Cholesterol is necessary for triacylglycerol-phospholipid emulsions to mimic the metabolism of lipoproteins

Emulsions with lipid compositions similar to the triacylglycerol-rich lipoproteins were metabolized similarly to natural chylomicrons or very-low-density lipoproteins when injected intravenously in rats. Radioactive labels tracing the emulsion triacylglycerols and cholesteryl esters were both removed rapidly from the blood stream, but the removal rate of triacylglycerols was faster than that of cholesteryl ester. Most of the removed cholesteryl ester label was found in the liver, but only a small fraction of the triacylglycerol label was found in this organ, consistent with hepatic uptake of the remnants of the injected emulsion. Emulsions otherwise identical but excluding unesterified cholesterol were metabolized differently. The plasma removal of triacylglycerols remained fast, but the cholesteryl esters were removed very slowly. Heparin stimulated lipolysis, but failed to increase the rate of removal of cholesteryl esters from emulsions lacking cholesterol. Evidently, emulsions lacking cholesterol were acted on by the enzyme lipoprotein lipase, but the resultant triacylglycerol-depleted remnant particle remained in the plasma instead of being rapidly taken up by the liver. Therefore, the presence of emulsion cholesterol is a critical determinant of early metabolic events, and the findings point to a similar role for cholesterol in the natural triacylglycerol-rich lipoproteins.     

Transfer of excess cholesterol from human red blood cells to plasma in vitro

This study examined the ability of plasma and plasma fractions from normolipidaemic subjects and plasma from a patient with homozygous familial high density lipoprotein deficiency (Tangier disease) to promote loss of excess cholesterol from red blood cells in vitro. Isolated high density lipoproteins were the most potent plasma fraction for removing excess cellular cholesterol. Lipoprotein-deficient plasma and human serum albumin, but not very low density lipoproteins and low density lipoproteins, also removed excess cholesterol from the red blood cells. The near absence of high density lipoproteins in plasma from the patient with Tangier disease did not result in an abnormally low rate of cholesterol loss from the enriched red blood cells. These results suggest that normal levels of high density lipoproteins are not vital for the removal of excess cholesterol from red blood cells by plasma.     

Triacylglycerol increase in plasma very low density lipoproteins in cyclophosphamide-treated rabbit: relationship with cholesteryl ester transfer activity

We have studied the cholesteryl ester transfer between HDL and VLDL in cyclophosphamide-treated rabbits, in order to explain the abnormal cholesteryl ester partition between these two lipoprotein classes. The hypertriglyceridemia caused by treatment with the drug was associated with cholesteryl ester- and triacylglycerol-rich VLDL and with HDL poor in esterified cholesterol but relatively enriched in triacylglycerol. These two lipoprotein classes were characterized by their chemical composition and by gel filtration chromatography. VLDL particles were slightly larger in size, compared with controls. Different transfer combinations were envisaged between these abnormal lipoproteins and control ones. The transfer study involved the plasma fraction of d greater than 1.21 g/ml containing the cholesteryl ester transfer protein (CETP). It appeared that the chemical composition of lipoproteins was responsible for the level of cholesteryl ester transfer between lipoproteins. Actually, when the cholesteryl ester acceptor lipoproteins (VLDL) were enriched in triacylglycerol, the transfer was enhanced. Therefore, the effect of lipolysis on the transfer has also been explored. Lipoprotein lipase seemed to enhance the transfer of cholesteryl ester from HDL to VLDL when these lipoproteins were normal, but an important decline was obtained when triacylglycerol-rich VLDL were lipolyzed. This study defines the relationship between lipoprotein chemical composition and transfer activity of cholesteryl ester from HDL to VLDL.     

Hepatic processing and biliary secretion of the cholesteryl esters from beta very-low-density lipoproteins in the rat

beta-Migrating very-low-density lipoproteins (beta-VLDL) are cholesteryl-ester-enriched lipoproteins which accumulate in the serum of cholesterol-fed animals or patients with type III hyperlipoproteinemia. In the rat, beta-VLDL are rapidly cleared by the liver and parenchymal liver cells form the major site for uptake. In this investigation, beta-VLDL were labeled with [3H]cholesteryl esters and the hepatic intracellular transport of these esters was followed. 2 min after injection, the major part of the [3H]cholesteryl esters is already associated with the liver and a significant proportion is recovered in endosomes. Up to 25 min after injection, an increase in radioactivity in the lysosomal compartment is noticed. This radioactivity initially represents cholesteryl esters, while from 25 min onward, radioactivity is mainly present in unesterified cholesterol. Between 45 min and 90 min after beta-VLDL injection, specific transfer of unesterified [3H]cholesterol to the endoplasmic reticulum is observed, while by 3 h the majority is located in this fraction. The appearance of radioactivity in the bile was rather slow as compared to the rapid initial uptake and processing, and up to 5 h after injection only 10% of the injected dose had reached the bile (mainly as bile acids). 72 h after injection, the amount of the injected radioactivity recovered in the bile had increased to 50%. Chloroquine treatment of the rats inhibited the hydrolysis of the cholesteryl esters and the appearance of radioactivity in the bile was retarded. It is concluded that beta-VLDL are rapidly processed by parenchymal liver cells and that the cholesteryl esters from beta-VLDL are hydrolyzed in the lysosomal compartment. Unesterified cholesterol remains associated with the endoplasmic reticulum for a prolonged time, although ultimately the majority will be secreted into the bile as bile acids. The effective operation of this pathway will prevent extrahepatic accumulation of cholesteryl esters from beta-VLDL, while the prolonged residence time of unesterified cholesterol in the endoplasmic reticulum might be important for regulation of low-density lipoprotein (LDL) receptors in liver and thus for LDL levels in the blood.     

Core modification of human low-density lipoprotein by artificial triacylglycerol emulsion

To determine whether an apolipoprotein-free artificial triacylglycerol emulsion can substitute for VLDL in studying cholesterol ester-triacylglycerol exchange processes between triacylglycerol-rich lipoproteins and cholesterol ester-rich lipoproteins, we used Intralipid to modify human plasma LDL. Intralipid was incubated with LDL in the presence of lipoprotein-poor plasma (d greater than 1.21 g/ml) at 37 degrees C. Intralipid served as an acceptor for cholesterol ester and as a donor of triacylglycerol, modifying the low-density lipoproteins so that triacylglycerol became the major core lipid in the particle - the contribution of cholesterol ester to LDL mass decreased from 38% to 18%, while that of triacylglycerol increased from 4.9% to 26%. On lipolysis most added LDL triacylglycerol (59-72%) was hydrolyzed, resulting in a smaller particle than the "native' LDL particle with net loss of cholesterol ester. Incubation of LDL with the original Intralipid emulsion resulted in modified LDL with a high relative weight of phospholipid (27.7%). On removal of excess phospholipid from Intralipid and incubation of the resultant "washed' Intralipid with LDL, the relative weight of phospholipid in modified LDL decreased to 20%, which was similar to that observed after incubation of LDL with VLDL. We demonstrate that artificial triacylglycerol emulsion can indeed substitute for VLDL in neutral lipid exchange processes, and further confirm that transfer of core cholesterol ester and triacylglycerol occurs independently of the apolipoproteins present in triacylglycerol-rich lipoproteins and LDL.     

Fluorescence studies of protein–sterol relationships in human plasma lipoproteins (Short Communication)

Cholesta-5,7,9(11)-trien-3beta-ol and its oleate ester were incorporated into human low-density lipoprotein and reconstituted high-density lipoprotein. The unesterified sterol was more efficient than its ester in quenching tryptophan fluorescence, especially in low-density lipoprotein. The results, which indicate that in such lipoproteins unesterified sterols are more closely associated with peptide than are esterified sterols, are used to assess possible structures for the lipoproteins.     

The effect of low density lipoproteins, cholesterol, and 25-hydroxycholesterol on apolipoprotein B gene expression in HepG2 cells.

The purpose of the present study was to examine the effects of exogenous cholesterol on the apolipoprotein (Apo) B gene expression in HepG2 cells. Pure cholesterol had no significant effect on either the cellular content of cholesteryl esters or the net accumulation of neutral lipids and ApoB in the culture medium. By contrast, addition of 25-hydroxycholesterol increased the net accumulation of cholesteryl esters in cells and medium by 2-3-fold and decreased that of unesterified cholesterol by 50% in both compartments. A 33% reduction in the cellular content of triglycerides was commensurate with a 40% increase in their accumulation in the medium. A significant 3-fold increase in the net accumulation of ApoB in the medium was predominantly due to enhanced secretion of newly synthesized ApoB as established by pulse-chase studies. The stimulation in ApoB secretion was accompanied by a 55% increase in cellular ApoB mRNA. Under these experimental conditions, the low density lipoprotein receptor activity was decreased by only 12-20%. Addition of progesterone prevented the effects of 25-hydroxycholesterol. The changes in the concentration of neutral lipids and ApoB were reflected in the composition of secreted "low-density" lipoproteins. These particles had increased percentage contents of cholesteryl esters and ApoB and a decreased percentage content of unesterified cholesterol in comparison with lipoproteins produced by control cells. The rate of ApoB production was not correlated with the triglyceride mass in the cells but was positively correlated with the cellular and secreted cholesteryl esters and secreted triglycerides. With the exception of unchanged cellular unesterified cholesterol and ApoB mRNA levels, plasma low density lipoprotein had similar, although less pronounced, effects on the production of neutral lipids and ApoB. These results demonstrate that in HepG2 cells the synthesis and secretion of ApoB and cholesteryl esters are tightly coupled and that 25-hydroxycholesterol increased the concentration of ApoB-containing lipoproteins primarily by stimulating their production rather than reducing their catabolism.     

Fate of intravenously administered particulate and lipoprotein cholesterol in the rat

Unesterified radioactive cholesterol, both bound to serum lipoproteins and dispersed in ethanol-saline, was injected into bile fistula and intact rats. Due to phagocytosis, mainly by the liver macrophages, intravenously injected cholesterol in ethanol-saline disappears from the bloodstream significantly faster than lipoprotein-bound cholesterol. Soon after the initial phagocytosis, the particulate isotopic cholesterol started to reappear in blood, reaching a maximal radioactivity in blood 10-24 hr after injection. Although the radioactive cholesterol reappears in serum in both esterified and unesterified form, it is likely that cholesterol is released from the phagocytic cells as unesterified cholesterol which is then esterified intravascularly or at other sites. In the bile fistula rats, somewhat more of the lipoprotein cholesterol than of the particulate cholesterol appeared in bile early after injection. However, cholesterol turnover calculated from a twopool model was the same for rats injected with lipoproteinbound or particulate cholesterol.     

Characterization of guinea pig plasma lipoproteins: the appearance of new lipoproteins in response to dietary cholesterol

Dietary cholesterol induces a hemolytic anemia in guinea pigs, accompanied by changes in the lipid composition of red cells and of plasma lipoproteins. This report presents a characterization of the lipoprotein species present in each main density class in both control and cholesterol-fed guinea pigs. Traces of a typical high density lipoprotein (HDL) were detected in control plasma. HDL from cholesterol-fed, anemic guinea pigs differed from control HDL in electron microscopic appearance and lipid and peptide composition. Long stacks of discs were observed in the electron microscope in addition to smaller, spherical particles characteristic of control HDL. Low density lipoproteins (LDL) from cholesterol-fed, anemic guinea pigs had two main populations, which were separated by gel chromatography. One population appeared in the electron microscope as large transparent discs and contained mainly unesterified cholesterol and phospholipids in a 2:1 molar ratio. The other population resembled control LDL in size and composition except for its high unesterified cholesterol content. Dietary cholesterol also altered the composition and decreased the electrophoretic mobility of very low density lipoproteins. Gel electrophoretic and immunochemical evidence indicates that a peptide (mol wt 35,000) appears in lipoproteins from cholesterol-fed, anemic guinea pigs that is undetectable in those of controls. Similarities between the cholesterol-induced lipoprotein abnormalities in guinea pigs and those reported in patients with obstructive jaundice, biliary cirrhosis, type III hyperlipoproteinemia, or familial lecithin:cholesterol acyltransferase deficiency are discussed.     

Group C Niemann-Pick disease: faulty regulation of low-density lipoprotein uptake and cholesterol storage in cultured fibroblasts

Incubation of mutant Niemann-Pick C fibroblasts with low-density lipoprotein (LDL) resulted in excessive internalization of lipoprotein and extensive cellular over-accumulation of unesterified cholesterol. The uptake of LDL by the mutant cells appeared to occur through the classic LDL receptor pathway and internalized lipoprotein was processed in lysosomes. Lipoprotein uptake into mutant cells was associated with delays in the initiation of established cellular cholesterol homeostatic responses. Subcellular fractionation of mutant Niemann-Pick C fibroblasts accumulating LDL-cholesterol showed excess unesterified sterol to be localized in the light lysosome-light membrane region of a Percoll gradient, and revealed that cholesterol storage was associated with a specific alteration in the normal profiles of lysosomal marker enzymes.     

Intestinal lipoprotein synthesis in control and hypercholesterolemic rats

Previous studies in our laboratory have shown that very-low-density lipoproteins (VLDL) synthesized by the intestine of the diet-induced hypercholesterolemic rat are enriched in cholesteryl esters and unesterified cholesterol compared with intestinal VLDL from control rats. In these studies, we isolated and characterized nascent intestinal Golgi intermediate-density lipoproteins (IDL, d 1.006-1.040 g/ml) and studied isotope incorporation into apoliproteins of Golgi VLDL from control and hypercholesterolemic rats. IDL were triacylglycerol-rich lipoproteins but contained more cholesteryl ester and protein than the corresponding Golgi VLDL fractions. IDL from hypercholesterolemic rats were enriched in cholesteryl esters to a greater extent than IDL from control rats. The apolipoprotein patterns of IDL fractions were the same as those of intestinal Golgi VLDL, consisting of apolipoproteins (apo) B-48, A-I and A-IV. Time-course isotope incorporation curves for apo A-I and A-IV in Golgi VLDL were similar, but they differed from curves for apo B-48. None of these curves was markedly altered in the hypercholesterolemic rat. We conclude that the major effect of increased dietary cholesterol on intestinal lipoprotein biosynthesis is to increase the percentage of cholesteryl esters in Golgi lipoproteins. Dietary cholesterol does not alter the apolipoprotein composition of Golgi lipoproteins, nor does it have a significant effect on the pattern of isotope incorporation into apolipoproteins of Golgi VLDL. The effect of cholesteryl ester enrichment on the subsequent metabolism of these particles in the circulation and the effect of these particles on hepatic lipoprotein production remain to be determined.     

Quantitation of the in vitro free cholesterol exchange of human red cells and lipoproteins

The exchange of free cholesterol in vitro between human red blood cells and low density lipoproteins (LDL) was quantified. The flux of sterol between LDL and red cells was relatively constant over a wide range of concentrations of free cholesterol in lipoproteins. In a system containing a suspension of red blood cells in a mixed solution of high density lipoproteins (HDL) and LDL, the fractional rate of exchange of HDL cholesterol was most rapid followed by LDL and lastly, by red cells. Increasing the ionic strength of the incubation media had no effect on the exchange of cholesterol between LDL and red cells. However, when both HDL and LDL were incubated with red cells in a buffer of increased ionic strength, total red cell cholesterol exchange was unaltered, but proportionately more exchange occurred with HDL and less with LDL. Addition of acetone to the buffer increased the exchange of cholesterol between LDL and red cells but produced no increment in red cell-HDL exchange.     

Uptake of endogenous cholesterol by a synthetic lipoprotein

The addition of cholesterol-poor phospholipid liposomes to canine plasma in vivo and in vitro substantially alters the distribution of phospholipids, apoproteins, and, especially, cholesterol. In vivo, intravenously injected phospholipid liposomes remain discrete particles, which are readily distinguished from the normally occurring lipoproteins by their buoyant density and electrophoretic mobility. They acquire unesterified cholesterol from endogenous sources, thereby producing an acute rise in the concentration of this sterol in plasma. The liposomes also accumulate endogenous proteins, one of which is identified as apolipoprotein A-I. In vitro, phospholipid liposomes incubated with plasma acquire unesterified cholesterol and apolipoprotein A-I at the expense of high-density lipoproteins (HDL), the major carrier of cholesterol in normal canine plasma. In exchange, the HDL particles are enriched in phospholipids and become larger. At sufficiently high concentrations, the liposomes nearly completely deplete HDL of its unesterified cholesterol. Thus, there are generated two types of particles, both rich in apolipoprotein A-I and phospholipid, but one (modified HDL) containing mainly esterified cholesterol in its core and the other (modified liposomes) containing mainly unesterified cholesterol at its surface. It is concluded that phospholipid liposomes produce important changes in the distribution of lipids and protein in canine plasma, particularly at the expense of HDL. These changes appear to favor the mobilization of tissue cholesterol into the plasma, and may have application to atherosclerosis.     

Changes of membrane phospholipid composition of human erythrocytes in hyperlipidemias. I. Increased phosphatidylcholine and reduced sphingomyelin in patients with elevated levels of triacylglycerol-rich lipoproteins.

The composition of red blood cell membrane and plasma phospholipids has been analyzed in patients with hyperlipidemias. In red cells of patients with elevated levels of triacylglycerol-rich lipoproteins, phosphatidylcholine (PC) was raised and sphingomyelin (SM) reduced, resulting in a 20% increase of the membrane PC/SM ratio. In plasma phospholipids of these patients PC and SM levels were also higher and lower, respectively and the plasma PC/SM ratio was elevated by more than 50%. Close positive correlations between plasma and membrane phospholipids were obtained for PC, SM and the PC/SM ratio in normolipidemic and hyperlipidemic donors. Plasmalogen phosphatidylethanolamine (PE), a supposed endogenous protector against lipid oxidation, was reduced by about 20% in red cell membrane lipids in hyperlipidemic patients. Also plasmalogen-PE in plasma tended to be reduced in hyperlipidemic donors. Plasma HDL levels were positively related to the content of plasmalogen PE in the red cell membrane. In conclusion, there are closely related increases in PC/SM ratios in plasma and the red cell membrane in patients with elevated levels of triacylglycerol-rich lipoproteins. It is speculated that decreases in red cell membrane plasmalogen-PE in hyperlipidemic patients could be related to impaired antioxidant protection, possibly as a consequence of reductions in plasma HDL levels.     

Interactions between model triacylglycerol-rich lipoproteins and high-density lipoproteins in rat, rabbit and man

There are inverse relationships between HDL cholesterol and plasma triacylglycerol concentrations in normal and in hypertriglyceridemic individuals. To investigate the interactions between triacylglycerol-rich lipid particles and HDL, a lipid emulsion model of the triacylglycerol-rich lipoproteins was prepared. When emulsion particles were incubated with rat high-density lipoproteins (HDL) in the presence of lipid transfer activity (d greater than 1.21 g/ml fractions) from rabbit or human plasma there was a rapid bi-directional exchange of cholesteryl oleate (CO) and phospholipid (PL) labels between lighter and heavier fractions of HDL and emulsion particles. The transfers of CO and PL labels between both light and heavy fractions of HDL and the emulsion particles were increased with increasing amounts of emulsion added to the incubations. Incubation with the d greater than 1.21 g/ml fraction from rat plasma resulted in only a small exchange of CO whereas PL exchange was similar to rabbit and human plasma. Retinyl palmitate label was not transferred from emulsion particles to the HDL fractions even in the presence of lipid transfer activity from rabbit or human plasma. The present study shows that the transfer protein-mediated exchanges of surface and core lipids between HDL and the triacylglycerol-rich lipoproteins are affected by the quantity of triacylglycerol-rich particles in the system. This mechanism may contribute to the inverse relationships between plasma triacylglycerol concentrations and HDL concentrations in normal and hypertriglyceridemic individuals.     

Effect of ketoconazole on the activity of key enzymes of biosynthesis of cholesterol and its esters in the liver of rats maintained on a cholesterol-enriched diet]

Ketoconazole in vivo has been studied for its effect on the activity of key enzymes of the cholesterol and its esters' biosynthesis in the liver and on the cholesterol concentration in certain fractions of blood lipoproteins in normal and cholesterol-fed rats. It is established that ketoconazole decreases cholesterol concentration in low-density lipoproteins and in very low-density lipoproteins as well as decrease the acyl-CoA-cholesterol acyl-transferase activity and increases the 3-hydroxy-3-methyl-glutaryl-CoA-reductase activity in the liver microsomes of intact and test animals. It is supposed that the possible cause of the observed changes can be a disturbance in regulation of basic links of cholesterol metabolism in the liver.     

Homeostasis of free cholesterol in the blood: a preliminary evaluation and modeling of its passive transport

The rate of noncatalyzed transfer of cholesterol (Chol) among lipoproteins and cells in the blood is of fundamental importance as a baseline to assess the role of active transport mechanisms, but remains unknown. Here we address this gap by characterizing the associa­tion of the Chol analog, ergosta-5,7,9(11),22-tetraen-3β-ol (DHE), with the lipoproteins VLDL, LDL, HDL₂, and HDL₃. Combining these results with data for the association of DHE with liposomes, we elaborated a kinetic model for the noncatalyzed exchange of free Chol among blood compartments. The computational results are in good agreement with experimental values. The small deviations are explained by the nonequilibrium distribution of unesterified Chol in vivo, due to esterification and entry of new unesterified Chol, and eventual effects introduced by incubations at low temperatures. The kinetic profile of the homeostasis of unesterified Chol in the blood predicted by the model developed in this work is in good agreement with the observations in vivo, highlighting the importance of passive processes.     

Hypertriglyceridemia is exacerbated by slow lipolysis of triacylglycerol-rich lipoproteins in fed but not fasted streptozotocin diabetic rats.

Hydrolysis by endothelial lipases of triacylglycerol-rich lipoproteins of diabetic origin were compared to lipoproteins of non-diabetic origin. The plasma lipoprotein fraction of density < 1.006 g/ml, including chylomicrons and VLDL, were incubated in vitro with post-heparin plasma (PHP) lipases. The lipoproteins of diabetic origin were hydrolysed at a significantly slower rate than lipoproteins from normal rats by the lipoprotein lipase component of PHP. However, if rats were fasted for 16 h prior to lipoprotein recovery, no differences in rates of VLDL hydrolysis were observed. Slower hydrolysis of lipoproteins of diabetic origin reflected a decrease in the apolipoprotein CII/CIII ratio and other changes in the apolipoprotein profile. To assess whether diabetic rats were less able to clear triacylglycerol independent of changes in the nature of the lipoproteins, we monitored the clearance of chylomicron-like lipid emulsions in hepatectomized rats. In vivo, emulsion triacylglycerol hydrolysis was not slowed due to diabetes. However, control and diabetic rats, which had been fasted for 16 h, cleared triacylglycerol at about twice the rate of fed rats. Triacylglycerol secretion rates in diabetic and control rats were similar, whether fed or fasted. We conclude that in streptozocin diabetic rats, hypertriglyceridemia was not due to overproduction of chylomicron- or VLDL-triacylglycerol, nor to decreased endothelial lipase activities. Rather, in fed diabetic rats, the triacylglycerol-rich lipoproteins are poorer substrates for lipoprotein lipase. This may lead to slower formation of remnants which would exacerbate slow remnant removal. VLDL of diabetic origin were hydrolysed as efficiently as VLDL from control donors, suggesting that in the fed state the lipolytic defect may be specific for chylomicrons.     

Studies on the mechanism of hypertriglyceridemia in Tangier disease. Determination of plasma lipolytic activities, k1 values and apolipoprotein composition of the major lipoprotein density classes

Mechanisms responsible for hypertriglyceridemia in Tangier disease were elucidated by an analysis of the plasma post-heparin lipolytic activities and the structural and metabolic properties of very low (VLDL) and low (LDL) density lipoproteins. The levels of lipoprotein lipase activity in six Tangier patients were significantly lower (P less than 0.001) than in 40 control subjects (8.1 +/- 3.3 (+/- S.D.) vs. 14.1 +/- 3.7 units/ml). In contrast, the levels of hepatic triacylglycerol lipase were higher (P less than 0.01) than in normal controls (14.4 +/- 3.9 vs. 9.3 +/- 4.0 units/ml). Because kinetic parameters such as Km or Vmax cannot be obtained with naturally occurring triacylglycerol-rich lipoproteins, the pseudo-first-order rate constant (k1) of triacylglycerol hydrolysis was used to assess the effectiveness of triacylglycerol-rich lipoproteins as substrates for lipoprotein lipase. The k1 values for Tangier VLDL (k1 = 0.017 +/- 0.002 min-1) were significantly lower (P less than 0.001) than the k1 values (0.036 +/- 0.008 min-1) for control VLDL. Both the Tangier and control LDL2 are similar in their resistance to the action of lipoprotein lipase, as shown by their low k1 values (0.002 +/- 0.001 and 0.001 +/- 0.001 min-1, respectively). The major compositional difference between the lipoproteins of Tangier disease and normal subjects was a significant increase in the percent content of apolipoprotein A-II in all lipoprotein particles with d less than 1.063 g/ml, with the greatest increase occurring in VLDL and the lowest in LDL2. These results were interpreted as indicating that, in Tangier disease, there is a lower reactivity of VLDL with lipoprotein lipase which may in part be attributed to the abnormal apolipoprotein composition. This finding, in conjunction with the reduced levels of lipoprotein lipase activity, may explain the hypertriglyceridemia in Tangier disease.     

Hep G2 cells as a resource for metabolic studies: lipoprotein, cholesterol, and bile acids

Hep G2, a liver cell line derived from a human hepatoblastoma that is free of known hepatotropic viral agents, has been found to express a wide variety of liver-specific metabolic functions. Among these functions are those related to cholesterol and triglyceride metabolism. Confluent Hep G2 monolayers express normal low-density lipoprotein (LDL) receptors and continue to internalize and metabolize chylomicrons, very low-density lipoproteins (VLDL), LDL, and high-density lipoproteins. In lipoprotein-free medium, apolipoproteins A-I, A-II, B, C, and E accumulate in the medium together with cholesterol, cholesteryl ester, triglyceride, and all the primary bile acids. The regulation of their synthesis and secretion is not fully known and their interrelationships have not been established. Because Hep G2 cells express these and other components of cholesterol and triglyceride metabolism, they are a microcosm for studying the central role of the liver.