Liver, lipoproteins and disease: II. Clinical relevance of disordered cholesterol metabolism in liver disease

The alterations in the concentration and composition of lipoproteins that occur in liver disease indicate the central role of the liver in lipoprotein metabolism. A number of studies have characterized plasma lipoproteins in patients with liver disease, although in most cases the underlying molecular defects responsible for the changes are still undetermined.     

Filtration of chylomicrons by the liver may influence cholesterol metabolism and atherosclerosis.

A fenestrated endothelial lining of sinusoids in rat liver has been shown to separate chylomicrons of different sizes following their injection into the portal vein. This sieving may have physiological importance, since during low dietary fat intake some intestinal lipoproteins are probably small enough to contact liver cells, but during high dietary fat loads most chylomicrons are too large to pass through the filter and must first be degraded to smaller remnants. The liver plays a central role in cholesterol metabolism since it catabolises dietary cholesterol which inhibits synthesis of cholesterol to be circulated as liver-derived very low density lipoproteins (VLDL) and low density lipoproteins. The sieving of chylomicrons, remnants and other lipoproteins by the sinusoidal endothelium of the liver may thus play an important role in lipid transport, affecting the balance of various lipoprotein moieties which in turn may affect the relationship of dietary lipids to various hyperlipidaemias and atherosclerosis.     

Lipoprotein physiology and its relationship to atherogenesis

The major plasma lipids, cholesterol, triglycerides, and phospholipids are transported as components of macromolecular complexes called lipoproteins. The major lipoprotein classes include the chylomicrons, which transport dietary lipids to the peripheral tissues and the liver; very low density and low density lipoproteins, which transport endogenously synthesized lipids from the liver to peripheral tissues; and high density lipoproteins, which appear to facilitate the reverse transport of cholesterol from peripheral tissues to the liver. The rates of synthesis and catabolism of the major lipoprotein classes are regulated, to a large degree, by one or more proteins, called apoproteins, that reside on the surface of the lipoproteins. This article describes normal lipoprotein metabolism and includes discussions of the role of abnormalities in lipoprotein transport in the atherogenic process.     

Effect of blood serum hormones and lipoproteins on levels of cAMP and cGMP in viable sections of rat liver

Cyclic nucleotides are universal intermediary agents of hormones in the target tissues, however mechanisms of the regulation of the cAMP and cGMP content in the cell are rather complex and still obscure. More data have appeared of late on the involvement of blood serum lipoproteins in the regulation of various intracellular processes including adenylate cyclase activity. Therefore the purpose of the study was to investigate the effect of adrenaline, hydrocortisone, glucagon, insulin and blood serum lipoproteins on the cAMP and cGMP content in rat liver surviving sections. An attempt was made to study a cooperative effect of the above hormones and lipoproteins of various classes. The results obtained have shown that adrenaline and glucagon raise the cAMP level in rat liver surviving sections. The effect of adrenaline is mediated by beta-adrenoreceptors. Insulin lowers an increases the level of cAMP in liver sections determined by the effect of glucagon. A decrease of the initial cAMP content in response to insulin occurs after a short lag period. High density lipoproteins (HDLP3) reduce the cAMP content in liver surviving sections. A cooperative effect of lipoproteins of a very low density and adrenaline (or hydrocortisone) in the regulation of the cAMP content in the rat liver was revealed. The cGMP content in rat liver surviving sections does not change under the influence of the above hormones and lipoproteins.     

Binding and uptake of native and modified low-density lipoproteins by human hepatocytes in primary culture

The binding and uptake of native low-density lipoproteins and malondialdehyde-treated low density lipoproteins by human hepatocytes in primary culture has been analyzed. Experiments with 125I-labeled malondialdehyde-treated low-density lipoproteins showed that cultured liver cells took up and degraded malondialdehyde-treated low-density lipoproteins, but the cell type(s) responsible for this action remain unclear. Immunofluorescent visualization of receptor-bound low-density lipoproteins revealed that low-density lipoprotein binding sites were distributed on the surface of nearly all cells of the culture. Binding sites for malondialdehyde-treated low-density lipoproteins were found in only 5% of the cultured cells, and these cells differed from hepatocytes in shape and size. Cultured hepatocytes internalized and native low-density lipoproteins, but not malondialdehyde-treated low-density lipoproteins, labeled with the fluorescent dye 3',3'-dioctadecylindocarbocyanine. About 15% of the cells that take up 3',3'-dioctadecylindocarbocyanine-labeled malondialdehyde-treated low-density lipoproteins could be identified as liver endothelial cells and macrophages, since they internalized formaldehyde-treated human albumin and fluorescent carboxylated microspheres. Our results indicate that human hepatocytes in primary culture express surface receptors for native low-density lipoproteins but not for modified low-density lipoproteins.     

Imaging of hepatic low density lipoprotein receptors by radionuclide scintiscanning in vivo.

The low density lipoprotein (LDL) receptor mediates the cellular uptake of plasma lipoproteins that are derived from very low density lipoproteins (VLDL). Most of the functional LDL receptors in the body are located in the liver. Here, we describe a radionuclide scintiscanning technique that permits the measurement of LDL receptors in the livers of intact rabbits. 123I-labeled VLDL were administered intravenously, and scintigraphic images of the liver and heart were obtained at intervals thereafter. In seven normal rabbits, radioactivity in the liver increased progressively between 1 and 20 min after injection, while radioactivity in the heart (reflecting that in plasma) decreased concomitantly. In Watanabe-heritable hyperlipidemic rabbits, which lack LDL receptors on a genetic basis, there was little uptake of 123I-labeled VLDL into the liver and little decrease in cardiac radioactivity during this interval. These findings demonstrate that the LDL receptor is necessary for the hepatic uptake of VLDL-derived lipoproteins in the rabbit. Two conditions that diminish hepatic LDL receptor activity, cholesterol-feeding and prolonged fasting, also reduced the uptake of 123I-labeled VLDL in the liver as measured by scintiscanning. The data suggest that radionuclide scintiscanning can be used as a noninvasive method to quantify the number of LDL receptors expressed in the liver in vivo.     

Plasma lipids and lipoproteins in liver disease.

There are many changes in the plasma, lipids, and lipoproteins in patients with liver disease. They have proved difficult to study but our understanding of these changes has increased greatly during recent years. In obstructive jaundice hyperlipidaemia is a fairly constant finding and this appears to be due to the regurgitation of phospholipid from the obstructed biliary tree. The plasma lipids tend to fall with parenchymal liver disease. The composition of the lipoproteins depends on the activity of the plasma enzyme lecithin: cholesterol acyl transferase. When LCAT activity is high the individual lipoprotein fractions are normal. When it is reduced all of the lipoprotein fractions are affected but the pattern found with obstruction is quite different from that found with parenchymal disease. The changes in plasma lipoproteins appear to be associated with change in the lipid composition of cellular membranes and this may have important functional implications.     

Plasma Lipoprotein Pattern in Relation to Liver Histology after Toxic Hepatitis and Experimental Biliary Obstruction in Rabbits

Plasma lipoproteins were studied in relation to liver histology in rabbits in the course of toxic hepatitis and compared to those after experimental biliary obstruction. The lipoprotein electrophoretic pattern became deeply abnormal during the acute phase of toxic hepatitis and correlated with the degree of liver injury, improving during recovery. Liver damage was more severe after carbon tetrachloride than after alcohol and milder after allylo-isopropyl-acetamide, a porphyrinogenic substance. Lipoprotein abnormalities were not followed by significantly reduced levels of cholesterol esters in the plasma. In comparison, animals with biliary obstruction developing milder liver damage presented gross abnormalities of plasma lipids and lipoproteins, followed by relative deficiency of cholesterol esterification. It is concluded that lipoprotein changes in acute liver injury, although nonspecific, are a sensitive index of liver damage and recovery. Serious acute liver injury can exist without significant fall in cholesterol esters.     

Cell-mediated lipoprotein transport: A novel anti-atherogenic concept

Lipoprotein transport is thought to occur in the plasma compartment of the blood, where lipoproteins are modulated by various enzymatic reactions. Subsequently, lipoproteins can migrate through the endothelial barrier to the subendothelial space or are taken up by the liver. The interaction between pro-atherogenic (apoB-containing) lipoproteins and blood cells (especially monocytes and macrophages) in the subendothelial space is well known. This lipoprotein-inflammatory cell interplay is central in the development of the atherosclerotic plaque. In this review, a novel interaction is described between lipoproteins and both leukocytes and erythrocytes in the blood compartment. This lipoprotein-blood cell interaction may also be related to the process of atherosclerosis by inducing inflammatory changes in the case of leukocytes (pro-atherogenic) and as an anti-atherogenic transport-system by adherence to erythrocytes. Triglyceride rich lipoprotein (TRL)-mediated leukocyte activation can lead to an inflammatory situation with generation of oxidative stress and the production of cytokines, ultimately resulting in acute endothelial dysfunction. Binding of apoB containing lipoproteins to erythrocytes may be a potential anti-atherogenic mechanism protecting the vessel wall from the pro-inflammatory effects of these lipoproteins and also playing a role in the removal of these particles from the circulation. One of the proposed mechanisms of this interaction implies complement activation on the lipoprotein surface and binding to the Complement Receptor 1 (CR1) on erythrocytes and leukocytes, followed by clearance by the liver.     

The biochemistry of lipoproteins

Lipids are transported in the blood in four major classes of lipoproteins. The triacylglycerol-rich lipoproteins are chylomicrons and very-low-density lipoproteins (VLDL) which are produced by the small intestine and liver, respectively. These lipoproteins mainly carry fatty acids to adipose tissue and muscle where the triacylglycerol is hydrolysed by lipoprotein lipase. The resulting particles that remain in the blood are chylomicron remnants and low-density lipoprotein (LDL), respectively. The remnant is taken up by the liver via endocytosis which is mediated by a specific receptor for apolipoprotein E (apoE). LDL, which are rich in cholesterol, can also be taken up by the liver or extrahepatic tissues by a receptor-mediated endocytosis that specifically recognises apoB or apoE. Nascent high-density lipoprotein (HDL) particles are secreted by the liver and intestine and then undergo modification to become HDL₃ and then HDL₂ as they acquire cholesterol ester. They facilitate the reverse transport of cholesterol back to the liver.Little is known of the hormonal regulation of lipoprotein uptake by the liver. Recently, we have shown that insulin and tri-iodothyronine (T₃) increase the specific binding of LDL to cultured hepatocytes whereas dexamethasone (a synthetic glucocorticoid) has the opposite effect. The changes in binding produced by insulin and dexamethasone are paralleled by alterations in the rate of degradation of apoB. These findings may in part explain the hypercholesterolaemia and increased risk of premature atherosclerosis that can be associated with poorly controlled diabetes or hypothyroidism.     

Metabolism of cholesteryl esters of very low density lipoproteins in the guinea pig.

Very low density lipoproteins from guinea pig plasma, endogenously labeled with 3H in both the esterified and free cholesterol moieties, were obtained from serum collected 20 hr after the intravenous injection of 3H-cholesterol into donor animals. When these lipoproteins were injected into recipient guinea pigs, the esterified 3H-cholesterol was rapidly cleared from the plasma; 24% was in the liver in 5 min and 54% in 15 min. A smaller fraction of the esterified cholesterol appeared in other plasma lipoprotein fractions, with 3H in the low density lipoproteins reaching a peak of 9%-18% of the injected esterified 3H-cholesterol between 30 and 60 min after the injection. The results indicate that most of the esterified cholesterol in very low density lipoproteins of guinea pig plasma is removed directly by the liver and a minor fraction is transferred to low density lipoproteins. The pattern of labeling of cholesteryl esters of high density lipoproteins in these experiments suggests that their low concentration in the guinea pig is accompanied by a rapid turnover rate.     

Plasma glycerol and hepatic synthesis of lipoproteins in the Zucker fa/fa rat

In order to determine if excess plasma glycerol originating in lipolysis increased the synthesis and secretion of lipoproteins in the liver of Zucker fa/fa rats, labelled (1-14C) glycerol was injected in vivo in the jugular vein, in the presence of (9,10-3H20 oleic acid and Triton WR 1339 to block plasma clearance of neoformed lipoproteins and to be secreted into the blood by the liver. Glycerol stimulated hepatic synthesis of lipids in the Zucker fa/fa rat. This resulted in an increased secretion by the liver of VLDL and lipoproteins comparable to chylomicrons.     

Abnormalities of Lipoprotein Levels in Liver Cirrhosis: Clinical Relevance

Progressive lipoprotein impairment occurs in liver cirrhosis and is associated with increased morbidity and mortality. The present review aims to summarize the current evidence regarding the prognostic value of lipoprotein abnormalities in liver cirrhosis and to address the need of a better prognostic stratification of patients, including lipoprotein profile assessment. Low levels of lipoproteins are usual in cirrhosis. Much evidence supports the prognostic role of hypolipidemia in cirrhotic patients. In particular, hypocholesterolemia represents an independent predictor of survival in cirrhosis. In cirrhotic patients, lipoprotein impairment is associated with several complications: infections, malnutrition, adrenal function, and spur cell anemia. Alterations of liver function are associated with modifications of circulating lipids. Decreased levels of lipoproteins significantly impact the survival of cirrhotic patients and play an important role in the pathogenesis of some cirrhosis-related complications.     

Accelerated clearance of low-density and high-density lipoproteins and retarded clearance of E apoprotein-containing lipoproteins from the plasma of rats after modification of lysine residues

Selective chemical modification of lysine residues of lipoproteins by acetoacetylation dramatically altered the metabolism of the lipoproteins without significantly altering other physical or chemical properties. Modification of 30-60% of the total lysine residues of iodinated rat or human low-density lipoproteins ((125)I-LDL) resulted in a rapid removal of these acetoacetylated lipoproteins from the plasma of rats. Within minutes after intravenous injection into intact rats, greater than 80% of the total injected dose disappeared from the plasma. The rapidly cleared acetoacetylated LDL appeared in the liver, and within 6-30 min as much as 50-80% of the total injected dose of modified LDL could be accounted for in the liver. Furthermore, it was possible to demonstrate in the isolated perfused rat liver that the Kupffer cells were responsible for the lipoprotein uptake. Human high-density lipoproteins (HDL(3)) were also rapidly removed from the plasma after acetoacetylation. In striking contrast, acetoacetylation (30-60%) of two E apoprotein-containing lipoproteins (rat HDL(1) and dog HDL(c)) retarded their removal from the plasma. The accelerated removal of modified LDL and HDL(3), in contrast to the retarded removal of modified HDL(1) and HDL(c), suggests that the recognition and removal process is specific for a property acquired by only certain lipoproteins after acetoacetylation. Moreover, these results suggest that lysine residues of the E apoprotein may play a functional role in the recognition process for the normal clearance of HDL(1) and HDL(c), a process that is interfered with after acetoacetylation.     

Liver, lipoproteins and disease: I. Biochemistry of lipoprotein metabolism

Cholesterol is a structural component of biological membranes and an immediate precursor for steroid hormones and bile acids. The liver is central to the production and removal of cholesterol-rich lipoproteins and bile acids. The basic biochemical aspects of hepatic lipoprotein and cholesterol metabolism and how abnormalities in liver function impair these metabolic pathways are reviewed.     

Lipoproteins containing apolipoprotein B-100 are secreted by the heart

It generally is assumed that lipoproteins containing apolipoprotein B (apo B) are secreted only by the intestine and the liver. However, we recently demonstrated that the human apo-B gene also is expressed in the hearts of human apo-B transgenic mice and in human heart tissue. Using metabolic labeling techniques, we showed that heart tissue from human apo-B transgenic mice and nontransgenic mice, as well as human heart tissue, synthesize and secrete apo-B-containing lipoproteins. The reason why the heart makes lipoproteins is unknown, but we hypothesized that the heart may use lipoprotein synthesis to unload surplus cellular lipids, particularly triglycerides, which are not immediately required for mitochondrial beta-oxidation.     

Secretion of cholesteryl ester-enriched very low density lipoproteins by the liver of cholesterol-fed rabbits

The output of lipids and lipoproteins by isolated perfused livers of normal-fed and cholesterol-fed rabbits has been examined. There was a comparable output of triglyceride by the livers of both groups, resulting in an accumulation of 40-50 mg triglyceride/liver/2 h in the perfusate in each case. The output of cholesteryl esters, however, was very much greater from the livers of cholesterol-fed (45 mg/liver/2 h) than from normal-fed (3.3 mg/liver/2 h) rabbits. The major lipoproteins in liver perfusates from both groups of animals were very low density lipoproteins (VLDL). In the perfusate of normal livers the VLDL were enriched with triglyceride and depleted of cholesteryl esters when compared with plasma VLDL from normal animals. VLDL in the perfusate of livers from cholesterol-fed rabbits, on the other hand, were markedly enriched with cholesteryl esters; cholesteryl esters accounted for 33% by mass of VLDL from cholesterol-fed livers and only 3.1% of VLDL from normal livers. The cholesteryl esters in the plasma lipoproteins of cholesterol-fed rabbits were relatively enriched with cholesteryl oleate when compared to those in normal plasma. Similarly, cholesteryl oleate predominated in the VLDL in the liver perfusate of the cholesterol-fed animals, consistent with an hepatic acyl CoA/cholesterol acyltransferase origin. Thus, cholesterol-feeding in the rabbit results in a marked increase in the hepatic secretion of cholesteryl esters as a component of VLDL.     

The lipemia of sepsis: triglyceride-rich lipoproteins as agents of innate immunity

Bacterial endotoxin (LPS) elicits dramatic responses in the host including elevated plasma lipid levels due to the increased synthesis and secretion of triglyceride (TG)-rich lipoproteins by the liver, and the inhibition of lipoprotein lipase. This cytokine-induced hyperlipoproteinemia, clinically termed the "lipemia of sepsis", was customarily thought to represent the mobilization of lipid stores to fuel the host response to infection. However, since lipoproteins can also bind and neutralize LPS, we hypothesize that TG-rich lipoproteins (VLDL and chylomicrons) are also components of an innate, non-adaptive host immune response to infection. Herein we review data demonstrating the capacity of lipoproteins to bind LPS, protect against LPS-induced toxicity, and modulate the overall host response to this bacterial toxin. Lastly, we propose a pathway whereby lipoprotein-bound LPS may represent a novel, endogenous mechanism for regulating the hepatic acute phase response.     

Regulation of plasma high-density lipoprotein levels by the ABCA1 transporter and the emerging role of high-density lipoprotein in the treatment of cardiovascular disease

High-density lipoproteins (HDL) protect against cardiovascular disease. HDL removes and transports excess cholesterol from peripheral cells to the liver for removal from the body. HDL also protects low-density lipoproteins (LDL) from oxidation and inhibits expression of adhesion molecules in endothelial cells, preventing monocyte movement into the vessel wall. The ABCA1 transporter regulates intracellular cholesterol levels in the liver and in peripheral cells by effluxing excess cholesterol to lipid-poor apoA-I to form nascent HDL, which is converted to mature alpha-HDL by esterification of cholesterol to cholesteryl esters (CE) by lecithin cholesterol acyltransferase. The hepatic ABCA1 transporter and apoA-I are major determinants of levels of plasma alpha-HDL cholesterol as well as poorly lipidated apoA-I, which interact with ABCA1 transporters on peripheral cells in the process of reverse cholesterol transport. Cholesterol in HDL is transported directly back to the liver by HDL or after transfer of CE by the cholesteryl ester transfer protein (CETP) by the apoB lipoproteins. Current approaches to increasing HDL to determine the efficacy of HDL in reducing atherosclerosis involve acute HDL therapy with infusions of apoA-I or apoA-I mimetic peptides and chronic long-term therapy with selective agents to increase HDL, including CETP inhibitors.     

Obstructive jaundice leads to accumulation of oxidized low density lipoprotein in human liver tissue

Oxidized low density lipoprotein (ox-LDL) molecule is one of the most important modified lipoproteins produced during the oxidative stress. Modified lipoproteins have been defined as being part of the immune inflammatory mechanisms in association with oxidant stress. We have reported the accumulation of ox-LDL in Balb/c mice liver after bile duct ligation previously. Here, we investigated this finding in human beings with obstructive jaundice. Our study demonstrates that obstructive jaundice results in tremendous accumulation of ox-LDL in the liver tissue of patients.