Rat high density lipoprotein subfraction (HDL3) uptake and catabolism by isolated rat liver parenchymal cells

Pyrene lipids as markers of peroxidative processes in different regions of low and high density lipoproteins

Three different pyrene derivatives, pyrene decanoyl phosphatidylcholine (P10PC), pyrene dodecanoyl sulfatide (P12CS) and cholesteryl pyrenyl hexanoate (P6Chol), were used to follow lipid peroxidation in low and high density lipoproteins. Probe-labelled lipoproteins were subjected to Cu2+ catalyzed peroxidation. In all cases the fluorescence of the probes progressively decreased due to the involvement of pyrene in the peroxidative reaction. Thus, we used the fluorescence decrease of P6Chol to monitor the lipid peroxidation in the hydrophobic core of LDL and HDL, and that of the amphipatic probes, P10PC and P12CS, to follow lipid peroxidation in the envelope of both lipoproteins. The possibility of following lipid peroxidation in individual lipoprotein regions could lead to more detailed information on the oxidative modifications that play an important role in the altered cholesterol homeostasis involved in the formation of atherosclerotic lesions. No differences were observed in the peroxidation kinetics of the hydrophobic core of HDL and LDL monitored with P6Chol. On the contrary kinetics obtained with P10PC and P12 CS demonstrated the HDL envelope to be more susceptible to Cu2+ -dependent lipid peroxidation than that of the LDL. This could be due to a greater radical generating capacity of the HDL envelope and can be explained on the basis of low vitamin E levels and large amounts of polyunsaturated fatty acids esterified on phospholipids determined in HDL, and on literature evidence that indicates HDL as the principal vehicle of circulating plasma lipids peroxides.

Increase in neutral cholesteryl ester hydrolase activity produced by extralysosomal hydrolysis of high-density lipoprotein cholesteryl esters in rat hepatoma cells (H-35)

The metabolism of high-density lipoprotein-associated cholesteryl esters (HDL-CE) in liver cells is not well understood. We studied the possible role of lysosomal and extralysosomal pathways on such metabolism by measuring the uptake and hydrolysis of HDL-CE in H-35 rat hepatoma cells. Incubation of cells with [3H]cholesteryl ester-labeled HDL led to the intracellular accumulation of both 3H-free cholesterol and [3H]cholesteryl ester. The ratio of 3H-free cholesterol/[3H]cholesteryl ester increased with an increase in incubation time even in the presence of chloroquine. Because chloroquine did not inhibit the conversion of cholesteryl ester to free cholesterol, the hydrolysis of HDL-CE may have been catalyzed by an extralysosomal enzyme, perhaps by neutral cholesteryl ester hydrolase (NCEH). When we incubated cells with increasing concentrations of HDL, NCEH activity increased. This increase in enzyme activity was not inhibited by the addition of chloroquine. A complex of dimyristoylphosphatidylcholine (DMPC)/apo HDL/cholesteryl ester enhanced the activity as well as native HDL. Neither the DMPC/apo HDL nor the DMPC/cholesteryl ester complex affected the activity, suggesting that apo HDL may be required for the uptake of HDL-CE. The present study demonstrated that the extralysosomal hydrolysis by NCEH is operating in the metabolism of HDL-CE in hepatoma cells.

Binding and degradation of lipoprotein(a) and LDL by primary cultures of human hepatocytes. Comparison with cultured human monocyte-macrophages and fibroblasts

Although lipoprotein(a) (Lp[a]) has structural similarities to low-density lipoprotein (LDL) that include the presence of apolipoprotein B100, there is some disagreement over the strength of its interaction with the LDL receptor and its cellular catabolism by the LDL receptor-mediated pathway. To clarify this subject we evaluated LDL receptor-mediated binding and degradation of Lp(a) and LDL in three human cell lines. The binding of 50 nmol/L Lp(a) at 37 degrees C to the LDL receptor of primary hepatocytes, macrophages, and fibroblasts was only 10%, 29%, and 29% of the respective value obtained with 50 nmol/L LDL. Analysis of 4 degrees C binding curves indicated that Lp(a) and LDL had equal affinities for the LDL receptor of fibroblasts, whereas maximal binding of Lp(a) was remarkably lower than that of LDL. LDL receptor-mediated degradation of 50 nmol/L Lp(a) in hepatocytes, macrophages, and fibroblasts was only 17%, 22%, and 26%, respectively, of the value obtained with 50 nmol/L LDL and varied greatly among the cells in that it was lowest in hepatocytes, an order of magnitude greater in macrophages, and two orders of magnitude greater in fibroblasts. In contrast, the nonspecific degradation rate of Lp(a) was similar to that of LDL in each of the three tested cell lines. However, the proportion of the degradation of Lp(a) that was nonspecific varied greatly, being 76%, 58%, and 33% in hepatocytes, macrophages, and fibroblasts, respectively. These studies indicate that not only is Lp(a) recognized by the LDL receptor but also that, in fibroblasts, Lp(a) and LDL have equal affinities for the LDL receptor, although Lp(a) has a much lower receptor occupancy than LDL. Additionally, they show that there are great cellular differences in the LDL receptor-mediated degradation of Lp(a). If these results can be extrapolated in vivo, where normal LDL levels are 40- to 50-fold higher than those of Lp(a), it would be unlikely that the hepatic LDL receptor is significantly involved in the degradation of Lp(a).

Dissection of compartments in rat hepatocytes involved in the intracellular trafficking of high-density lipoprotein particles or their selectively internalized cholesteryl esters

The trafficking of apolipoprotein E-deficient high-density lipoprotein particles and of their component cholesteryl esters in rat hepatocytes was studied. Human high-density lipoprotein 3, labeled with two nondegradable, intracellularly trapped tracers in their apolipoprotein A-I and their cholesteryl esters, were injected into rats, and five subcellular hepatocytic fractions were isolated at various time intervals. In control experiments with homologous lipoproteins, doubly labeled rat high-density lipoproteins depleted of apolipoprotein E were used. In endosomes and lysosomes the two labels were recovered at near unity, indicating that high-density lipoproteins are endocytosed as particles, transported to early and late endosomes and finally subjected to lysosomal degradation. No significant amounts of label were found in receptor-recycling endosomes. In contrast to label of those of low-density lipoproteins, label of component protein and cholesteryl esters of high-density lipoproteins from isolated endosomes floated at different densities in gradient ultracentrifugation, indicating early disintegration of high-density lipoprotein particles. In contrast to the endocytic organelles, in the whole liver, label of high-density lipoprotein-associated cholesteryl esters exceeded the label of high-density lipoprotein-associated apolipoprotein A-I twofold to threefold. This finding is compatible with selective uptake of high-density lipoprotein cholesteryl esters in addition to uptake of high-density lipoprotein particles. The excess cholesteryl esters accumulated in a nonendosomal fraction, whose major proteins differed from the integral proteins of endosomes. These data suggest two distinct intracellular routes of hepatocytic high-density lipoprotein trafficking in vivo. High-density lipoproteins free of apolipoprotein E are internalized intact by hepatocytes, are predominantly transported to early and late endosomes and are finally subjected to lysosomal degradation. High-density lipoprotein particles do not undergo retroendocytosis in hepatocytes. In addition, high-density lipoprotein-associated cholesteryl esters can be taken up by hepatocytes selectively. They, however, accumulate in a nonendosomal, nonlysosomal compartment.

Effect of lithocholic acid feeding on plasma lipoproteins and binding of radioiodinated human lipoproteins to hepatic membranes in rats

1. Male Sprague-Dawley rats fed diets containing 0.25% lithocholic acid for 6 weeks exhibited elevated serum cholesterol. 2. The rats were fed diets containing 5 or 20% fat with and without the lithocholate and/or oxytetracycline-HCl. 3. The cholesterol elevation was associated with high density lipoprotein (HDL) and not very low density lipoprotein (VLDL) or low density lipoprotein (LDL). 4. Specific binding of human [125I]HDL to hepatic membranes was lowered in lithocholate-fed rats, but binding of human [125I]LDL to these membranes was not affected.

Characteristics of the binding of human and bovine high-density lipoproteins by bloodstream forms of the African trypanosome, Trypanosoma brucei brucei

Bloodstream forms of Trypanosoma brucei brucei are unable to synthesize cholesterol but appear to bind and take up plasma low-density lipoproteins (LDL) from their host. Whether cholesterol homeostasis of this unicellular parasite also requires interactions with host high-density lipoprotein (HDL) particles is unknown. Equilibrium binding of radioiodinated apolipoprotein E-depleted human HDL3 (d = 1.125-1.21 g/ml) and bovine HDL (d = 1.063-1.21 g/ml) by T.b.brucei was rapid (less than 30 min) at 4 degrees C and was characterized by a saturable, specific component. There were five times the number of high-affinity binding sites for human HDL3 as for bovine HDL (64,000 vs. 11,500 per trypanosome) and their binding affinity was greater with an equilibrium dissociation constant (Kd) of 157 nM compared to 315 nM for bovine HDL). Binding of rat and rabbit HDL3 was similar to bovine HDL. By contrast, equilibrium binding of human LDL was slower (approximately 6 h) and the number of high-affinity binding sites (Kd = 23 nM) was much lower for this ligand (660 per trypanosome). Total binding of HDL3 was independent of divalent cations and was only slightly inhibited by heparin, but when the trypanosomes were preincubated with trypsin or pronase the binding was markedly reduced. After 30 min at 37 degrees C, binding of bovine HDL and human HDL3 was 10-20% higher than at 4 degrees C; after 45 min trypanolysis occurred with human HDL3 but not with bovine HDL. Chemical modification of HDL3 by treatment with cyclohexanedione, by acetylation or by reductive alkylation had little effect on its ability to compete with [125I]labelled HDL3 for binding by the parasite. Nitrosylation of HDL3 with tetranitromethane increased its binding ability, suggesting that trypanosomes might possess scavenger receptors, and native HDL3 was less effective than nitrosylated HDL3 in displacing bound [125I]labelled nitrosylated HDL3. These findings suggest that, in addition to a receptor for LDL, T.b.brucei has other lipoprotein binding sites which separately recognize HDL from permissive host species such as bovine, trypanolytic HDL such as human HDL3, and more negatively charged HDL particles such as nitrosylated HDL3.

Visualization of the interaction of native and modified lipoproteins with parenchymal, endothelial and Kupffer cells from human liver

The interaction of low density lipoprotein, acetylated low density lipoprotein and apolipoprotein E-free high density lipoprotein with parenchymal, endothelial and Kupffer cells of human liver was visualized. For this purpose, the fluorescent phospholipid analog 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate was used to label the lipoproteins. The involvement of both parenchymal and nonparenchymal cells in the uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-labeled low density lipoprotein and acetylated low density lipoprotein was studied using in vitro perfusion of human liver tissue blocks. In addition, primary hepatocyte cultures were used to visualize the interaction with 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-labeled apolipoprotein E-free high density lipoprotein and (modified) low density lipoprotein. 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-low density lipoprotein showed a time-dependent and concentration-dependent interaction with both hepatocytes and Kupffer cells, although the intensity of the interaction with parenchymal cells varied strongly among the liver donors. Uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-low density lipoprotein by both cell types was strongly inhibited by the presence of excess unlabeled low density lipoprotein in the (perfusion) medium. Methylation and hydroxyacetaldehyde treatment of low density lipoprotein prevented the uptake of low density lipoprotein. This indicated that the uptake of low density lipoprotein by Kupffer and parenchymal cells was mediated by the low density lipoprotein receptor. 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-acetylated low density lipoprotein was mainly taken up in situ by liver endothelial cells and by a minor population of Kupffer cells. Polyinosinic acid, a known inhibitor of the scavenger receptor, prevented the uptake by liver endothelial cells. Therefore human liver endothelial cells express active scavenger receptors on their surface. Apolipoprotein E-free 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-high density lipoprotein was found to be associated with the membrane of cultured liver parenchymal cells but was not taken up intracellularly, indicating a cholesterol exchange process occurring extracellularly at the plasma membrane. The cellular localization of lipoprotein receptors and uptake of the various classes of lipoproteins are comparable with the situation in rats.

Chemical cross-linking alters high-density lipoprotein to be recognized by a scavenger receptor in rat peritoneal macrophages

Rat peritoneal macrophages possess a surface receptor for high-density lipoprotein (HDL). To obtain the functional aspect of the HDL receptor, the present study was undertaken to modify HDL with three different cross-linkers; dimethylsuberimidate, disuccinimidylsuberate and dithiobissuccinimidylpropionate (DSP) and determine their effect on the ligand activity for the HDL receptor. Upon modification at a low reagent concentration, DSP was found to be most effective in cross-linking of HDL apolipoproteins. The ligand activity of DSP-HDL for the HDL receptor was reduced by greater than 60%. Experiments with these macrophages at 37 degrees C showed; (i) the amounts of the cell-associated [125I]DSP-HDL as 3.5-fold higher than [125I]HDL; (ii) the cell-association of [125I]DSP-HDL was effectively (greater than 70%) inhibited by unlabeled DSP-HDL, whereas HDL showed a partial inhibition (30%); (iii) [125I]DSP-HDL underwent chloroquine-sensitive intracellular degradation; and (iv) DSP-HDL induced a 3-fold increase in the incorporation of [14C]oleic acid into cholesteryl oleate when compared with unmodified HDL. Experiments at 0 degrees C showed that the cellular binding of [125I]DSP-HDL was competed by acetylated low-density lipoprotein and dextran sulfate. These findings indicate that DSP-HDL is recognized as a ligand by a scavenger receptor of rat peritoneal macrophages, a notion consistent with HDL modified with tetranitromethane (Kleinherenbrink-Stins, M.F. et al. (1989) J. Lipid Res. 39, 511-520).

Characterization in vitro of interaction of human apolipoprotein E-free high density lipoprotein with human hepatocytes

Characterization of the interaction of iodinated apolipoprotein (apo) E-free high density lipoprotein (HDL) with cultured human hepatocytes provides evidence for a saturable, Ca2(+)-independent, high affinity binding site with an apparent km value of 20 micrograms/ml of apolipoprotein. Nitrated HDL and low density lipoprotein (LDL) did not compete for the binding of HDL, in contrast to very low density lipoprotein (VLDL). It is suggested that VLDL competition is exerted by the presence of apo Cs. Degradation of HDL was relatively low and in some cases not detectable. In cases where degradation was found, inhibitors of the lysosomal pathway of protein degradation had no effect, while LDL degradation was inhibited more than 80%. In the presence of 10 microM of monensin, the cell-association of HDL was unaffected, but the degradation was inhibited by 30%. Under similar conditions, LDL association was inhibited by 40% and LDL degradation, by 90%. Incubation of human hepatocytes with fluorescently labeled HDL (Dil-HDL) revealed (in contrast to Dil-LDL) mainly strong membrane-bound fluorescence and hardly any labeling of small intracellular vesicles. It is concluded that human hepatocytes possess a specific high affinity site for human HDL with recognition properties similar to those described earlier on rat hepatocytes. No evidence that the binding of HDL is actively coupled to uptake and lysosomal degradation could be obtained, indicating that binding of LDL and HDL to human hepatocytes is coupled differently to intracellular pathways.

Effect of apoprotein cross-linking on the metabolism of human HDL3 in rat

Apo E-free human high-density lipoprotein (HDL3) was labeled with 125I in apoprotein and with 3H in cholesteryl linoleyl ether (a non-hydrolyzable analogue of cholesteryl ester). The labeled HDL3 was modified by cross-linking of apoproteins with dimethylsuberimidate (DMS) to inhibit binding to HDL specific receptors. The control and the DMS HDL3 were characterized with respect to their rate of clearance from rat blood, in vivo binding to major rat organs and in vitro binding to purified rat liver plasma membranes. Both 125I and 3H labels from control HDL3 were cleared from rat blood monoexponentially, but 3H at a faster rate than 125I (3H t1/2 = 3.0-4.1 h; 125I t1/2 = 7.0-7.7 h). This difference is consistent with reports of the nonendocytotic selective uptake of HDL-associated cholesteryl ester. DMS modification did not affect the rate of 3H clearance whereas it increased the rate of 125I clearance (HDL3 t1/2 = 7.7 h; DMS HDL3 t1/2 = 4.1 h). Both in vivo binding to rat organs and in vitro binding to rat liver membranes confirmed that DMS modification inhibited the specific binding of HDL, but also suggested that the modification produced saturable binding of HDL to a separate class of sites. Thus, the present data do not rule out the involvement of direct HDL-cell interaction in the selective uptake of HDL cholesteryl ester. However, results suggest that the binding of HDL to its specific cell surface sites is not necessary for this uptake.

Detection and characterization of anionic polypeptide fraction binding sites in rat liver plasma membranes and cultured hepatocytes

The binding of human 125I-labeled 'anionic polypeptidic fraction' (APF) to purified rat liver plasma membranes was studied. The dissociation constant for this binding was 3.0 micrograms protein/mg membrane protein. Binding was competitively inhibited by unlabeled human APF, but not by human LDL (low density lipoproteins). When unlabeled HDL3 was added, binding of labeled APF was competitively reduced to a level between that of unlabeled APF and unlabeled LDL. Experiments with cultured rat hepatocytes confirmed those obtained with liver membranes and suggested the presence in rat liver of saturable APF-binding sites which seem to be specific for APF. The physiologic significance of these APF binding sites is discussed in relation to the fate of cholesterol in the liver.

Effects of monocyte macrophages stimulation on hepatic lipoprotein receptors

The first series of in vivo experiments were designed to investigate the effects of monocytic macrophages (MM phi) stimulation by zymosan in cholesterol-fed rats. We found that the MM phi stimulation significantly decreased plasma very-low-density lipoprotein-cholesterol, low-density lipoprotein-cholesterol but not high-density lipoprotein-cholesterol. The hepatic and aortic cholesterol levels were also significantly decreased; meanwhile, the biliary total bile acid and fecal sterol excretion were significantly enhanced. These results were beneficial to the prevention and regression of atherosclerosis. The second series of in vitro experiments led to the discovery that zymosan did not have effect on HDL and LDL binding, uptake and degradation of hepatic parenchymal and nonparenchymal cells isolated from normal rats, but did have significant effects on those isolated from cholesterol-fed rats. The experiments of Kupffer cells modulating hepatocytes also demonstrated that hepatocyte HDL receptor activity was significantly enhanced by conditioned medium from acetylated LDL plus zymosan added to Kupffer cells. Bmax of 125I-labeled HDL specific binding was increased from 237.8 to 295.2 ng/mg cell protein. The Ka value was not affected, indicating that there might be an increment in receptor number, but not receptor affinity. Cholesterol-loaded zymosan-stimulated Kupffer cells might secrete a soluble mediator affecting hepatocyte HDL receptor activity. Zymosan and other MM phi-stimulating reagents are promising in the exploration of a new approach for prevention and treatment of hypercholesterolemia and atherosclerosis.

Characterization of the specific binding of rat apolipoprotein E-deficient HDL to rat hepatic plasma membranes

We have used a preparation of rat liver plasma membranes to study the binding of rat apolipoprotein E-deficient HDL to rat liver. The membranes were found to bind HDL by a saturable process that was competed for by excess unlabeled HDL. The binding was temperature-dependent and was 85% receptor-mediated when incubated at 4, 22 and 37 degrees C. The affinity of the binding site for the HDL was consistent at all temperatures, while the maximum binding capacity increased at higher temperatures. The specific binding of HDL to the membranes did not require calcium and was independent of the concentration of NaCl in the media. The effect of varying the pH of the media on HDL binding was small, being 30% higher at pH 6.5 than at pH 9.0. Both rat HDL and human HDL3 were found to compete for the binding of rat HDL to the membranes, whereas rat VLDL remnants and human LDL did not compete. At 4 degrees C, complexes of dimyristoylphosphatidylcholine (DMPC) and apolipoproteins A-I, A-IV and the C apolipoproteins, but not apolipoprotein E, competed for HDL binding to the membranes. At 22 and 37 degrees C, all DMPC-apolipoprotein complexes competed to a similar extent, DMPC vesicles that contained no protein did not compete for the binding of HDL. These results suggest that the rat liver possesses a specific receptor for apolipoprotein E-deficient HDL that recognizes apolipoproteins A-I, A-IV and the C apolipoproteins as ligands.

Inhibition of the binding of low density lipoproteins to liver membrane receptors by rat serum phosphorylcholine binding protein

Rat serum phosphorylcholine binding protein (PCBP) is characterized by its Ca2+ dependent property to bind phosphorylcholine ligand. PCBP immobilized on sepharose has been shown to selectively bind human plasma apo B and E containing lipoproteins. The present report describes an inhibitory effect of PCBP on the binding of human 125I-LDL to LDL receptors on estradiol treated rat liver membranes. Pre-incubation of liver membranes with PCBP did not affect the binding of 125I-LDL to the membranes. Gel filtration analysis of the incubation products from the LDL-receptor assay showed a concentration dependent binding of 125I-PCBP to LDL. The inhibitory effect of PCBP is likely due to the formation of LDL-PCBP complex and not due to the binding of PCBP to the LDL receptor site.

Specific high affinity binding and degradation of high density lipoproteins by isolated epithelial cells of human small intestine

The interaction of high density lipoproteins (HDL) with isolated epithelial cells of human small intestine (enterocytes) was studied. 125I-HDL3 (density = 1,125 to 1,126 g/cm3) exhibit a high-affinity (Kd = 8.3 X 10(-8). Bmax = 886 ng/mg cell protein), saturable and reversible binding to isolated enterocytes. In the presence of excess unlabeled HDL3, the cell surface-bound 125I-HDL3 are released into the medium nondegraded. Treatment of cells with pronase does not affect 125I-HDL3 binding. The binding is accompanied with internalization and degradation of 125I-HDL3. Chloroquine inhibits the degradation and increases 125I-HDL3 uptake. A threefold excess of HDL3 and HDL2 inhibits the binding and degradation of 125I-HDL3 by 60%, whereas a 20-fold excess of low density lipoproteins (LDL), only by 20%. HDL3 (20 to 1,000 micrograms/mL) stimulates the synthesis of sterols and inhibits sterol ester synthesis in enterocytes. The obtained results make it possible to assume that epithelial cells of the small intestine may participate in the catabolism of HDL in human organism.

Specific saturable binding of rat high-density lipoproteins to rat kidney membranes

The binding of rat 125I-labelled high-density lipoprotein (HDL) to rat kidney membranes was studied using HDL fractions varying in their apolipoprotein E content. The apolipoprotein E/apolipoprotein A-I ratio (g/g) in the HDL fractions ranged from essentially 0 to 1.5. All these HDL preparations showed the same binding characteristics. The saturation curves, measured at 0 degrees C in the presence of 2% bovine serum albumin, consisted of two components: low-affinity non-saturable binding and high-affinity binding (Kd about 40 micrograms of HDL protein/ml). Scatchard analyses of the high-affinity binding suggest a single class of non-interacting binding sites. These sites could be purified together with the plasma membrane marker enzyme 5'-nucleotidase. The binding of rat HDL to rat kidney membranes was not sensitive to high concentrations of EDTA, relatively insensitive to pronase treatment and influenced by temperature. The specific binding of rat HDL was highest at acid pH and showed an additional optimum at pH 7.5. On a total protein basis unlabelled rat VLDL competed as effectively as unlabelled rat HDL for binding of 125I-labelled rat HDL to partially purified kidney membranes. Rat LDL, purified by chromatography on concanavalin A columns and human LDL did not compete. Unlabelled human HDL was a much weaker competitor than unlabelled rat HDL and the maximal specific binding of 125I-labelled human HDL was only 10% of the value for 125I-labelled rat HDL.

Characteristics of lipoprotein receptors of the isolated liver parenchymal cells prepared from the streptozotocin-induced diabetic rats

Characteristics of lipoprotein receptors of the isolated liver parenchymal cells prepared from the streptozotocin-induced diabetic rats were investigated. Streptozotocin-induced diabetic rats fed 1.0% cholesterol showed the exaggerated hypercholesterolemia as compared to control rats fed 1.0% cholesterol. The present study was designed to elucidate the role of lipoprotein receptor mechanisms of liver parenchymal cells in the diabetic dyslipoproteinemia. 125I-labeled lipoproteins (rat beta-VLDL, human LDL2 or rat HDL3) were incubated with liver parenchymal cells isolated by liver perfusion using collagenase. According to the Scatchard analysis, the apparent dissociation constant (kd) and maximum beta-VLDL binding (Bmax) for the higher affinity binding site in the diabetic rats (n = 6) were (11.9 +/- 5.1) X 10(2) ng/ml and 307.5 +/- 145.2 ng/10(6) cells, respectively. These binding characteristics of the diabetic rats were not significantly different from the control rats. Furthermore, there were no significant differences in the binding characteristics of human LDL2 and rat HDL3 between the diabetic rats and the control rats. The data presented suggest that significant role of alteration of lipoprotein receptor characteristics in liver parenchymal cells is not played in the diabetic dyslipoproteinemia.

Discrepancies in the catabolic pathways of human and rat high-density lipoprotein apolipoprotein A-I in the rat

The in vivo metabolism in the rat of radioiodinated human and rat high-density lipoprotein was compared with a double-label procedure using 125I and 131I. While rat high-density lipoprotein showed a biphasic serum decay, human high-density lipoprotein was characterized by a monoexponential serum decay. No differences were observed between the serum decay of human high-density lipoprotein-2 and -3 subfractions, isolated by rate zonal ultracentrifugation. The catabolic sites of human and rat high-density lipoprotein were analysed using the lysosomal cathepsin inhibitor leupeptin. Radioiodinated rat high-density lipoprotein was catabolized by the kidneys and by the liver. In contrast, radioiodinated human high-density lipoprotein was catabolized almost exclusively in the liver. No difference in the catabolic sites of human high-density lipoprotein-2 and -3 subfractions was observed. The catabolic sites of human high-density lipoprotein apolipoprotein A-I in the rat were further analysed using the O-(4-diazo-3-[125I]iodobenzoyl) sucrose label. Compared with rat high-density lipoprotein apolipoprotein A-I, the kidneys played a minor role in the catabolism of human high-density lipoprotein apolipoprotein A-I. It is concluded that in the rat the catabolic pathways of the apolipoprotein A-I moieties of rat and human high-density lipoproteins are different, indicating that homologous high-density lipoproteins should be used for the investigation of in vivo metabolism.

Characterization of a human hepatic receptor for high density lipoproteins

Characterization of the membrane receptor for the low density lipoproteins (LDL) has led to insights into cellular receptor physiology as well as mammalian lipid transport. Result with LDL have stimulated the search for specific receptors for other plasma lipoproteins. Receptors for high density lipoproteins (HDL) have been identified in human fibroblasts and smooth muscle cells. Specificity for this receptor has been difficult to define since normal HDL contains several apolipoproteins, and particles containing apolipoproteins B and E have been shown to compete for HDL binding. In the present study, we demonstrate that HDL isolated from a patient devoid of apolipoprotein E was bound specifically by human hepatic membranes. This binding reached saturation within 2 hours and was EDTA-resistant. Assuming a single receptor model, we found that 2.9 x 10(15) receptors/mg membrane protein bound with an affinity KD = 3.5 x 10(-7) M at 0 to 4 degrees C and KD = 1.9 x 10(-7) M at 37 degrees C. The binding was effectively competed with intact HDL3, with HDL3 that had undergone selective arginine and lysine residue modification, and with antibodies to apolipoproteins A-I and A-II. However, LDL, asialofetuin, and HDL3 which had undergone tyrosine modification by nitration, and anti-apolipoprotein B did not compete with apo A-I HDL binding. In contrast to LDL binding, the human hepatoma cell line, HEPG2, increased HDL binding with cholesterol loading that was specific for HDL3. Thus, hepatic tissue can modulate its recognition of HDL. Finally, hepatic membranes from a patient lacking normal hepatic LDL receptors bound apo A-I HDL normally. These data indicate that a saturable, specific regulatable receptor for apo E-free HDL is present in human liver.

Reversible high affinity uptake of apo E-free high density lipoproteins in cultured pig hepatocytes

We examined the high affinity binding, uptake, and degradation of apo E-free 125I-high density lipoprotein (HDL) in cultured pig hepatocytes. At steady state, the cells degraded 9.4% of cell-associated 125I-HDL/hour, compared with 41.7%/hour for 125I-LDL. Pulse-chase experiments at 4 degrees C revealed that high affinity 125I-HDL binding was reversible. Similar experiments at 37 degrees C revealed that about 70% of the cell-associated 125I-HDL was released as a macromolecule; the remainder was degraded to acid-soluble products. In contrast, over 75% of the 125I-LDL that was released had been degraded to acid soluble products. The amount of macromolecular 125I-HDL released at 37 degrees C was similar to the amount that was bound to the cell surface, as estimated from measurements of trypsin-releasable radioactivity. Density gradient ultracentrifugation and SDS-polyacrylamide gel electrophoretic analysis of macromolecular 125I-HDL released to the medium revealed an increase in density, and the apparent partial proteolysis of apo A-I (Mr 25,000) to products of apparent Mr 12,000-14,000. The findings suggest that high affinity 125I-HDL uptake had a reversible component in which HDL was concentrated temporarily at the cell surface, modified, and then released as a somewhat denser lipoprotein particle. Measurement of 125I-HDL and 125I-LDL degradation in cell homogenates revealed no difference in the inherent susceptibility of the two lipoproteins to proteolysis by lysosomal enzymes. The overall slower rate of degradation of 125I-HDL compared to 125I-LDL was therefore due in part to the smaller fraction of HDL that was committed to irreversible catabolism. The rate of catabolism of this fraction, however, was considerable. Cells pulsed at 4 degrees C and subsequently warmed to 37 degrees C released one-half the acid-soluble products from 125I-HDL within about 4 hours, compared with 2 hours for cells pulsed with 125I-LDL. These findings indicate that HDL was internalized, transported to lysosomes, and degraded at about one-half the rate of LDL.

Binding and degradation of human high-density lipoproteins by human hepatoma cell line HepG2

The catabolism of human HDL was studied in human hepatoma cell line HepG2. The binding of 125I-labeled HDL at 4 degrees C was time-dependent and reached completion within 2 h. The observed rates of binding of 125I-labeled HDL at 4 degrees C and uptake and degradation at 37 degrees C indicated the presence of both high-affinity and low-affinity binding sites for this lipoprotein density class. The specific binding of 125I-labeled HDL accounted for 55% of the total binding capacity. The lysosomal degradation of 125I-labeled HDL was inhibited 25 and 60% by chloroquine at 50 and 100 microM, respectively. Depolymerization of microtubules by colchicine (1 microM) inhibited the degradation of 125I-labeled HDL by 36%. Incubation of cells with HDL caused no significant change in the cellular cholesterol content or in the de novo sterol synthesis and cholesterol esterification. Binding and degradation of 125I-labeled HDL was not affected by prior incubation of cells with HDL. When added at the same protein concentration, unlabeled VLDL, LDL and HDL had similar inhibitory effects on the degradation of 125I-labeled HDL, irrespective of a short or prolonged incubation time. Reductive methylation of unlabeled HDL had no significant effect on its capacity to inhibit the 125I-labeled HDL degradation. The competition study indicated no correlation between the concentrations of apolipoproteins A-I, A-II, B, C-II, C-III, E and F in VLDL, LDL and HDL and the inhibitory effect of these lipoprotein density classes on the degradation of 125I-labeled HDL. There was, however, some association between the inhibitory effect and the levels of apolipoprotein D and C-I.

A year-long study of changes induced by thyroidectomy in the plasma lipid and lipoprotein spectrum in the European badger

Hypothyroidism is associated with hypercholesterolemia and increased risk for atherosclerotic disease. The European badger exhibits large seasonal changes in thyroid activity and the annual minimum of plasma thyroxine level in this species occurs at the same period of the year (i.e. late fall) as a pronounced hypercholesterolemia. We examined the plasma lipid and lipoprotein spectrum in a group of thyroidectomized male badgers every month for a year. Non-operated animals were used as controls. Our analyses included measurement of plasma lipid levels, density gradient ultracentrifugation of lipoproteins, electrophoresis of lipoproteins and apolipoproteins, and histological studies. Maximal differences between the two groups of animals were observed during spring, occurring concomitantly with the annual maximum of plasma thyroxine concentration in control badgers. Comparison with the latter animals revealed a permanent hypercholesterolemia and hyperphospholipidemia in thyroidectomized badgers, while their lipoprotein spectrum was characterized by the continual presence of elevated concentrations of cholesterol-rich lipoproteins of d congruent to 1.015 - 1.027 g/ml. The ratio of triglyceride/cholesteryl ester content in such lipoproteins remained constant throughout the year, resembling that noted in intact animals during late fall. Other features distinguishing the lipoprotein spectrum in thyroidectomized badgers were: (1) higher levels of lipoproteins with d 1.027 - 1.065 g/ml and d 1.065 - 1.100 g/ml, and (2) a cholesteryl ester enrichment of both these lipoprotein subclasses. The two groups of animals shared a heterogeneity of low density lipoprotein subfractions isolated on density gradients, together with the presence of apolipoproteins with molecular weights respectively typical of human apolipoproteins A-I and B throughout the low density range. Arterial walls and heart tissues from intact and thyroidectomized animals were free of atherosclerotic lesions at the end of the experimental period.

Renal handling of plasma high density lipoprotein

Renal tubular reabsorption and hydrolysis of plasma high density lipoprotein (HDL3) were studied. Rabbit proximal straight nephron segments were microperfused in vitro with iodinated HDL3. Progressive luminal uptake and cellular accumulation of radiolabeled material were observed during an initial phase, followed by a reduction in sequestration and the appearance of 125I-label in the bathing medium. To detect proteolysis, collected perfusates and bathing media were analyzed for trichloracetic acid soluble radioactivity. 125I-HDL3 in the luminal fluid was intact, but metabolites appeared in the bathing medium. Electron microscopic radioautography demonstrated endocytic uptake of 125I-HDL3 at the luminal membrane of the proximal tubule and movement of grains into lysosome-like dense bodies. Incubation of radiolabeled HDL3 in the presence of renal homogenates resulted in proteolytic activity with an acidic pH optimum. Analytical cell fractionation studies indicated that hydrolysis of the protein component is associated with lysosomes derived from proximal kidney tubules. Collectively, the data show that plasma HDL3 can be reabsorbed in the proximal nephron by a mechanism involving endocytosis at the luminal membrane, followed by proteolysis at lysosomes.

Characterization of high-density lipoprotein binding sites in rat liver and testis membranes

The binding of human 125I-labeled HDL3 to purified rat liver and testis plasma membranes was studied. About 50-65% of the total HDL binding in these membranes was abolished by 1% bovine serum albumin in the incubation medium. The remaining albumin-insensitive binding sites, determined in the presence of albumin were associated with plasma membranes; a good correlation was found between the 125I-labeled HDL3 binding and the 5'-nucleotidase activities of the membrane fractions. The binding sites in these tissues were saturable, specific for HDL (not competed for by LDL) and had similar affinities for 125I-labeled HDL3 (Kd, 11.8 micrograms protein/ml for liver and 12.7 micrograms protein/ml for testis membranes); the maximum binding capacity of the testis membranes was higher (1.3 vs. 0.7 microgram protein/mg membrane protein). Egg phosphatidylcholine complexes of both human apolipoprotein A-I and apolipoprotein C's competed for the HDL-binding sites, but phosphatidylcholine vesicles alone did not. Chemical modification of the lysine and arginine residues of apolipoproteins did not affect the interaction of HDL3 with its binding sites. Despite the fact that the HDL-binding sites in these tissues are not specific for apolipoprotein A-I, they may have important physiological roles in lipid transport, as they appear to recognize apolipoprotein-phospholipid complexes.

Cigarette smoking impairs hepatic uptake of high density lipoproteins

The effect of chronic inhalation of cigarette smoke on hepatic uptake of high density lipoproteins (HDL) in White Carneau pigeons was examined. Four treatment groups included: 1) Shelf Control birds fed a chow diet and retained in their cages; 2) Sham pigeons fed a cholesterol-saturated fat diet and exposed to fresh air by a smoking machine; 3) Low nicotine-low carbon monoxide (LoLo) animals also fed the cholesterol diet and exposed to low concentrations of these cigarette smoke products; and 4) High nicotine-high carbon monoxide (HiHi) birds fed the cholesterol diet and subjected to high concentrations of these components. Livers from both smoke exposed groups contained significantly more triglyceride than those from Sham animals while livers from HiHi birds alone had elevated concentrations of protein. Liver slices from LoLo and HiHi pigeons incorporated significantly less HDL 3H free and esterified cholesterol and HDL 14C apoprotein from media during in vitro incubation than livers from Sham birds. Impaired hepatic uptake of HDL suggests a permanent alteration in liver function resulting from chronic exposure to tobacco smoke and may represent one mechanism by which cigarette smoking attenuates HDL's anti-atherogenic properties.

Binding properties of high-density lipoprotein subfractions and low-density lipoproteins to rabbit hepatocytes

Primary cultures of rabbit hepatocytes which were preincubated for 20 h in a medium containing lipoprotein-deficient serum subsequently bound, internalized and degraded 125I-labeled high-density lipoproteins2 (HDL2). The rate of degradation of HDL2 was constant in incubations from 3 to 25 h. As the concentration of HDL2 in the incubation medium was increased, binding reached saturation. At 37 degrees C, half-maximal binding (Km) was achieved at a concentration of 7.3 micrograms of HDL2 protein/ml (4.06 X 10(-8)M) and the maximum amount bound was 476 ng of HDL2 protein/mg of cell protein. At 4 degrees C, HDL2 had a Km of 18.6 micrograms protein/ml (1.03 X 10(-7)M). Unlabeled low-density lipoproteins (LDL) inhibited only at low concentrations of 125I-labeled HDL2. Quantification of 125I-labeled HDL2 binding to a specific receptor (based on incubation of cells at 4 degrees C with and without a 50-fold excess of unlabeled HDL) yielded a dissociation constant of 1.45 X 10(-7)M. Excess HDL2 inhibited the binding of both 125I-labeled HDL2 and 125I-labeled HDL3, but excess HDL3 did not affect the binding of 125I-labeled HDL3. Preincubation of hepatocytes in the presence of HDL resulted in only a 40% reduction in specific HDL2 receptors, whereas preincubation with LDL largely suppressed LDL receptors. HDL2 and LDL from control and hypercholesterolemic rabbits inhibited the degradation of 125I-labeled HDL2, but HDL3 did not. Treatment of HDL2 and LDL with cyclohexanedione eliminated their capacity to inhibit 125I-labeled HDL2 degradation, suggesting that apolipoprotein E plays a critical role in triggering the degradative process. The effect of incubation with HDL on subsequent 125I-labeled LDL binding was time-dependent: a 20 h preincubation with HDL reduced the amount of 125I-labeled LDL binding by 40%; there was a similar effect on LDL bound in 6 h but not on LDL bound in 3 h. The binding of 125I-labeled LDL to isolated liver cellular membranes demonstrated saturation kinetics at 4 degrees C and was inhibited by EDTA or excess LDL. The binding of 125I-labeled HDL2 was much lower than that of 125I-labeled LDL and was less inhibited by unlabeled lipoproteins. The binding of 125I-labeled HDL3 was not inhibited by any unlabeled lipoproteins. EDTA did not affect the binding of either HDL2 or HDL3 to isolated liver membranes. Hepatocytes incubated with [2-14C]acetate in the absence of lipoproteins incorporated more label into cellular cholesterol, nonsaponifiable lipids and total cellular lipid than hepatocytes incubated with [2-14C]acetate in the presence of any lipoprotein fraction. However, the level of 14C-labeled lipids released into the medium was higher in the presence of medium lipoproteins, indicating that the effect of those lipoproteins was on the rate of release of cellular lipids rather than on the rate of synthesis.

Human high density lipoprotein (HDL3) binding to rat liver plasma membranes

The binding of human 125I-labeled HDL3 to purified rat liver plasma membranes was studied. 125I-labeled HDL3 bound to the membranes with a dissociation constant of 10.5 micrograms protein/ml and a maximum binding of 3.45 micrograms protein/mg membrane protein. The 125I-labeled HDL3-binding activity was primarily associated with the plasma membrane fraction of the rat liver membranes. The amount of 125I-labeled HDL3 bound to the membranes was dependent on the temperature of incubation. The binding of 125I-labeled HDL3 to the rat liver plasma membranes was competitively inhibited by unlabeled human HDL3, rat HDL, HDL from nephrotic rats enriched in apolipoprotein A-I and phosphatidylcholine complexes of human apolipoprotein A-I, but not by human or rat LDL, free human apolipoprotein A-I or phosphatidylcholine vesicles. Human 125I-labeled apolipoprotein A-I complexed with egg phosphatidylcholine bound to rat liver plasma membranes with high affinity and saturability, and the binding constants were similar to those of human 125I-labeled HDL3. The 125I-labeled HDL3-binding activity of the membranes was not sensitive to pronase or phospholipase A2; however, prior treatment of the membranes with phospholipase A2 followed by pronase digestion resulted in loss of the binding activity. Heating the membranes at 100 degrees C for 30 min also resulted in an almost complete loss of the 125I-labeled HDL3-binding activity.

Regulation of high density lipoprotein levels

Current concepts of the structure and metabolism of high density lipoproteins are presented and factors that influence their levels in human beings are surveyed.

Relative contributions of parenchymal and non-parenchymal (sinusoidal) liver cells in the uptake of chylomicron remnants

The relative contributions of parenchymal cells and non-parenchymal (sinusoidal) cells to the in vivo hepatic uptake of chylomicron remnants was measured 30 min after intravenous injection into rats. The chylomicron remnants were labeled with [3H]leucine, which was almost exclusively present in apolipoprotein B. The isolated non-parenchymal cells (a mixture of Kupffer cells and endothelial cells) contained 6.7 times more apolipoprotein B radioactivity per mg cell protein than the isolated parenchymal cells. It was calculated that the contributions of non-parenchymal and parenchymal liver cells to the total hepatic uptake of chylomicron remnants are 35% and 65%, respectively.

High density lipoprotein utilization by dispersed rat luteal cells

Utilization of lipoproteins by cells prepared by collagenase dispersion of ovaries of immature gonadotropin-primed rats was studied. Human and rat HDL increased basal progestin secretion and incorporation of [14C]oleate into cellular sterol esters 2-fold during a 2 h incubation, with maximal stimulation occurring at a lipoprotein sterol concentration of 125 micrograms/ml. This concentration of HDL cholesterol also increased progestin production by cells stimulated with dibutyryl cyclic AMP. Human LDL or cholesterol-rich lipid dispersions had little effect upon either progestin secretion or sterol esterification at similar sterol concentrations. However, addition of delipidated human HDL apolipoproteins to the cholesterol-rich lipid dispersions markedly enhanced progestin production. Incubation of the dispersed cells in the presence of 25 micro M ML-236B, which blocked cellular de novo sterol synthesis by over 90%, had no effect upon progestin secretion. Specific uptake of human 125I-labeled HDL by the dispersed cells was observed. Analysis fo 125I-labeled HDL uptake as a function of lipoprotein concentration indicated that the uptake process was saturated at HDL levels of 200-400 micrograms protein/ml. The amount of HDL specifically associated with the cells at saturating levels after 1 h of incubation was sufficient to account for the increased progestin synthesis and sterol ester storage observed during this time. During the incubations cell-specific degradation of the 125I-labeled HDL apolipoprotein appeared to be minimal. We conclude that lipoprotein-carried cholesterol is an important substrate for rat luteal cells and that these cells possess a specific mechanism for the uptake of HDL.

The binding of high density lipoproteins to isolated rat hepatocytes

The association of 125I-labelled rat high density lipoproteins (125-I-HDL) to suspended rat hepatocytes was studied at 4 degrees C. 125 I-HDL associated to isolated hepatocytes by two processes-one of high and one of low affinity. The cell-association of 125I-HDL exhibited saturation kinetics and was inhibited to varying degrees with both rat and human lipoproteins such as VLDL, LDL and HDL but not by lipoprotein deficient serum or asialo-fetuin. The cell-association of 125I-HDL did not require divalent cations and could be reduced by pronase treatment of the cells. The binding site was clearly different from the receptor for LDL in extrahepatic cells since heparin and apolipoprotein E did not compete and cholesterol ester labelled HDL. The number of binding sites for HDL at 4 degrees C was 2.2 X 10(6) per cell and the association constant (Ka) 8.2 X 10(6) (mol/l)-1. Experiments with HDL labelled with [3H] cholesterol by means of lecithin: cholesterol acyltransferase (LCAT, EC 2.3.1.43) in the cholesterol ester moiety suggested that the same mechanism was responsible for the cell-association of HDL prepared this way.