the conjugates that phospholipids may act as the fatty acids reservoir in the CoA-fortified enzyme system. However, it is also known that cholesterol esterification is very sensitive to the fluidity of the microsomal membrane (12). The reduced conjugation produced when... 

There are conflicting data on whether the availability of cholesterol and/or cholesteryl esters directly influences apo B secretion. Several studies have suggested that cholesterol supply can regulate VLDL secretion. For example, VLDL production in animals and man is decreased by statin treatment, and inhibition of cholesterol synthesis by a statin, an inhibitor of the rate-limiting step of cholesterol biosynthesis (Chapter 14), reduced VLDL secretion in perfused rat livers (M. Heimberg, 1990) and primary hepatocytes. However, this effect of statins can perhaps be ascribed to increased expression of LDL receptors rather than to a reduction in cholesterol synthesis (Section 7.1). Depletion of cholesterol in rodent hepatocytes by the ABCAl-dependent lipidation of apo A1 (Chapter 19) also decreases VLDL secretion (R. Lehner, 2004). Furthermore, the secretion of apo BlOO-containing VLDLs is increased in primary hepatocytes derived from Niemann-Pick Cl-deficient mice. Niemann-Pick Cl-deficiency causes a severe defect in trafficking of unesterified cholesterol out of the lysosomal/endosomal pathway and consequently, Niemann-Pick Cl-deficient hepatocytes accumulate 5- to 10-fold more unesterified cholesterol than do wild-type hepatocytes. In hepatocytes from Niemann-Pick Cl-deficient mice, cholesterol synthesis is increased and the rate of cholesterol esterification and the amount of the transcriptionally active form of SREBP-1 are also increased (J.E. Vance, 2007). However, because of multiple alterations in lipid metabolism in these hepatocytes, increased VLDL secretion cannot be attributed specifically to increased synthesis of cholesterol or cholesteryl esters. 

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. 

Cholesterol is the major sterol in the body and occurs mainly as the nonesterified free form, which is a fundamental component of cell membranes and the precursor for steroid hormones and bile acids. Cholesteryl esters present in the tissues and plasma are mainly formed by cholesterol esterification with long chain fatty acids these cholesterol esters act as a storage pool. Most of the requirements for cholesterol are met by endogenous synthesis, mainly in the liver, with the exogenous supplementation from the diet. 

Ferrous ion-induced Hpid peroxidation of rat liver mitochondria was accelerated by phosphate (Yamamoto et al. 1974). Preincubation of rat liver microsomes with iron (Fe)/ascorbate (50 pM/ 200 pM), known to induce peroxidation, resulted in a significant inhibition of (i) the rate-limiting enzyme in cholesterol biosynthesis, HMG-CoA reductase (46 %, P <0.01, (ii) the crucial enzyme control-Hng the conversion of cholesterol in bile acids, cholesterol 7a-hydroxylase (48%, P <0.001), and (iii) the central enzyme for cholesterol esterification, acyl-CoAxholesterol acyltransferase (ACAT, 80%, P <0.0001) (Brunet etal. 2000). The disturbances of these key enzymes coincided with a high rate of malondialdehyde production (350%, P <0.007) and the loss of polyunsaturated fatty adds (36.19 1.06% vs. 44.24 0.41% in controls, P <0.0008). While a-tocopherol simultaneously neutrahsed lipid peroxidation, preserved microsomal fatty acid status, and restored ACAT activity, it was not effective in preventing Fe/ascorbate-induced inactivation of both HMG-CoA reductase (44%, P <0.01) and cholesterol 7a-hydroxylase (71%, P< 0.0001). 

In addition, cholesterol accumulation in macrophages, mediated by modified lipoproteins (e. g., acetylated low-density lipoprotein, AcLDL), stimulates these cells to synthesize and secrete apolipoprotein (Apo) E/phospholipid discs. During the intraplasmatic cholesterol esterification process mediated by lecithinxholesterol acyltransferase (LCAT), HDL are assumed to incorporate unesterified cholesterol from the cell surface and Apo E from secreted Apo E/phospholipid discs and thereby mediate reverse cholesterol transport from peripheral cells back to the liver. The resulting cholesteryl ester- and Apo E-enriched HDLi are transported to the liver where they may be recognized by a hepatic Apo E receptor. 

PPARs enhance cholesterol efflux and stimulate critical steps of the reverse cholesterol transport pathway (reviewed in ref. 507). PUFA and activating PPAR increase hepatic cholesterol uptake. PPARy induces expression of SR B1 in rat hepatocytes, liver EC, and Kuppfer cells (508). PPARa activation in human macrophages and foam cells results in an enhanced availability of free cholesterol for efflux through the ABCAl pathway by reducing cholesterol esterification rates and ACATl activity (509). 

Cholesterol esterification in liver (Goodman et al., 1964 Erickson and Cooper, 1980) as well as in many other types of tissues (see Spector et al., 1979 Erickson and Cooper, 1980, for references) is catalyzed by the enzyme acyl-CoA cholester-ol acyltransferase (EC 2.3.1.26 ACAT), which reacts free cholesterol with a fatty acyl-CoA to form cholesteryl ester. This membrane-bound enzyme is found mainly in liver microsomes and has been partially characterized. 

Cholesterol esterification (and hydrolysis) takes place at a number of other sites, e.g. liver, pancreas, small intestine, skin, aorta,15 16 ovaries (corpora lutea and interstitial tissuel7) and adipose tissue.18 Liver and arterial tissue are of significance in atherogenesis since both are involved in the uptake and degradation of plasma cholesterol esters. 

Although sterols are present in most mammalian body tissues, the proportion of sterol ester to free sterol varies markedly. For example, blood plasma, especially that of Man, is rich in sterols and like most plasma lipids they are almost entirely found as components of the lipoproteins about 60-80% of this sterol is esterified. In the adrenals, too, where cholesterol is an important precursor of the steroid hormones, over 80% of the sterol is esterified. However, in brain and other nervous tissues, where cholesterol is a majof component of myelin, virtually no cholesterol esters are present. Cholesterol esters are formed by the action of a microsomal acyl-CoA cholesterol acyl transferase (AC AT) which is present in most cells. Under normal conditions the enzyme is considered rate-limiting for cholesterol esterification. It is regulated by progesterone and may be modulated by phosphorylation/dephosphorylation like HMG-CoA reductase. Under conditions where cells take up a large amount of cholesterol, such as via LDL receptors, ACAT is induced. The enzyme is particularly important in intestine and is relatively low in liver where the lipoproteins made for secretion into the serum contain little if any cholesterol ester. 

The use of LDL and other lipoproteins in drug targeting has been reviewed [170,172], Damle et al. [173] have shown that radiopharmaceuticals, such as iopanoic acid, a cholecystographic agent, could be incorporated in chylomicron remnants by esterification with cholesterol and used for liver imaging. About 87% of the chylomicron remnant-loaded iopanoic acid accumulated in the liver within 0.5 hour after administration, compared with 31% accumulated using a... 

HDL particles extract cholesterol from peripheral membranes and, after esterification of cholesterol to a fatty acid, the cholesteryl esters are delivered to the liver (to make bile salts) or steroidogenic tissues (precursor of steroids). 

Esterification of cholesterol When cholesterol is taken up by HDL, it is immediately esterified by the plasma enzyme phos-phatidylcholine cholesterol acyltransferase (PCAT, also known as LCAT, in which "L" stands for lecithin). This enzyme is synthesized by the liver. PCAT binds to nascent HDLs, and is activated by apo A-l. PCAT transfers the fatty acid from carbon 2 of phosphatidyl-... 

The metabolism of HDL probably involves interaction with both hepatic and peripheral cells, as well as with other lipoproteins. HDL may remove cholesterol from tissues, the "scavenger hypothesis (11,12). The cholesterol may then be esterifed by the action of lecithin cholesterol acyl transferase. HDL may provide cholesterol to the liver for bile acid synthesis (13) and some HDL may be catabolized by the liver in the process. HDL has not been found to interfere with the binding of LDL in cultured human fibroblasts (6). However, in cultured human arterial cells, porcine or rat hepatocytes, and rat adrenal gland, there appears to be some competition of HDL with LDL binding sites, suggesting the presence of a "lipoprotein-binding" site (14). 

Studies by Bell and coworkers have shown that DEHP also can alter sterologenesis in rodents, which may have an impact on steroid-dependent functions, such as reproductive functions. For example, feeding female rats DEHP at an estimated dose of 500 mg/kg/day for 13 days significantly inhibited sterologenesis from 14C-mevalonate in liver and adrenal minces (Bell 1980). DEHP also inhibited cholesterol synthesis in the liver from male rats and rabbits as well as in rats testes (Bell 1982). In a subsequent study, Bell and Buthala (1983) demonstrated that the inhibition of cholesterol synthesis in the liver was due to a reduction in the activity of microsomal acylCoA cholesterol acyltransferase, an enzyme responsible for the esterification of cholesterol. 

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. 

ACAT transfers amino-acyl groups from one molecule to another. ACAT is an important enzyme in bile acid synthesis, and catalyses the intracellular esterification of cholesterol and formation of cholesteryl esters. ACAT-mediated esterification of cholesterol limits its solubility in the cell membrane and thus promotes accumulation of cholesterol ester in the fat droplets within the cytoplasm this process is important in preventing the toxic accumulation of free cholesterol that would otherwise damage ceU-membrane structure and function. Most of the cholesterol absorbed during intestinal transport undergoes ACAT-mediated esterification before incorporation into chylomicrons. In the liver, ACAT-mediated esterification of cholesterol is involved in the production and release of apo-B-containing lipoproteins. 

The ll-palm-A -THC can be hydrolyzed to II-OH-A -thc by cholesterol esterase and triacylglycerol lipase but not by phospholipase A, acetylesterase or phosphotransacetylase (16). An attempt to modify the retention of fatty acid-conjugated DDT metabolites was carried out by injecting the DDT-treated rats with sodium salt of various bile acids, heparin or lecithin of which all were known to affect the esterification or ester hydrolysis by the cholesterol esterase system. The results Indicated a significant decrease in the retention of the conjugated DDT metabolites in the rat liver and spleen (17). 

---