Receptor-dependent and receptor-independent degradation of low density lipoprotein in normal rabbits and in receptor-deficient mutant rabbits

Low density lipoprotein (LDL) catabolism was studied using WHHL rabbits, an inbred strain deficient in LDL receptor activity and, thus, an animal model for homozygous familial hypercholesterolemia. WHHL and normal rabbits were injected with [14C]sucrose-LDL and the tissue sites of LDL degradation were determined 24 h later. On degradation of [14C]sucrose-LDL, the [14C]sucrose ligand remains trapped within tissues as a cumulative measure of degradation. The fractional catabolic rate of [14C]sucrose-LDL in Watanabe heritable hyperlipidemic (WHHL) rabbits was reduced (0.024 +/- 0.010 versus 0.063 +/- 0.026 h-1) but, by virtue of the increased plasma pool, total LDL flux was increased (33.5 +/- 9.6 versus 10.6 +/- 4.4 mg of LDL protein/kg/day). Liver was the predominant site of catabolism in both WHHL and normal rabbits (52.7 +/- 6.9 and 56.6 +/- 6.2% of total degradation). About 90% of hepatic catabolism was attributable to parenchymal cells in both cases. Thus, Kupffer cells, a major component of the reticuloendothelial system, do not play a major role in LDL catabolism in WHHL rabbits. Despite receptor deficiency, the relative contribution of various tissues to overall LDL degradation was not greatly altered and the absolute rate of delivery of LDL to all tissues was increased with the exception of the adrenal. Thus, there was no evidence that the increased degradation occurred in any special subset of "scavenger" cells. Nevertheless, local scavenger cell uptake may be critically important, especially in atherogenesis. If it is assumed that receptor-independent degradation occurs at the same rate in the tissues of WHHL and normal rabbits and that catabolism in the absence of receptors is a linear function of concentration, then one can estimate the fraction of uptake in normal tissues mediated by receptors. The difference in the fraction of the plasma LDL pool cleared per unit of time in normal and WHHL rabbits would reflect the contribution of receptors to fractional clearance. By this calculation, receptor-mediated degradation in normal rabbits was 62% overall, 63% in liver, 92% in adrenal, and 83% in gut.     

Cholesterol synthesis in vivo and in vitro in the WHHL rabbit, an animal with defective low density lipoprotein receptors

These studies were undertaken to measure rates of synthesis of digitonin-precipitable sterols in vivo and in vitro in control rabbits (New Zealand (NZ) control) and in homozygous Watanabe heritable hyperlipidemic rabbits (WHHL) that lack receptors for low density lipoproteins (LDL). The plasma cholesterol concentration in NZ control fetuses equaled 79 mg/dl, rose to 315 mg/dl 12 days after birth, and fell to 80 mg/dl in young adult animals. At these same ages, cholesterol concentrations in the WHHL animals equal 315, 625, and 715 mg/dl, respectively. The rate of whole animal sterol synthesis in vivo, expressed as the mumol of [3H]water incorporated into sterols per hr per kg of body weight, was lower in the WHHL animals than in the NZ controls both in the fetuses (108 vs 176) and in the adult animals (48 vs 66). In adult NZ controls the content of newly synthesized sterols (rate of sterol synthesis) per g of tissue was highest in the liver (538 nmol/g per hr), adrenal gland (438), small bowel (371), and ovary (225) while lower rates of synthesis were found in 15 other tissues. In the WHHL rabbits a higher content of [3H]sterols was found only in the adrenal gland (2,215) while synthesis was suppressed in the liver (310), colon, lung, and kidney, and was unchanged in the remaining organs. These findings were confirmed by measurements of rates of sterol synthesis in the same tissues in vitro. When whole organ weight was taken into consideration, the tissues that were the major contributors to whole body sterol synthesis in both types of rabbits were liver, small bowel, skin, and carcass. However, it was the lower rate of synthesis in the liver of the WHHL animals that alone accounted for the lower rate of whole animal sterol synthesis seen in these rabbits. These studies demonstrate that in WHHL animals that lack LDL receptors and that have very high levels of circulating LDL cholesterol, the rate of cholesterol synthesis in nearly all tissues is normal but in the liver is significantly suppressed. Only the adrenal gland manifested enhanced synthesis. Such findings suggest that in the WHHL rabbit where LDL receptor activity is reduced and plasma LDL levels rise, mechanisms other than receptor-mediated LDL uptake may act to deliver cholesterol to the cells of the various organs and to the liver.     

Total cholesterol,low density lipoprotein cholesterol,and high density lipoprotein cholesterol and coronary heart disease in Scotland.

OBJECTIVE--To investigate long term changes in total cholesterol, high density lipoprotein cholesterol, and low density lipoprotein cholesterol concentrations and in measures of other risk factors for coronary heart disease and to assess their importance for the development of coronary heart disease in Scottish men. DESIGN--Longitudinal study entailing follow up in 1988-9 of men investigated during a study in 1976. SETTING--Edinburgh, Scotland. SUBJECTS--107 men from Edinburgh who had taken part in a comparative study of risk factors for heart disease with Swedish men in 1976 when aged 40. INTERVENTION--The men were invited to attend a follow up clinic in 1988-9 for measurement of cholesterol concentrations and other risk factor measurements. Eighty three attended and 24 refused to or could not attend. MAIN OUTCOME MEASURES--Changes in total cholesterol, high density lipoprotein cholesterol, and low density lipoprotein cholesterol concentrations, body weight, weight to height index, prevalence of smoking, and alcohol intake; number of coronary artery disease events. RESULTS--Mean serum total cholesterol concentration increased over the 12 years mainly due to an increase in the low density lipoprotein cholesterol fraction (from 3.53 (SD 0.09) to 4.56 (0.11) mmol/l) despite a reduction in high density lipoprotein cholesterol concentration. Body weight and weight to height index increased. Fewer men smoked more than 15 cigarettes/day in 1988-9 than in 1976. Blood pressure remained stable and fasting triglyceride concentrations did not change. The frequency of corneal arcus doubled. Alcohol consumption decreased significantly. Eleven men developed clinical coronary heart disease. High low density lipoprotein and low high density lipoprotein cholesterol concentrations in 1976, but not total cholesterol concentration, significantly predicted coronary heart disease (p = 0.05). Almost all of the men who developed coronary heart disease were smokers (91% v 53%, p less than 0.05). CONCLUSION--Over 12 years the lipid profile deteriorated significantly in this healthy cohort of young men. Smoking, a low high density lipoprotein concentration and a raised low density lipoprotein concentration were all associated with coronary heart disease in middle aged Scottish men, whereas there was no association for total cholesterol concentration. The findings have implications for screening programmes.     

Is hypertriglyceridemic very low density lipoprotein a precursor of normal low density lipoprotein?

The precursor-product relationship of very low density (VLDL) and low density lipoproteins (LDL) was studied. VLDL obtained from normal (NTG) and hypertriglyceridemic (HTG) subjects was fractionated by zonal ultracentrifugation and subjected to in vitro lipolysis. The individual subfractions and their isolated lipolysis products, as well as IDL and LDL, were rigorously characterized. A striking difference in the contribution of cholesteryl ester to VLDL is noted. In NTG subfractions, the cholesteryl ester to protein ratio increases with decreasing density (VLDL-I----VLDL-III). This is the expected result of triglyceride loss through lipolysis and cholesteryl ester gain through core-lipid transfer protein action. In HTG subfractions there is an abnormal enrichment of cholesteryl esters that is most marked in VLDL-I and nearly absent in VLDL-III. Thus, the trend of the cholesteryl ester to protein ratios is reversed, being highest in HTG-VLDL-I and lowest in VLDL-III. This is incompatible with the precursor-product relationship described by the VLDL----IDL----LDL cascade. In vitro lipolysis studies support the conclusion that not all HTG-VLDL can be metabolized to LDL. While all NTG subfractions yield products that are LDL-like in size, density, and composition, only HTG-VLDL-III, whose composition is most similar to normal, does so. HTG VLDL-I and VLDL-II products are large and light populations that are highly enriched in cholesteryl ester. We suggest that this abnormal enrichment of HTG-VLDL with cholesteryl ester results from the prolonged action of core-lipid transfer protein on the slowly metabolized VLDL mass. This excess cholesteryl ester load, unaffected by the process of VLDL catabolism, remains entrapped within the abnormal particle. Therefore, lipolysis yields an abnormal, cholesteryl ester-rich product that can never become LDL.     

Role of acyl-coenzyme A:cholesterol acyltransferase-1 in the control of hepatic very low density lipoprotein secretion and low density lipoprotein receptor expression in the mouse and hamster

Cholesteryl esters present in nascent very low density lipoproteins are generated in a reaction catalyzed by acyl CoA:cholesterol acyltransferase (ACAT). To examine the effect of cholesteryl esters on the secretion of apoB-containing lipoproteins, we transiently overexpressed human (h) ACAT-1 in the livers of low density lipoprotein (LDL) receptor(-/-) mice using adenovirus-mediated gene transfer. Overexpression of hACAT-1 increased hepatic total and esterified cholesterol but did not reduce hepatic free cholesterol due to a compensatory increase in the rate of de novo cholesterol synthesis. Overexpression of hACAT-1 markedly increased the plasma concentration and hepatic secretion of apoB-containing lipoproteins but had no effect on the clearance of very low density lipoprotein-apoB from plasma indicating that cholesteryl esters play an important role in regulating the assembly and secretion of apoB-containing lipoproteins. ACAT activity has been implicated in the regulation of the LDL receptor pathway by dietary fatty acids. It has been hypothesized that unsaturated fatty acids, by enhancing ACAT activity, reduce the amount of free cholesterol in a putative regulatory pool that feeds back on LDL receptor expression. We directly tested this hypothesis in hamsters by transiently overexpressing hACAT-1 in the liver. Enhanced cholesterol esterification in the liver resulted in a compensatory increase in de novo cholesterol synthesis but no induction of LDL receptor expression suggesting that fatty acids regulate LDL receptor expression via a mechanism independent of ACAT.     

Receptor-mediated catabolism and tissue uptake of human low density lipoprotein in the cholesterol-fed, atherosclerotic rabbit

The LDL receptor pathway, which was delineated in cultured cells, is now known to operate in vivo. In this study we have measured the plasma clearances and tissue uptakes of native and chemically modified (1,2-cyclohexanedione-treated or reductively methylated) LDL in rabbits in order to determine the response of the pathway to a high-cholesterol diet. 1 week on the diet increased circulating LDL and suppressed its receptor-mediated plasma clearance and uptake into all tissues. The fractional catabolic rate of the lipoprotein via the receptor-independent route also fell. Continuation of the feeding program for 12 weeks accentuated these changes and virtually eliminated receptor uptake into all tissues so that the plasma decay curves of native and cyclohexanedione-treated LDL were superimposable. Lipoprotein assimilation by the aorta, however, did not follow this general trend. This tissue, after 12 weeks, was variably infiltrated by atheromatous deposits and the appearance of these lesions was associated with a substantial increase in the relative uptakes of both native and chemically modified (cyclohexanedione-treated and reductively methylated) LDL. We concluded (a) that expansion of tissue cholesterol pools virtually abolishes LDL receptor activity in rabbits; and (b) that LDL assimilation (both apparently receptor-mediated and receptor-independent) paradoxically increases at sites where the aorta is affected by atheromatous lesions.     

Differences in the metabolism of oxidatively modified low density lipoprotein and acetylated low density lipoprotein by human endothelial cells: inhibition of cholesterol esterification by oxidatively modified low density lipoprotein

The rate of degradation of oxidatively modified low density lipoprotein (Ox-LDL) by human endothelial cells was similar to that of unmodified low density lipoprotein (LDL), and was approximately 2-fold greater than the rate of degradation of acetylated LDL (Ac-LDL). While LDL and Ac-LDL both stimulated cholesterol esterification in endothelial cells, Ox-LDL inhibited cholesterol esterification by 34%, demonstrating a dissociation between the degradation of Ox-LDL and its ability to stimulate cholesterol esterification. Further, while LDL and Ac-LDL resulted in a 5- and 15-fold increase in cholesteryl ester accumulation, respectively, Ox-LDL caused only a 1.3-fold increase in cholesteryl ester mass. These differences could be accounted for, in part, by the reduced cholesteryl ester content of Ox-LDL. However, when endothelial cells were incubated with Ac-LDL in the presence and absence of Ox-LDL, Ox-LDL led to a dose-dependent inhibition of cholesterol esterification without affecting the degradation of Ac-LDL. This inhibitory effect of Ox-LDL on cholesteryl ester synthesis was also manifest in normal human skin fibroblasts incubated with LDL and in LDL-receptor-negative fibroblasts incubated with unesterified cholesterol to stimulate cholesterol esterification. Further, the lipid extract from Ox-LDL inhibited cholesterol esterification in LDL-receptor negative fibroblasts. These findings suggest that the inhibition of cholesterol esterification by oxidized LDL is independent of the LDL and scavenger receptors and may be a result of translocation of a lipid component of oxidatively modified LDL across the cell membrane.     

Constitutive receptor-independent low density lipoprotein uptake and cholesterol accumulation by macrophages differentiated from human monocytes with macrophage-colony-stimulating factor (M-CSF)

Recently, we have shown that macrophage uptake of low density lipoprotein (LDL) and cholesterol accumulation can occur by nonreceptor mediated fluid-phase macropinocytosis when macrophages are differentiated from human monocytes in human serum and the macrophages are activated by stimulation of protein kinase C (Kruth, H. S., Jones, N. L., Huang, W., Zhao, B., Ishii, I., Chang, J., Combs, C. A., Malide, D., and Zhang, W. Y. (2005) J. Biol. Chem. 280, 2352-2360). Differentiation of human monocytes in human serum produces a distinct macrophage phenotype. In this study, we examined the effect on LDL uptake of an alternative macrophage differentiation phenotype. Differentiation of macrophages from human monocytes in fetal bovine serum with macrophage-colony-stimulating factor (M-CSF) produced a macrophage phenotype demonstrating constitutive fluid-phase uptake of native LDL leading to macrophage cholesterol accumulation. Fluid-phase endocytosis of LDL by M-CSF human macrophages showed non-saturable uptake of LDL that did not down-regulate over 48 h. LDL uptake was mediated by continuous actin-dependent macropinocytosis of LDL by these M-CSF-differentiated macrophages. M-CSF is a cytokine present within atherosclerotic lesions. Thus, macropinocytosis of LDL by macrophages differentiated from monocytes under the influence of M-CSF is a plausible mechanism to account for macrophage foam cell formation in atherosclerotic lesions. This mechanism of macrophage foam cell formation does not depend on LDL modification or macrophage receptors.     

Turnover and tissue sites of degradation of glucosylated low density lipoprotein in normal and immunized rabbits

Immunological mechanisms have been implicated in the atherogenic process since immunoglobulins are frequently found in the atherosclerotic aorta. We have previously shown that modifications of homologous low density lipoproteins (LDL) make it immunogenic. In particular we have demonstrated that immunization with homologous nonenzymatically glucosylated LDL (glcLDL) results in the generation of antibodies specific to the derivatized lysine residue, and that such antibodies do not react with native LDL epitopes. In the present study we immunized rabbits with reductively glucosylated rabbit LDL and then determined the effects of the circulating antibodies on the rates of plasma clearance and on the sites of degradation of LDL in which varying degrees of glucosylation had been achieved. In normal chow-fed animals, the plasma clearance of glcLDL was retarded in proportion to the extent of lysine derivatization. In contrast, in immunized animals the clearance of glcLDL was greatly accelerated. When 10% or more of lysine residues were derivatized, clearance of glcLDL was accelerated 50- to 100-fold. Even when only 5% of lysines were derivatized, plasma clearance was accelerated 2- to 3-fold. Cholesterol feeding inhibited LDL clearance from plasma and decreased LDL uptake of LDL receptor-rich tissues. In a similar manner, glucosylation of LDL inhibited its ability to bind to the LDL receptor and redirected sites of LDL degradation away from LDL receptor-rich tissues. Thus degradation of glcLDL by liver and adrenal was markedly diminished. The presence of antibodies to glcLDL also redirected sites of degradation of the modified LDL, primarily to the reticuloendothelial cells of the liver. There was no evidence for specific targeting of glcLDL-immunoglobulin complexes to the aorta; instead they were targeted to the liver. These data suggest that the presence of humoral antibodies to modified LDL acts to rapidly remove such LDL from plasma and specifically targets such complexes to reticuloendothelial cells, primarily in the liver. In this manner such antibodies may serve a useful purpose.     

Sialic acid content of low density lipoprotein and its relation to lipid concentrations and metabolism of low density lipoprotein and cholesterol

A low sialic acid content in low density lipoprotein (LDL) has been associated with atherogenicity and coronary artery disease (CAD) in many but not all studies. We investigated associations of the sialic acid-to-apolipoprotein B (apoB) ratio of LDL with lipoprotein lipid concentrations, kinetics of LDL, metabolism of cholesterol, and the presence of CAD in 98 subjects (CAD(+), n = 56; CAD(-), n = 42). The sialic acid ratios of total, dense, and very dense LDL were lower in the CAD(+) than CAD(-) subjects, especially at high sialic acid ratios. The LDL sialic acid ratio was inversely associated with respective lipid and apoB concentrations and positively with lipid-to-apoB ratios of LDL. The transport rates (TRs) for total and dense LDL apoB were negatively associated with their sialic acid ratios. The sialic acid ratio of dense LDL, but not that of total LDL, was inversely correlated with serum levels of cholesterol precursor sterols, indicators of cholesterol synthesis, and positively with serum levels of plant sterols, indicators of cholesterol absorption. In addition, the TR for dense LDL was positively correlated with cholesterol synthesis.In conclusion, a low LDL sialic acid ratio was associated with CAD, high numbers of small LDL particles, and a high TR for LDL apoB, and in dense LDL also with high synthesis and low absorption of cholesterol.     

Provision of cholesterol to lymphocytes by high density and low density lipoproteins. Requirement for low density lipoprotein receptors

The capacity of lipoprotein fractions to provide cholesterol necessary for human lymphocyte proliferation was examined. When endogenous synthesis of cholesterol was blocked, proliferation of mitogen-stimulated normal human lymphocytes was markedly inhibited unless an exogenous source of sterol was supplied. All lipoprotein fractions with the exception of high density lipoprotein subclass 3 were able to provide cholesterol for lymphocyte proliferation. Each of the lipoprotein subfractions capable of providing cholesterol was also able to regulate endogenous sterol synthesis in cultured human lymphocytes. Provision of cholesterol by lipoproteins required the interaction of apolipoprotein B or apolipoprotein E with specific receptors on normal lymphocytes. Apolipoprotein modification by acetylation or methylation, which markedly reduced the ability to regulate sterol biosynthesis, also diminished the capacity of lipoproteins to provide cholesterol. In addition, depletion of apolipoprotein B- and apolipoprotein E-containing particles from high density lipoprotein decreased its ability to suppress cholesterol synthesis and prevented it from providing cholesterol to proliferating lymphocytes. Monoclonal antibodies directed against the receptor-recognition sites on apolipoprotein B and apolipoprotein E were used to define the specific apolipoproteins required for the provision of cholesterol to lymphocytes by the various lipoprotein fractions. The antibody to apolipoprotein B inhibited cholesterol provision by both low density lipoprotein (LDL) and other lipoprotein fractions. The antibody to apolipoprotein E did not decrease provision of cholesterol by LDL but did inhibit the capacity of other fractions to provide cholesterol. In addition, a monoclonal antibody against the ligand binding site on the LDL receptor inhibited provision of cholesterol to normal lymphocytes by all lipoproteins. Finally, lymphocytes lacking LDL receptors were unable to obtain cholesterol from any lipoprotein fraction. These studies demonstrate that LDL receptor-mediated interaction with apolipoprotein B or apolipoprotein E is essential for the provision of cholesterol to normal human lymphocytes from all lipoprotein sources.     

The metabolism of esterified cholesterol in rabbit plasma low density lipoproteins

The metabolism of esterified cholesterol in plasma low density lipoproteins (LDL) has been studied in rabbits. LDL labelled with ³H in the esterified and free cholesterol moieties was isolated from the serum of donar rabbits which has been injected with [³H]mevalonic acid, and subsequently either incubated at 37°C in vitro with unlabelled rabbit serum or unlabelled rabbit lipoprotein fractions, or reinjected into other rabbits.In vitro there was found to be a transfer of 40–60% of the esterified [³H]-cholesterol out of LDL into both the very low density lipoprotein (VLDL) and high density lipoprotein (HDL) fractions which could not be explained in terms of net transfer of esterified cholesterol mass. In the incubations of labelled LDL with either of the other unlabelled lipoprotein fractions, transfers were apparent only if the dialysed 1.21 g/ml infranatant of rabbit serum was also present. The transfer of esterified [³H]cholesterol out of LDL was enhanced when lecithin:cholesterol acyltransferase was active.After reinjecting labelled LDL into other rabbits, it was found that more than half of the esterified [³H]cholesterol removed from the recipient LDL fraction during the first 30 min was not lost from the plasma compartment, but rather was recovered in HDL. There was only minimal in vivo transfer of LDL esterified [³H]cholesterol into VLDL.It has been concluded that in vitro the esterified cholesterol in LDL exchanges with that in both the VLDL and HDL, and that in vivo the esterified cholesterol pools in LDL and HDL may represent parts of a progressively equilibrating plasma pool.     

Reduced uptake of cholesterol esterase-modified low density lipoprotein by macrophages.

Changes in low density lipoprotein (LDL) lipid composition were shown to alter its interaction with the LDL receptor, thus affecting its cellular uptake. Upon incubation of LDL with 5 units/ml cholesterol esterase (CEase) for 1 h at 37 degrees C, there was a 33% reduction in lipoprotein cholesteryl ester content, paralleled by an increment in its unesterified cholesterol. CEase-LDL, in comparison to native LDL, was smaller in size, possessed fewer free lysine amino groups (by 14%), and demonstrated reduced binding to heparin (by 83%) and reduced immunoreactivity against monoclonal antibodies directed toward epitopes along the LDL apoB-100. Incubation of CEase-LDL with the J-774 macrophage-like cell line resulted in about a 30% reduction in lipoprotein binding and degradation in comparison to native LDL, and this was associated with a 20% reduction in macrophage cholesterol mass. Similarly, CEase-LDL degradation by mouse peritoneal macrophages, human monocyte-derived macrophages, and human skin fibroblasts was reduced by 20-44% in comparison to native LDL. CEase-LDL uptake by macrophages was mediated via the LDL receptor and not the scavenger receptor. CEase activity toward LDL was demonstrated in plasma and in cells of the arterial wall such as macrophages and endothelial cells. Thus, CEase modification of LDL may take place in vivo, and this phenomenon may have a role in atherosclerosis.     

Intimal smooth muscle cells up-regulate beta-very low density lipoprotein-mediated cholesterol accumulation by enhancing beta-very low density lipoprotein uptake and decreasing cholesterol efflux

To clarify the mechanism of smooth muscle cell (SMC)-derived foam cell formation, we investigated beta-very low density lipoprotein (beta-VLDL) cholesterol metabolism in vascular medial SMCs (M-SMCs) from normal rabbits compared with intimal SMCs (I-SMCs) from normal rabbits fed a high-cholesterol diet and LDL receptor-deficient rabbits. For both types of I-SMCs, uptake of [3H]cholesteryl oleate labeled beta-VLDL increased 1.6 times and release of [3H]cholesterol decreased 40% compared with M-SMCs. M-SMCs took up part of the beta-VLDL through the LDL receptor but I-SMCs did not. mRNAs for the VLDL receptor and the LDL receptor relative with 11 ligand binding repeats were expressed at similar levels in all SMCs. M-SMCs expressed more LDL receptor-related protein than I-SMCs. Ligand blotting analysis revealed greater 125I-beta-VLDL binding to a 700-kDa protein in I-SMCs compared with M-SMCs. I-SMCs had higher activities of acid cholesterol esterase and acyl-CoA:cholesterol acyltransferase, and lower activity of neutral cholesterol esterase than M-SMCs in both the absence and the presence of beta-VLDL. These results indicate that I-SMCs accumulate more cholesteryl ester than M-SMCs by taking up more beta-VLDL and by effluxing less cholesterol.     

Comparison of plasma clearance of low density lipoprotein with beta-very low density lipoprotein or acetoacetylated low density lipoprotein in cholesterol-fed rabbits

The plasma clearance and tissue distribution of radioiodinated low-density lipoprotein (LDL), beta-very low density lipoprotein (beta-VLDL), and acetoacetylated LDL were studied in cholesterol-fed rabbits. Radioiodinated LDL ([125I]LDL) was cleared more slowly than either [125I]beta-VLDL or acetoacetylated-[125I]LDL and its fractional catabolic rate was one-half that of [125I]beta-VLDL and one-ninth that of acetoacetylated-[125I]LDL. Forty-eight hours after the injection of the labeled lipoproteins, the hepatic uptake was the greatest among the organs evaluated with the uptake of [125I]LDL being one-third that of either [125I]beta-VLDL or acetoacetylated-[125I]LDL. The reduction in the hepatic uptake of LDL due to a down-regulation of the receptors would account for this retarded plasma clearance.     

Interaction of cholesterol ester and triglyceride in human plasma very low density lipoprotein.

The properties of human plasma very low density lipoproteins (VLDL), low density lipoproteins (LDL), and their extracted lipids were compared using calorimetric, X-ray scattering, and polarizing microscopy techniques. Intact LDL, and cholesterol esters isolated from LDL and VLDL each undergo reversible changes in their physical state around body temperature. These transitions are associated with ordered liquid crystalline to liquid phase changes of the cholesterol esters. In contrast to LDL, VLDL has no reversible transitions and shows no evidence of ordered liquid crystalline structures between 10 and 45 degrees C. Therefore, unlike LDL, VLDL does not contain a separate cholesterol ester region capable of undergoing cooperative melting. Solubility studies at 37 degrees C of cholesterol esters and triglyceride isolated from VLDL show that even at a weight ratio of 1:1, which greatly exceeds the relative amount of cholesterol esters in VLDL, cholesterol ester is completely soluble in triglyceride. Thus, the cholesterol ester in VLDL is not sequestered in a separate domain within VLDL, but is dissolved in the liquid core of the particle.