Apparent loss of apolipoprotein B immunoreactivity of isolated low density lipoprotein during storage: consequences for the quantification of apolipoprotein B in human plasma

Cryopreservation and long-term storage of human low density lipoproteins

A simple and effective technique of long-term storage of human low density lipoproteins (LDL) has been developed. The technique involves the addition of 1.4 mol/l (or 10% by volume) dimethyl sulphoxide directly to a solution of the freshly isolated LDL in high salt buffer, and subsequent freezing and storage for up to 2 years at -70 degrees C. We have shown that freshly isolated LDL, "preserved" as described above, are able to keep their native properties for a long period, i.e.: a) electrophoretic behaviour in non-denaturing (or with sodium dodecyl sulphate, 1 milligram) polyacrylamide 2-16% gradient gel electrophoresis; b) immunoreactivity of apolipoprotein B (analyzed by radial immunodiffusion, electroimmunoassay and immunoturbidimetric assay); c) immunogeneity of apolipoprotein B; d) an average size of LDL particles (analyzed by electron microscopy); e) ability to bind with B,E-receptors of human skin fibroblasts. The technique can also be applied to radiolabelled LDL samples. Taking into consideration the labour- and time-consuming procedure of obtaining and characterizing LDL, and the preferred use of single well-characterized LDL preparation, we recommend that the above technique of LDL long-term storage be applied in various clinical and biomedical studies.

The binding of human lipoprotein lipase treated VLDL by the human hepatoma cell line HepG2

It has been suggested that besides the LDL-receptor, hepatocytes possess an apo E or remnant receptor. To evaluate which hepatic lipoprotein receptor is involved in VLDL remnant catabolism, we studied the binding of VLDL remnants to HepG2 cells. Native VLDL was obtained from type IIb hyperlipidemic patients and treated with bovine milk lipoprotein lipase (LPL). This LPL-treated VLDL (LPL-VLDL) was used as representative for VLDL remnants. Our results show that LPL-VLDL binds with high affinity to HepG2 cells. Competition experiments showed that the binding of 125I-labelled LPL-VLDL is inhibited to about 30% of the control value by the simultaneous addition of an excess of either unlabelled LDL or LPL-VLDL. Preincubation of HepG2 cells with LDL resulted in a reduction of the binding of LDL and LPL-VLDL to 34 and 55% of the control value, whereas preincubation of the cells with heavy HDL (density between 1.16 and 1.21 g/ml) stimulated the binding of LDL and LPL-VLDL to about 230% of the control value. Preincubation of the cells with insulin (250 nM/l) also stimulated the binding of both LDL and LPL-VLDL (175 and 143% of the control value, respectively). We conclude that LPL-VLDL binds to the LDL-receptor of HepG2 cells and that no evidence has been obtained for the presence on HepG2 cells of an additional receptor that is involved in the binding of VLDL remnants.

Estimation of turnover of low density lipoproteins by simplified methods

This study was carried out to evaluate simplified methods for estimating parameters of kinetics of low density lipoproteins (LDL). Initially, LDL turnover studies were carried out in 78 patients, and fractional catabolic rates (FCRs) for LDL were determined from die-away curves of plasma radioactivity and from urine-to-plasma (U/P) ratios of radioactivity. When U/P ratios were calculated on a daily basis and throughout the study and averaged by a weighting procedure, the weighted U/P ratios were highly correlated with plasma decay FCRs (r = 0.92, P less than 0.001). One simplified method involved calculation of FCR for LDL from the U/P ratio on the seventh day after injection of radioactivity. Seventh-day U/P ratios also were correlated significantly with plasma decay FCRs (P less than 0.001), but the strength of correlation between the two was relatively weak (r = 0.52). Finally, another correlation was found to be significant. This was the plasma decay FCR vs. the fraction of injected dose disappearing from plasma during the first 24 h (r = 0.86). This comparison was extended to 140 turnover studies, and the correlation remained high (r = 0.90, P less than 0.0001). Thus, estimation of FCR for LDL from the fraction of dose disappearing at 24 h appears superior to that determined from the 7th-day U/P ratio.

Regulation of low density lipoprotein receptor activity in primary cultures of human hepatocytes by serum lipoproteins

The low density lipoprotein receptor activity was measured in primary cultures of human hepatocytes. The receptor-mediated association and degradation of low density lipoprotein increased gradually up to 140 and 190%, respectively, upon incubation of the cells with increasing amounts of whole serum (up to 100%). Preincubation of the cells with low density lipoprotein resulted in a weak downregulation of the receptor-mediated association of low density lipoprotein (only 35% reduction at 100 micrograms low density lipoprotein per ml). However, preincubation with high density lipoproteins with density between 1.16 and 1.20 gm per ml (heavy high density lipoprotein) resulted in a more than 2-fold stimulation of the receptor-mediated association of low density lipoprotein. This heavy high density lipoprotein-mediated stimulation could not be antagonized by a simultaneous addition of low density lipoprotein during that preincubation. We conclude that, in primary cultures of human hepatocytes, the downregulation of the low density lipoprotein receptor activity by low density lipoprotein is weak and completely overruled by heavy high density lipoprotein. If these results for human hepatocytes in vitro hold true for hepatocytes in vivo, our results might explain why in vivo liver cells still display low density lipoprotein receptor activity notwithstanding the exposure of these cells to physiological concentrations of low density lipoprotein.

Stimulation of the LDL receptor activity in the human hepatoma cell line Hep G2 by high-density serum fractions

The regulation of the LDL receptor activity in the human hepatoma cell line Hep G2 was studied. In Hep G2 cells, in contrast with fibroblasts, the LDL receptor activity was increased 2.5-fold upon increasing the concentration of normal whole serum in the culture medium from 20 to 100% by volume. Incubation of the Hep G2 cells with physiological concentrations of LDL (up to 700 micrograms/ml) instead of incubation under serum-free conditions resulted in a maximum 2-fold decrease in LDL receptor activity (10-fold decrease in fibroblasts). Incubation with physiological concentrations of HDL with a density of between 1.16 and 1.20 g/ml (heavy HDL) resulted in an approximately 7-fold increase in LDL receptor activity (1.5-fold increase in fibroblasts). This increased LDL receptor activity is due to an increase in the number of LDL receptors. Furthermore, simultaneous incubation of Hep G2 cells with LDL and heavy HDL (both 200 micrograms/ml) resulted in a 3-fold stimulation of the LDL receptor activity as compared with incubation in serum-free medium. 3-Hydroxy-3-methylglutaryl-CoA reductase activity was also stimulated after incubation of Hep G2 with heavy HDL (up to 3-fold). The increased LDL receptor activity in Hep G2 cells after incubation with heavy HDL was independent of the action of lecithin:cholesterol acyltransferase during that incubation. However, previous modification of heavy HDL by lecithin:cholesterol acyltransferase resulted in an enhanced ability of heavy HDL to stimulate the LDL receptor activity. Our results indicate that in Hep G2 cells the heavy HDL-mediated stimulation of the LDL receptor activity overrules the LDL-mediated down-regulation and raises the suggestion that in man the presence of heavy HDL and the action of lecithin:cholesterol acyltransferase in plasma may be of importance in receptor-mediated catabolism of LDL by the liver.

Quantitation of apolipoprotein B by polyclonal and monoclonal antibodies

A non-competitive sandwich enzyme linked immunosorbent assay for the quantitation of apolipoprotein B with polyclonal and monoclonal antibodies was developed. Polyclonal antibodies were used as 'coater'. In the assay with polyclonal antibodies, the same antibody was used as conjugate with alkaline phosphatase. For studies with monoclonal antibodies, enzyme conjugated anti-mouse immunoglobulin had to be used, since monoclonal antibodies lost their reactivity upon enzyme conjugation. Two murine monoclonal antibodies were employed: MAB B-1 with specificity for apolipoproteins (Apo) B-48 and B-100 and MAB B-5 with specificity for B-100 (Radioimmunoassay Inc.). In a reference group Apo B values of 0.82 +/- 0.20 g/l were measured with polyclonal antibodies, 0.68 +/- 0.19 g/l and 0.95 +/- 0.33 g/l with MAB B-1 and MAB B-5. In pure hypercholesterolemia, a similar increase was found with all three antibodies, while in combined hyperlipoproteinemia MAB B-5 gave greater than 40% lower values. Differences were also found with respect to the correlation between Apo B and cholesterol or triglycerides.

High-affinity uptake and degradation of acetylated low density lipoprotein by confluent human vascular endothelial cells

Confluent human vascular endothelial cells take up and degrade acetylated low density lipoproteins (Ac-LDL) via a high-affinity binding process, comparable to that for native LDL. The degradation of 125I-labelled Ac-LDL by endothelial cells was inhibited by the addition of unlabelled Ac-LDL but not by the addition of unlabelled LDL. Similarly, the degradation of 125I-labelled LDL could be inhibited by unlabelled LDL but not by unlabelled Ac-LDL. Unlabelled apolipoprotein E-free HDL did not compete with the degradation of either 125I-labelled LDL or Ac-LDL. Electron microscopical studies, using gold-labelled LDL and gold-labelled Ac-LDL, showed that at 4 degrees C both LDL and Ac-LDL bind to indented regions of the endothelial cell plasma membrane. At 37 degrees C both LDL and Ac-LDL were taken up and associated with lysosomes. Although morphologically identical, we conclude that the binding of LDL and Ac-LDL to human endothelial cells proceeds via 2 different high-affinity receptors. The uptake and degradation of Ac-LDL by human endothelial cells was about 25 and 15%, respectively, of that of native LDL. The uptake and degradation of either LDL or Ac-LDL did not lead to a massive increase in cellular cholesterol content.

A comparative study of the effects of acipimox and clofibrate in type III and type IV hyperlipoproteinemia

Acipimox, an analogue of nicotinic acid, is a hypolipidemic drug with antilipolytic activity. Ten patients with type III and 10 with type IV hyperlipoproteinemia participated in a comparative open cross-over study of the effect of acipimox (750 mg/day) and clofibrate (2 g/day) on lipoproteins, apoliproproteins and postheparin lipase activities during 6 weeks. During acipimox treatment 2 type III patients complained of flushing, resulting in one drop-out. In the type III patients serum cholesterol decreased 30% (P less than 0.01) during treatment with acipimox and 24% (P less than 0.01) with clofibrate, and serum triglycerides 48% (P less than 0.01) and 34% (P less than 0.01), respectively. In the type IV patients serum cholesterol remained unchanged and serum triglycerides decreased 34% (P less than 0.05) and 35% (P less than 0.01), respectively. HDL cholesterol increased during treatment with both drugs in both groups between 6 and 15% (P less than 0.05) mainly due to a rise in HDL3 cholesterol (d greater than 1.100 g/ml). LDL cholesterol increased significantly during treatment with clofibrate, but not with acipimox. There were no or slight changes in the apoproteins A and B. Postheparin lipoprotein lipase increased during clofibrate treatment and hepatic lipase decreased during acipimox treatment. We concluded that acipimox in a dose of 750 mg/day has a similar hypolipidemic effect as 2 g clofibrate daily in type III and IV hyperlipoproteinemia.

Low density lipoprotein metabolism by endothelial cells from human umbilical cord arteries and veins

Binding and metabolism of low density lipoprotein (LDL) and acetylated LDL were examined in endothelial cells from human umbilical cord arteries and veins. Both high and low affinity LDL interactions were observed. High affinity LDL binding and catabolism were increased five- to sevenfold after preincubation for 18 hours in LPDS containing medium. Subconfluent cells degraded, endocytosed, and bound 1.5 to 2.7 times more LDL by high affinity interaction than confluent cells, when endothelial cell growth supplement (ECGS) was present in the culture system. In the absence of ECGS, these ratios were somewhat less. Low affinity LDL metabolism was less affected by the state of confluency. Binding of LDL and acetylated LDL by venous endothelial cells was more than two- and threefold, respectively, than that by comparable arterial cells. However, the difference in LDL binding was not reflected in an altered LDL catabolism. There apparently is a population of low affinity binding sites not involved in LDL catabolism. LDL metabolism was identical in cells, which were cultured in medium supplemented with 20% to 100% serum or hirudin- or heparin-treated platelet-poor plasma. Without preincubation in LPDS, high affinity adsorptive endocytosis mediated the main part of LDL uptake only at low LDL concentrations (5 to 20 micrograms protein/ml). However, at physiological LDL concentrations (550 micrograms/ml), we estimated that this process mediated only 17% of the LDL uptake. We calculated that fluid endocytosis and low affinity adsorptive endocytosis of LDL accounted for the remaining 12% and 70%, respectively, of the LDL uptake at physiological LDL concentrations.

The metabolism in vitro of human low-density lipoprotein by the human hepatoma cell line Hep G2

The human hepatoma cell line Hep G2 was studied with respect to metabolism of human low-density lipoprotein (LDL). The Hep G2 cells bind, take up and degrade human LDL with a high-affinity saturable and with a low-affinity non-saturable component. The high-affinity binding possesses a KD of 25 nM-LDL and a maximal amount of binding of about 70 ng of LDL-apoprotein/mg of cell protein. The high-affinity binding, uptake and degradation of LDL by Hep G2 cells is dependent on the extracellular Ca2+ concentration and is down-regulated by the presence of fairly high concentrations of extracellular LDL. Incubation of the Hep G2 cells with LDL results in suppression of the intracellular cholesterol synthesis. It is concluded that the human hepatoma cell line Hep G2 possesses specific LDL receptors similar to the LDL receptors demonstrated on extrahepatic tissue cells.