Detection of lipid transfer protein activity in rabbit liver perfusate

CETP activity in liver perfusates and plasma from rabbits hypo- or hyperresponsive to dietary cholesterol

We investigated the relationship between the development of hypercholesterolemia in rabbits and cholesteryl ester transfer protein (CETP) activity secretion by their perfused livers. Two inbred strains of rabbits were compared which differ markedly in their hypercholesterolemic response to dietary cholesterol. Feeding a high-cholesterol (0.3%) diet, increased plasma and liver cholesterol level in the two strains, the increments being 15 mM and 30 mumol/g greater in the hyperresponders, respectively. The high-cholesterol diet caused an about 2-fold increased hepatic secretion of CETP activity, but there was no difference between the two rabbit strains. Feeding a lower amount of dietary cholesterol (0.08%) also caused higher cholesterolemic (2 mM) and hepatocholesterolic (28 mumol/g) responses in hyper- than in hyporesponsive rabbits. The activity of hepatic CETP secretion was not increased by the low-cholesterol diet, and there was no difference between hypo- and hyperresponsive rabbits. Cholesterol feeding increased plasma CETP activity by 90% in both rabbit strains, but there was no difference between the strains. Our combined data suggest that with increasing plasma cholesterol levels hepatic CETP secretion may be increased in a parabolic manner, reaching its maximum rate for before plasma cholesterol concentrations are maximal. There were no differences in hepatic CETP activity secretion of plasma CETP activity levels between the genetically different strains of hypo- and hyperresponsive rabbits.

Cholesteryl ester transfer activity in liver disease and cholestasis, and its relation with fatty acid composition of lipoprotein lipids

Liver disease is accompanied by major qualitative and quantitative disturbances in plasma lipoprotein metabolism, the extent and intensity of which depend on the degree of parenchymal damage, cholestasis, or both. The main objective of this study was to determine the cholesteryl ester transfer CETP activity and its association with the lipoprotein neutral lipid composition in patients with either liver cirrhosis or cholestasis, as compared to normal controls. Lipoproteins were isolated by ultracentrifugation, lipids and apolipoproteins were measured by conventional methods, and the fatty acid composition was established by gas chromatography; CETP activity in lipoprotein-deficient plasma was measured by determining the transfer of [3H]cholesteryl esters from HDL to VLDL. Lipoprotein lipase and hepatic lipase activities were measured in post-heparin plasma by radiochemical methods. In patients with liver cirrhosis, low levels of VLDL, HDL, apo B, and Lp(a) were observed, as well as a change in the composition of HDL particles, with increases in the relative proportion of triglyceride and free cholesterol. Respectively, the last two changes could be attributed in part to the low hepatic lipase activity observed in this study, and to the low lecithin:cholesterol acyltransferase activity previously observed by others. In patients with cholestasis, a moderate hyperlipidemia due to the elevation of LDL was found. In contrast, HDL and apo A-I levels were very low reflecting a low number of HDL particles, which also had altered compositions with increases in the triglyceride and free cholesterol contents relative to apo A-I and esterified cholesterol, respectively. As regards the fatty acid composition of lipoprotein lipids, the two groups of patients showed, in general, a lower proportion of linoleic acid and a compensating higher proportion of oleic acid as compared to the controls, changes that were observed in both cholesteryl esters and triglycerides. In contrast, the proportions of oleic and palmitoleic acids in phospholipids were increased, whereas that of stearic acid was decreased in patients as compared to controls. In patients with liver cirrhosis, as well as in controls, no changes were observed in the fatty acid compositions of cholesteryl ester, triglycerides, or phospholipids among the different lipoproteins, which probably reflects the equilibration reached by the action of CETP. In patients with cholestasis, no differences were observed in fatty acid composition among the lipoprotein phospholipids but, interestingly, cholesteryl esters from VLDL had a significantly lower linoleic acid content than those from HDL, whereas triglycerides from VLDL had significantly higher oleic acid and lower linoleic acid contents than those from HDL. This distinct fatty acid composition of the neutral lipids between lipoproteins was associated with a significant decrease (25%) in the cholesteryl ester transfer activity in patients with cholestasis. We suggest that fat malabsorption due to the biliary defect may induce a decrease in cholesteryl ester transfer protein synthesis or section, which in turn would slow the equilibration of the neutral lipids among plasma lipoproteins.

Ethanol-induced redistribution of cholesteryl ester transfer protein (CETP) between lipoproteins

Since alcohol drinking reduces the concentration and activity of plasma cholesteryl ester transfer protein (CETP), we investigated the effects of alcohol on its synthesis and secretion by perfusing rabbit livers for 4 hours in the absence or presence of ethanol. The quantity of CETP mRNA in the perfused livers did not differ between the control and ethanol (25 mmol/L or 50 mmol/L) perfusions. CETP activity was determined by incubating [3H]cholesteryl ester-labeled human LDL and unlabeled human HDL with the perfusion medium after removing the endogenous VLDL (secreted by the perfused liver) by ultracentrifugation. CETP activity in the perfusion medium increased at a linear rate that was not affected by ethanol. When the VLDL was removed by precipitation with polyethylene glycol or a heparin-Sepharose column instead of ultracentrifugation, practically no CETP activity was detected in the ethanol perfusions, whereas these procedures did not affect CETP activity in the control perfusions. Inhibition of ethanol oxidation by 4-methylpyrazole resulted in CETP activity similar to that of the controls. We conclude that ethanol does not affect the synthesis or secretion of CETP, but its oxidation may alter the distribution of CETP in lipoproteins. CETP seems to be present in VLDL as well as in HDL, and since VLDL is more rapidly catabolized with HDL, this may explain the low plasma CETP concentration associated with alcohol consumption.

Cholesteryl ester transfer protein: its role in plasma lipid transport

1. The cholesteryl ester transfer protein (CETP) is a hydrophobic glycoprotein which acts in plasma to redistribute cholesteryl esters and triglyceride between plasma lipoproteins. 2. CETP also plays an important role in determining the composition and particle size distribution of high density lipoproteins (HDL). 3. Activity of CETP may be regulated in four ways: By factors which influence the concentration of CETP in plasma; by the activity of CETP inhibitor proteins; by variations in the concentrations and compositions of donor and acceptor lipoproteins and by factors which influence the interaction of CETP with plasma lipoproteins. 4. The mechanism of action of CETP is uncertain. Two models have been proposed: (i) a shuttle model in which CETP physically transports lipids between lipoprotein particles and (ii) a ternary complex model in which CETP forms a bridge between two lipoprotein particles, enabling them to exchange lipids. 5. Evidence is accumulating that CETP may be a pro-atherogenic factor.

Lecithin-cholesterol acyltransferase and lipid transfer protein activities in liver disease

The activities of lecithin-cholesterol acyltransferase (LCAT) and lipid transfer protein (LTP) were assayed using sensitive radioassay methods in controls (n = 113) and in patients with various liver diseases (n = 72). Plasma LCAT activity decreased with progression of hepatocellular damage. Plasma LTP activity in controls was 216 +/- 68 nmol/mL/h, and there were no significant differences between controls and patients with chronic hepatitis ([CH], 193 +/- 70), compensated liver cirrhosis (LC) with or without hepatocellular carcinoma ([HCC], 197 +/- 48 and 193 +/- 62, respectively), or decompensated liver cirrhosis ([dLC], 182 +/- 65). In acute viral hepatitis, LTP activity decreased significantly; however, the degree of reduction was not as dramatic as that for LCAT. There was no correlation between LCAT and LTP activity both in controls and patients with various liver diseases. LCAT activity was positively correlated with serum albumin (r = .52, P < 0.1) and cholinesterase (r = .37, P < .01) levels, and inversely correlated with serum bilirubin level (r = -.38, P < 0.1); there was no correlation between plasma LTP activity and these parameters of liver function. That plasma LTP activity did not change with hepatocellular damage may indicate that the liver in humans may not be the primary site of LTP production.

Dyslipoproteinaemia of liver disease

Hepatic diseases differ from most other causes of secondary dyslipidaemia in that the circulating lipoproteins are not only present in abnormal amounts but they frequently also have abnormal composition, electrophoretic mobility and appearance. Pre-beta and alpha bands can be absent on electrophoresis in all types of liver disease although material in the VLDL and HDL ranges can be isolated in the ultracentrifuge. Cholestatic liver disease has been the most extensively studied and the hyperlipidaemia can be extreme with marked elevations of free cholesterol and phospholipids. This results largely from the presence of LP-X, an abnormal LDL, with a vesicular structure that appears in rouleaux formation under the electron microscope. It is virtually specific for cholestasis and familial LCAT deficiency. The LDL, however, is heterogeneous and may also contain a large triglyceride-rich particle (LP-Y) as well as more normal-looking particles, which are none the less depleted in cholesteryl esters and rich in triglycerides. Indeed, when patients with cholestasis are hypertriglyceridaemic the excess triglyceride is to be found predominantly in these two LDL fractions rather than in VLDL. HDL in cholestasis may contain disc-like particles, similar to those newly secreted by the liver and intestine, as well as more normal-looking spherical particles. In extrahepatic obstruction concentrations of HDL and its major apolipoproteins, apoAI and apoAII, are frequently reduced, although a subfraction rich in apoE is often found. In all but the latest stages of chronic intrahepatic cholestasis due to primary biliary cirrhosis, however, HDL, especially HDL2, concentrations are increased, probably due to the presence of a circulating inhibitor of HL. Many of these lipoprotein changes found in cholestasis resemble those of familial LCAT deficiency, although the hyperlipidaemia is not usually so severe in the latter condition. Indeed, in patients with cholestasis but well-preserved LCAT activity many of the characteristic lipoprotein changes, such as LP-X, LP-Y and discoidal HDL, may not be seen. In acute hepatocellular disease, such as alcoholic or viral hepatitis, it is not unusual for the patient to go through a cholestatic phase and many of the same lipoprotein changes may be seen. In cirrhosis without cholestasis the patients are not usually significantly hyperlipidaemic and in advanced cases cholesterol and apoB levels may be reduced. Although LCAT activity and the proportion of plasma cholesterol esterified may also be markedly reduced, LP-X is not usually seen, possibly because the flux of free cholesterol and phospholipid (lecithin), the LCAT substrates, is relatively low. Discoidal HDLs are often present.(ABSTRACT TRUNCATED AT 400 WORDS)

Uptake and metabolism of high-density lipoproteins by cultured rabbit hepatocytes

The selective uptake and internalization of core components of high-density lipoproteins (HDL) were examined in primary monolayer cultures of rabbit hepatocytes. Using [14C]sucrose as a surface marker covalently attached to apolipoprotein and [3H]cholesteryl linoleyl ether as a core marker, there was a 5-6-fold greater internalization of cholesteryl ether than sucrose-labeled apolipoprotein during 48 h of culture. The rate of uptake of [3H]cholesteryl linoleyl ether was 263 +/- 29 ng apo HDL/mg cell protein per h during the initial 8 h of culture, but averaged 101 +/- 32 ng apo HDL/mg cell protein per h over the 48 h culture period. Concomitant with this apparent selective uptake of cholesteryl ester core, there was a change in the HDL size distribution, with the appearance of a distinct population of smaller 4.3 nm radius particles in addition to the originally predominant particles of 4.9 nm radius. This was associated with a significant reduction of cholesteryl ester as a percentage of lipoprotein mass from 15.5 +/- 1.2 to 11.0 +/- 1.2 (P less than 0.001) and a reduction in cholesteryl ester:protein mass ratio from 0.30 +/- 0.01 to 0.19 +/- 0.01 (P less than 0.001). There was no change in the mass ratio of HDL triacylglycerol to protein. Thus rabbit hepatocytes in culture exhibit the capacity to selectively extract cholesteryl ester from HDL and produce smaller HDL particles.

Plasma lipid transfer activity in rabbits: effects of dietary hyperlipidemias

Studies were performed in the rabbit to investigate the relationship between plasma lipids and the cholesteryl ester/triglyceride transfer activity of lipoprotein-deficient plasma (d greater than 1.21). The time courses for diet-induced changes in plasma lipids and the lipid (cholesteryl ester/triglyceride) transfer activity of lipoprotein-deficient plasma (LTA) were determined in rabbits fed a variety of hyperlipidemic diets. LTA was not altered within 36 h after a single high-cholesterol/oil meal despite a near doubling of the concentration of plasma cholesterol. With high cholesterol and/or high-fat feeding, LTA increased and reached new steady state levels within about 10 days with little additional change for up to 87 days even when plasma cholesterol continued to increase. For all diets, the greatest increments in LTA occurred about 5 days after initiation of experimental feeding. A low-cholesterol (0.05-0.2%, w/w) diet and a cholesterol-free, high-coconut oil diet (14%, w/w) were associated with comparable increases in both plasma cholesterol and LTA. A marked, concomitant increase in plasma triglycerides was not associated with a further increase in LTA in hypercholesterolemic rabbits. These data indicate that in the rabbit LTA increases in a parabolic manner with moderate and gross, diet-induced increases in plasma cholesterol, and the time courses for the diet-induced changes in LTA are similar for a variety of hyperlipidemic diets.

Enzymes involved in plasma cholesterol transport

Regulation of plasma cholesterol transport is to a large extent a function of factors that regulate plasma cholesterol esterification and the transfers of cholesteryl esters between plasma lipoprotein fractions. Plasma cholesterol esterification is catalysed by the action of lecithin: cholesterol acyltransferase on lipids on the surface of HDL, while the transfers of cholesteryl esters require activity of a specific lipid transfer protein. Esterification of the cholesterol on the surface of HDL generates a concentration gradient down which unesterified cholesterol moves from tissues into the plasma. Once within the plasma and esterified, the newly formed cholesteryl esters are incorporated initially into the core of HDL particles before being redistributed to other classes of lipoproteins. The end result of these processes of esterification and transfer is that most of the cholesterol in human plasma is accommodated within the core of LDL, where its transport is a function of the highly regulated uptake by tissues of intact LDL particles. The capacity of HDL to act as substrates for lecithin: cholesterol acyltransferase varies inversely with HDL particle size. Thus, factors such as the concentration of triglyceride-rich lipoproteins and activities of the lipid transfer protein, hepatic lipase, lipoprotein lipase and the HDL conversion protein, which are known to influence HDL particle size, may also be important as regulators of plasma cholesterol esterification.

Synthesis and secretion of plasma cholesteryl ester transfer protein by human hepatocarcinoma cell line, HepG2

We have examined the synthesis, secretion, and functional and physical characteristics of a lipid transfer protein synthesized by a human hepatocellular carcinoma line. We found that this protein shares immunochemical determinants and many other properties with the lipid transfer protein, LTP-I, which has been purified from human plasma. We conclude that the human liver cell line, HepG2, synthesizes and secretes LTP-I. Thus, hepatocytes may be the source of LTP-I in human plasma.

The role of lipid transfer proteins in plasma lipoprotein metabolism

Human plasma contains a number of proteins that promote movement of lipids between lipoprotein fractions. One of these proteins, designated lipid transfer protein, is known to promote bidirectional transfers of cholesteryl esters, triglyceride, and phospholipids between all plasma lipoprotein fractions. This report briefly reviews the role of lipid transfer protein in plasma cholesterol transport and in the regulation of the particle size distribution of high-density lipoproteins. Studies are described that show that the small particle size of high-density lipoproteins in human subjects with hypertriglyceridemia is a result of the combined actions of lipid transfer protein and hepatic lipase.

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.