Liver cholesterol ester metabolism

Scheme 4.21 Blocking of unproductive metabolic pathways by fluorination as a design tool for an orally active inhibitor of cholesterol absorption [53a]. The result of this rational approach (SCH 58235) is 50 times more active than the conceptual starting compound SCH 48461. (ED50 refers to reduction of liver cholesterol esters in hamsters).

The purification and properties of LCAT, together with a discussion of its mechanism of reaction are given by Marcel (1982). A number of disease states involve LCAT activity. Familial LCAT deficiency has been described (Glomset and Norum, 1973) and patients with this rare complaint have been thoroughly investigated. Many of the abnormalities seem in such patients have been found in those with cholestasis also. A discussion of cholesterol ester metabolism in relation to other liver diseases and dyslipoproteinaemia has been reported (Marcel, 1982). Similarly, the metabolism of cholesterol esters in relation to arteries and arterial disease has been fully discussed (Kritchevsky and Kothari, 1978). Mammalian steroid sulphates have been reviewed by Farooqui (1981). 

CI-976 was synthesized as a fatty acid anilide derivative designed to mimic fatty acyl-CoA, the fatty acid donor for ACAT enzymes, and it has been most extensively studied in this regard as a competitive ACAT inhibitor (Field et al., 1991 Roth et al., 1992). Various animal studies have shown that CI-976 lowers plasma low density Upoprotein (LDL)-cholester-ol and raises high density lipoprotein-cholesterol by inhibiting both hver and intestinal ACAT activities CI-976 also lowers liver cholesterol esters (CE) and decreases CE secretion (Carr et al., 1995 Krause et al., 1993). The metabolic fate of CI-976 has been studied in both whole animals and isolated hepatocytes, and it is oxidized to numerous metabolites, likely by cytochrome P450 pathways (Sinz et al., 1997). The biological activities of these metabolites are unknown. 

FIGURE 9. Endogenous lipoprotein metabolism. In liver cells, cholesterol and triglycerides are packaged into VLDL particles and exported into blood where VLDL is converted to IDL. Intermediate-density lipoprotein can be either cleared by hepatic LDL receptors or further metabolized to LDL. LDL can be cleared by hepatic LDL receptors or can enter the arterial wall, contributing to atherosclerosis. Acetyl CoA, acetyl coenzyme A Apo, apolipoprotein C, cholesterol CE, cholesterol ester FA, fatty acid HL, hepatic lipase HMG CoA, 3-hydroxy-3-methyglutaryl coenzyme A IDL, intermediate-density lipoprotein LCAT, lecithin-cholesterol acyltransferase LDL, low-density lipoprotein LPL, lipoprotein lipase VLDL, very low-density lipoprotein. 

Thioesters play a paramount biochemical role in the metabolism of fatty acids and lipids. Indeed, fatty acyl-coenzyme A thioesters are pivotal in fatty acid anabolism and catabolism, in protein acylation, and in the synthesis of triacylglycerols, phospholipids and cholesterol esters [145], It is in these reactions that the peculiar reactivity of thioesters is of such significance. Many hydrolases, and mainly mitochondrial thiolester hydrolases (EC 3.1.2), are able to cleave thioesters. In addition, cholinesterases and carboxylesterases show some activity, but this is not a constant property of these enzymes since, for example, carboxylesterases from human monocytes were found to be inactive toward some endogenous thioesters [35] [146], In contrast, allococaine benzoyl thioester was found to be a good substrate of pig liver esterase, human and mouse butyrylcholinesterase, and mouse acetylcholinesterase [147],... 

Chylomicrons and VLDL are primarily triglyceride particles, although they each have small quantities of cholesterol esters. Chylomicrons transport dietary trig lyceride to adipose tissue and muscle, whereas VLDL transport triglyceride synthesized in the liver to these same tissues. Both chylomicrons and VLDL have apoC-II, apoE, and apoB (apoB-48 on chylomicrons and apoB-IOO on VLDL). The metabolism of these particles is shown in Figure H5-5. 

Cholesterol can be derived from two sources—food or endogenous synthesis from ace-tyl-CoA. A substantial percentage of endogenous cholesterol synthesis takes place in the liver. Some cholesterol is required for the synthesis of bile acids (see p. 314). In addition, it serves as a building block for cell membranes (see p. 216), or can be esterified with fatty acids and stored in lipid droplets. The rest is released together into the blood in the form of lipoprotein complexes (VLDLs) and supplies other tissues. The liver also contributes to the cholesterol metabolism by taking up from the blood and breaking down lipoproteins that contain cholesterol and cholesterol esters (HDLs, IDLs, LDLs see p.278). 

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...

The lipidome profile of mice liver homogenates of free cholesterol, low cholesterol, and high cholesterol diets showed the influence between dietary cholesterol intake and atherosclerosis (17). To get individual metabolite fingerprints, they measured near 300 metabolites such as di- and triglycerides, phosphatidylcholines, LPCs, and cholesterol esters in plasma samples by LC-MS/MS. It was observed that when dietary cholesterol intake was increased, the liver compensated for elevations in plasma cholesterol by adjusting metabolic and transport processes related to lipid metabolism, which... 

It is, of course, also possible that the esterified cholesterol formed in HDL may be removed from plasma by some process other than uptake of the whole HDL particle or LTP-I-mediated transfer to other lipoprotein particles, but this possibility has not been fully investigated. Some evidence that there may be other pathways than these for the removal from plasma of HDL esterified cholesterol comes from the studies of Class et al. (G5, G6), who showed that cholesteryl ether incorporated in rat HDL as a tracer for cholesteryl ester was taken up in vivo by the rat liver (and by other organs) fester than apoA-I tracer (see Section 4.1.2). These studies are complicated by the relatively high concentration of apoE in rat HDL (compared, for instance, to man) and the unknown effect of apoE on HDL cholesteryl ester metabolism in the rat. Further studies on the removal of esterified cholester-... 

The liver synthesizes two enzymes involved in intra-plasmic lipid metabolism hepatic triglyceride lipase (HTL) and lecithin-cholesterol-acyltransferase (LCAT). The liver is further involved in the modification of circulatory lipoproteins as the site of synthesis for cholesterol-ester transfer protein (CETP). Free fatty acids are in general potentially toxic to the liver cell. Therefore they are immobilized by being bound to the intrinsic hepatic fatty acid-binding protein (hFABP) in the cytosol. The activity of this protein is stimulated by oestrogens and inhibited by testosterone. Peripheral lipoprotein lipase (LPL), which is required for the regulation of lipid metabolism, is synthesized in the endothelial cells (mainly in the fatty tissue and musculature). 

In liver cirrhosis, there are several changes reduction in LCAT (with cholesterol ester fall ), HDL and LDL as well as in VLDL, with a corresponding change in their distribution pattern occurrence of hypertriglycerid-aemia and atypical lipoproteins reduction in phospholipid synthesis (28), possibly with greatly impaired structure and function of the biomembranes. Hepatic extraction of bile acids is reduced with the result that they reach the peripheral circulation - even in the early stages of cirrhosis - and give rise to increased serum values. Bile acids have cholestatic and cytotoxic effects. When bile acid metabolism is markedly compromised, enteral absorption of fat-soluble vitamins is impeded, so that A, D, E and K hypovitaminoses may be observed. 

A cholesterol ester forms In the liver of rats given an oral dose (250 mg/kg for seven days) of the hypolipidemic drug l-(4-carboxyphenoxy)-10-(4-chlorophenoxy)decane ( 2J ). This xenobiotic cholesterol ester represented about 11% of the total lipid In the liver and was neither further metabolized nor transported by lipoproteins. An additional 1% of the total liver lipids consisted of hybrid trlacylglycerols containing this xenobiotic. The authors of this work suggest that the hypocholesterolemlc activity of the drug In rats results from hepatic accumulation of the xenobiotic cholesterol ester which appears to promote the hydrolysis of natural cholesterol esters and thereby facilitate clearance of low-density lipoproteins. 

The pyrethroid insecticide fenvalerate, (a-cyano-3-phenoxybenzyl-2-(4-chlorophenyl)isovalerate, contains two centers of chirality in its structure (designated as the 2 and a positions Fig. 19) and therefore four stereoisomers, two pairs of enantiomers are possible. Initial evaluation of the mixture, by addition to the diet of a number of species, resulted in granulomatous changes in the liver, lymph nodes, and spleen. Separation and evaluation of the individual stereoisomers indicated that the toxidty was associated with one of the four, the 2i ,a5-stereoisomer, and subsequent metabolic studies found the cause to be associated with the formation and disposition of a cholesterol ester of (i )-2-(4-chlorophenyl)isovalerate (Fig. 19). A metabolic transformation shown to be stereospedfic in mice, only the 2i ,a5-stereoisomer yielding the ester both in vitro and in vivo [159]. 

Dietary pectin affects lipid metabolism, especially that of cholesterol. One of the explanations proposed to explain an action of pectin on cholesterol metabolism is through its ability to bind bile acids and bile salts. However, pectin also has the property of forming a gel in water. This gel lowers the intestinal absorption of cholesterol and thereby decreases liver cholesterol. Recently, evidence has been obtained that the presence of pectin in a cholesterol diet increases the excretion of cholesterol esters. Results from the administration of cholesterol-4-l C in the diet showed that the presence of pectin slows gastric emptying and results in more labeled cholesterol as well as cholesterol esters in all segments of the gut. 

Much information is available about the metabolism of chylomicron cholesteryl esters taken up by the liver in association with the chylomicron remnant. This information may be relevant to the issue of chylomicron retinyl ester metabolism in the liver, about which much less direct information is on hand. Hepatic uptake of chylomicron cholesteryl esters occurs without hydrolysis of the cholesteryl esters (Goodman, 1965 (Juarfordt and Goodman, 1967 Stein et al., 1969). In studies with chylomicrons containing doubly labeled cholesteryl esters injected intravenously into rats, Quarfordt and Goodman (1967) observed that 80-90% of the chylomicron cholesteryl esters were removed by the liver without hydrolysis. In the liver, the newly absorbed cholesteryl esters underwent slow but extensive hydrolysis, to the extent of about 60% after 1 h and about 85-90% after 3.5 h. Subsequent to hydrolysis, most of the labeled free cholesterol slowly left the liver and was extensively redistributed in die body. Thus, 24 h later, only 20-28% of the labeled cholesterol found in the entire animal body was present in the liver. Since newly absorbed retinol, which is retained in the liver, is only mobilized slowly (see below), it is clear that following ester hydrolysis the hepatic metabolism of chylomicron cholesterol and retinol diverge in a major way. 

The role of linoleate in cholesterol deposition and transport is not entirely clear. Kelsey and Longenecker (1941) proved that 62% of the plasma cholesterol of cattle occurred in combination with linoleate. It is only natural to postulate that, in the absence of EFA, cholesterol is deposited in the liver, because there is insufficient linoleate available to transport it to other tissues for metabolism and excretion. However, it has been shown that, in such conditions, the increased cholesterol is deposited in the liver as an ester. The cholesterol esters in the liver of rats have been proved to consist almost entirely of those of saturated and oleic acids only approximately 10% of the cholesterol occurs in combination with linoleic acid, irrespective of whether or not the diet contains EFA (Achaya et al., 1954a). It would thus appear that Unoleic acid is of prime importance in the control of the distribution and deposition of cholesterol in the rat. Whether or not the same situation obtains in the case of man is a moot question. 

The second enzyme which is intimately concerned with the metabolism of cholesterol is acyl-CoA choIesterol acyltransferase, or ACAT. ACAT is a membrane-bound, microsomal enzyme that catalyzes the formation of iong chain fatty-acyl cholesterol esters in rat liver and in other tissues. This enzyme is important in regulating the concentration of unesterified cholesterol in the cell and, thus, it provides a mechcinism for the removal of a potentially hcirmful excess of unesterified cholesterol. I will now present briefly the evidence that the short-term... 

The role of the liver in the metabolism of chylomicron cholesterol ester is even more extensive than in the metabolism of triglycerides, as only about 20% of the chylomicron triglyceride (Ontko and Zil-versmit, 1967), but more than 70% of the cholesterol ester is taken up by the liver (Quarfordt and Goodman, 1967). 

The liver plays a decisive role in the cholesterol metabolism. The liver accounts for 90% of the overall endogenic cholesterol and its esters the liver is also impli-cated in the biliary secretion of cholesterol and in the distribution of cholesterol among other organs, since the liver is responsible for the synthesis of apoproteins for pre-p-lipoproteins, a-lipoproteins, and P-lipoproteins which transport the secreted cholesterol in the blood. In part, cholesterol is decomposed by intestinal micro-flora however, its major part is reduced to coprostanol and cholestanol which, together with a small amount of nonconverted cholesterol, are excreted in the feces. 

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