Control Low Density Lipoprotein Research Articles

Conversion of human low density lipoprotein into a very low density lipoprotein-like particle in vitro.

The lipid substrate specificity of Manduca sexta lipid transfer particle (LTP) was examined in in vitro lipid transfer assays employing high density lipophorin and human low density lipoprotein (LDL) as donor/acceptor substrates. Unesterified cholesterol was found to exchange spontaneously between these substrate lipoproteins, and the extent of transfer/exchange was not affected by LTP. By contrast, transfer of labeled phosphatidylcholine and cholesteryl ester was dependent on LTP in a concentration-dependent manner. Facilitated phosphatidylcholine transfer occurred at a faster rate than facilitated cholesteryl ester transfer; this observation suggests that either LTP may have an inherent preference for polar lipids or the accessibility of specific lipids in the donor substrate particle influences their rate of transfer. The capacity of LDL to accept exogenous lipid from lipophorin was investigated by increasing the high density lipophorin:LDL ratio in transfer assays. At a 3:1 (protein) ratio in the presence of LTP, LDL became turbid (and aggregated LDL were observed by electron microscopy) indicating LDL has a finite capacity to accept exogenous lipid while maintaining an overall stable structure. When either isolated human non B very low density lipoprotein (VLDL) apoproteins or insect apolipophorin III (apoLp-III) were included in transfer experiments, the sample did not become turbid although lipid transfer proceeded to the same extent as in the absence of added apolipoprotein. The reduction in sample turbidity caused by exogenous apolipoprotein occurred in a concentration-dependent manner, suggesting that these proteins associate with the surface of LDL and stabilize the increment of lipid/water interface created by LTP-mediated net lipid transfer. The association of apolipoprotein with the surface of modified LDL was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, and scanning densitometry revealed that apoLp-III bound to the surface of LDL in a 1:14 apoB:apoLp-III molar ratio. Electron microscopy showed that apoLp-III-stabilized modified LDL particles have a larger diameter (29.2 +/- 2.6 nm) than that of control LDL (22.7 +/- 1.9 nm), consistent with the observed changes in particle density, lipid, and apolipoprotein content. Thus LTP-catalyzed vectorial lipid transfer can be used to introduce significant modifications into isolated LDL particles and provides a novel mechanism whereby VLDL-LDL interrelationships can be studied.

Glycated low density lipoprotein catabolism is increased in rabbits with alloxan-induced diabetes mellitus.

Hyperglycaemia in diabetes mellitus is responsible for the process of non-enzymatic glycosylation of different proteins. Since we did not find elevated glycated apolipoprotein B levels in diabetic patients, an altered glycated apolipoprotein B metabolism was suspected in diabetic patients. Experiments in normal rabbits showed that non-reductive (in vitro) glycated low density lipoprotein (gly-LDL) was cleared at a slower rate than control LDL and thus stayed longer in the circulation (vascular mean residence time: 10 vs 8 h, p less than 0.001). The body mean residence time for gly-LDL was 22 h vs 17 h for control LDL. In diabetic animals the catabolic parameters of both LDL preparations changed towards a faster clearance, the effect being greatest for gly-LDL (total mean residence times of gly-LDL pre-diabetic: 19 h, diabetic: 16 h; control LDL pre-diabetic and diabetic: 14 h). The difference in clearance between glycated and control LDL was thus strongly reduced. Virtually no antibody complexed to gly-LDL could be measured. The results suggest an increased activity of the non-receptor mediated pathway in diabetes mellitus, possibly co-responsible for an increased atherosclerotic risk.

Effects of Shosaikoto (kampo medicine) on lipid metabolism in macrophages.

We investigated effects of Shosaikoto treatment on cholesterol metabolism in macrophages. Although macrophages, harvested from mice treated with Shosaikoto, took up a small amount of control low density lipoprotein (LDL) (thiobarbituric acid-reactive substance (TBA-RS) value was 0.27 pmol/mg of protein) as control macrophages, they took up more LDL modified with CuSO4 (TBA-RS value was 6.12 pmol/mg of protein) than control macrophages. Degradation of both control LDL and oxidized LDL was enhanced in Shosaikoto treated macrophages. In the presence of control LDL or in the absence of LDL, incorporation of [3H]oleic acid into chlesteryl oleate was significantly reduced in Shosaikoto treated macrophages. This suggests that acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity in macrophages was partly inhibited by Shosaikoto treatment. On the other hand, in the present of oxidized LDL, cholesteryl ester accumulated in Shosaikoto treated macrophages as much as in controls. However, cholesteryl oleate efflux from macrophages in the presence of high density lipoprotein (HDL) was enhanced in Shosaikoto treated macrophages. These result indicate that Shosaikoto facilitates oxidized LDL catabolism in macrophages, resulting in the augmentation of oxidized LDL uptake and the elimination of cholesterol from macrophages by HDL. These Shosaikoto effects may prevent foam cell formation and the progression of atherosclerotic lesions.

Lipoproteins activate acyl-coenzyme A:cholesterol acyltransferase in macrophages only after cellular cholesterol pools are expanded to a critical threshold level.

Activation of acyl-CoA:cholesterol actyltransferase (ACAT) in macrophages by lipoproteins is a key event in atheroma foam cell formation. To help elucidate the mechanisms whereby lipoproteins stimulate ACAT, the early cellular events of lipoprotein-induced ACAT stimulation were studied in mouse peritoneal macrophages. As a function of increasing lipoprotein-cholesterol influx to the cell during the first few hours of incubation, ACAT activity was markedly stimulated by beta-very low density lipoprotein (beta-VLDL) and acetyl-low density lipoprotein (acetyl-LDL) only after lipoprotein-cholesterol influx reached a threshold level of approximately 25% above the basal cell cholesterol content. In contrast, LDL stimulated ACAT only minimally at this level of lipoprotein-cholesterol influx. In further experiments, the source of ACAT cholesterol substrate during the initial stimulation of ACAT was shown to be a mixture of cellular (approximately 75%) and lipoprotein-cholesterol (approximately 25%) in proportions that approximated the proportions of originally cellular and lipoprotein-cholesterol in the cell. Thus, lipoprotein-cholesterol rapidly mixed with most or all of cellular cholesterol before ACAT esterification. Additional studies showed that LDL caused significant efflux of cellular cholesterol, thus providing at least a partial explanation for the relatively weak ACAT stimulatory potential of LDL. To support this idea, LDL that was modified to decrease its ability to induce net cellular cholesterol efflux stimulated ACAT 2-fold greater than control LDL when matched for lysosomal LDL-cholesterol influx. Moreover, when the effective efflux potentials of beta-VLDL and acetyl-LDL were increased, ACAT stimulation was markedly decreased despite unchanged lipoprotein-cholesterol influx. Thus, macrophage ACAT is stimulated not directly by the influx of newly hydrolyzed lipoprotein-cholesterol but rather by net expansion of cellular cholesterol pools to a particular threshold level. This scheme has potentially important implications regarding the cellular and molecular mechanisms of foam cell formation.

Ingestion of eicosapentaenoic acid-ethyl ester renders rabbit LDL less susceptible to Cu 2+-catalyzed-oxidative modification

The present study revealed that low density lipoprotein (LDL) obtained from rabbits fed highly purified eicosapentaenoic acid-ethyl ester (EPA-E) (EPA-LDL) was significantly less susceptible to oxidative modification catalyzed by Cu2+ as compared to LDL from rabbits fed a normal diet (control LDL). In a comparison of fatty acid composition of LDL, the contents of EPA and docosapentaenoic acid were significantly increased in EPA-LDL as compared to control LDL. The content of arachidonic acid (AA) was the same in both LDL groups. The contents of total cholesterol, free cholesterol, triglyceride, and phospholipid in LDL were unchanged by EPA-E ingestion. These data raise the possibility that EPA may protect LDL from oxidative modification, leading to a reduction of oxidized LDL in atherosclerotic lesions, and that this may be how EPA exert its anti-atherosclerotic action.

Detection of two species of low density lipoprotein particles in cholesteryl ester transfer protein deficiency.

By equilibrium density gradient ultracentrifugation, we analyzed the chemical composition and particle size of low density lipoproteins (LDLs) in 16 subfractions separated from the LDL fractions (1.019 less than d less than 1.063 g/ml) of two hyperalphalipoproteinemic patients who had a deficiency of cholesteryl ester transfer protein (CETP). The LDLs of these patients comprised a group of heterogeneous lipoprotein particles distributed almost equally in a wide density range from d = 1.025 g/ml to d = 1.053 g/ml, whereas LDLs from normal controls were a homogeneous group of lipoprotein particles distributed in a narrow density range from d = 1.030 g/ml to d = 1.046 g/ml. The LDL in each subfraction derived from the patients' plasma samples was poor in cholesteryl ester and rich in triglycerides and apolipoproteins. Each subfraction of normal control LDL contained only one species of homogeneous LDL particles, which progressively decreased in size with an increase in the density of the fraction. In contrast, each subfraction of patient LDL contained two species of LDL particles: smaller LDLs existed, in addition to those that were found to be identical to the normal control LDL particles observed in the corresponding subfractions. The intermediate density lipoproteins of the two patients were also composed of two species of lipoproteins. From these results, we speculate that two metabolic pathways may exist in the LDL formation process. In this process, the transfer of cholesteryl ester from high density lipoproteins by CETP may convert the smaller lipoprotein particles to the larger ones, forming the homogeneous LDL species.

Calcium Antagonists Prevent Monocyte and Endothelial Cell-Induced Modification of Low Density Lipoproteins

Low density lipoprotein (LDL) incubated in the presence of the calcium antagonists verapamil, nifedipine and flunarizine were more resistant than control LDL to human monocyte- or endothelial cell-induced modification, as assessed by electrophoretic mobility in agarose gel, thiobarbituric acid reactive substance content, and degradation by J774 macrophages. The efficiency of the drugs was: flunarizine greater than nifedipine greater than verapamil. Moreover, a 24 h preculture with calcium antagonists significantly impaired the ability of cells to modify LDL in the absence of the drugs. All the studied drugs also inhibited copper-induced autooxidation of LDL. None of the studied calcium antagonists, at concentrations up to 10(-4) M, significantly reacted with free radicals as assessed by the 1,1-diphenyl-2-picrylhydrazyl test. It is suggested that such a protective effect of calcium antagonists against LDL peroxidation could play a role in the previously reported antiatherogenic effect of these drugs.

Conformation of apolipoprotein B-100 in the low density lipoproteins of tangier disease. Identification of localized conformational response to triglyceride content.

The low density lipoproteins (LDL) from patients with Tangier disease are enriched in triglycerides, 27% of LDL mass versus 7% for normal LDL. To study whether this unique LDL core lipid composition affects the surface disposition of apolipoprotein (apo) B-100, we analyzed the LDL by protease digestion and in competitive radioimmunoassays. Limited proteolytic digestion of Tangier LDL by Staphylococcus aureus V8 protease generated a prominent fragment of 120 kDa (cleavage site at residue 1076), which was not visible in similarly digested normal LDL. In competitive radioimmunoassay, Tangier LDL bound weakly to the apoB-specific monoclonal antibody MB20, compared with control LDL. We localized the MB20 epitope between residues 1031 and 1084 of apoB-100, probably very near residue 1076. DNA sequencing of exon 21 of apoB genomic clones (coding for residues 1014-1084) from a Tangier patient revealed no difference from the normal DNA sequence, thus eliminating a protein polymorphism as a basis for the altered protease sensitivity and antibody binding. When the triglyceride contents of Tangier LDL were reduced to 10% of mass by incubation with normal high density lipoproteins, production of the 120-kDa fragment by proteolysis decreased and MB20 binding increased in affinity, implying a change toward normal conformation of apoB-100. Thus, using two independent techniques, proteolytic digestion and binding of monoclonal antibodies, we have demonstrated an alternative conformation of apoB-100 in the vicinity of residue 1076, which reflects the content of triglycerides in the LDL particle.

Macrophage proteases can modify low density lipoproteins to increase their uptake by macrophages

When low density lipoprotein (LDL) was incubated with sonicated macrophages at acidic pH, its protein moiety was partially degraded by cathepsins B and D. The reisolated LDL was taken up by intact macrophages up to about 20 times as fast as control LDL. LDL proteolysis and its enhanced uptake could be inhibited almost entirely by the selective protease inhibitors leupeptin and pepstatin. If macrophages in atherosclerotic lesions were to release acidic proteases (either by exocytosis or following cell death) and these were to modify LDL, this may help to explain why so much cholesteryl ester accumulates in these cells.

Hydrophobic Surfactant Inhibits Hypercholesterolemia in Pair-fed Rabbits on a Cholesterol-Free, Low-Fat Diet

Poloxalene, a hydrophobic surfactant, is known to prevent hypercholesterolemia in animals fed a high-fat, high-cholesterol diet. It has not been demonstrated, however, whether this agent is of benefit when hypercholesterolemia is induced in animals by means other than the feeding of a high-fat diet. In this study, hypercholesterolemia was produced in rabbits by feeding a low-fat, cholesterol-free diet with dietary protein supplied by casein for a period of 8 weeks. Controls were given this diet without poloxalene and experimentals were given the diet with poloxalene. Total serum cholesterol levels increased in both groups, but the rise was greater for the control group. Lipoprotein analysis performed at the conclusion of the study showed significantly greater low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol levels in the control group as compared to the experimental group. Total protein and apolipoprotein B (apo B) were also greater in control LDL. It was concluded that poloxalene favorably affects this model of hypercholesterolemia as total serum cholesterol, LDL cholesterol, and LDL apo B were all less and the HDL cholesterol to LDL cholesterol ratio was higher in surfactant-treated rabbits.

A membrane fluidizing factor in sera from Duchenne muscular dystrophy patients: effect on lymphocyte membranes of incubation in patient and control lipoproteins.

Sera from Duchenne muscular dystrophy (DMD) patients showed a membrane fluidizing effect on DMD and control lymphocytes. To look for the cause of this membrane fluidizing effect, human lymphocytes from healthy subjects were incubated in sera and in different serum fractions from DMD patients and healthy control subjects, and membrane fluidity was determined by steady state fluorescence polarization of the probe diphenyl hexatriene. DMD sera and total DMD lipoproteins showed a similar membrane fluidizing effect (p less than 0.02) after incubation, whereas lipoprotein deficient serum did not show any effect on membrane fluidity. Control and DMD HDL showed a membrane fluidizing effect, the fluidizing effect of DMD HDL being slightly higher as compared to control HDL. Control LDL and DMD LDL showed a membrane rigidifying effect, the rigidifying effect of DMD LDL being significantly lower as compared to control LDL (p less than 0.02). These data demonstrate that the membrane fluidizing effect of DMD serum is lipoprotein associated, LDL being the most important mediator of this effect.

Large buoyant LDL-like particles in hepatic lipase deficiency.

Hepatic lipase (HL) is thought to play a role in processing very low density lipoprotein to low density lipoprotein (LDL). To analyze the relationship between HL and LDL, the density, size, and chemical composition of LDL isolated from 18 normal subjects and from three subjects with reduced or absent levels of HL activity were compared. In an HL-deficient subject, the major peak of apoprotein (apo) B-containing lipoproteins ('LDL') had a density of 1.023 g/ml and a diameter of 26.4 nm compared to male control subjects (1.044 +/- 0.006 g/ml and 25.3 +/- 0.3 nm). Two half-sisters of the HL-deficient subject with half the normal levels of HL activity had LDL that also were more buoyant and slightly larger than the LDL isolated from female control subjects. The peak density and average diameter of LDL were correlated with HL activity, consistent with the hypothesis that HL influenced formation and physical characteristics of typical LDL. Apo B-100 was the major apoprotein in the 'LDL' isolated from the HL-deficient subject and contained a greater proportion of triglyceride compared to the control subjects' LDL. The absence of HL appears to prevent the production of classical LDL. Our data support the hypothesis that HL helps determine normal LDL characteristics.

Platelet-Modified Low Density Lipoprotein Induces Macrophage Cholesterol Accumulation and Platelet Activation

Low density lipoprotein (LDL), modified by chemical or biological means, was shown to induce macrophage cholesterol accumulation. The cholesterol and protein contents of LDL were decreased (by 10 and 15%, respectively) by incubation of the LDL for 2 h at 37 degrees C with normal washed platelet suspension or with platelet-conditioned medium; these decreases were not affected by platelet activation. The platelet-modified LDL caused a greater increase (by up to 15%) in collagen-induced, in vitro platelet aggregation than control LDL. Incubation of mouse peritoneal macrophages with platelet-modified LDL for 18 h at 37 degrees C resulted in an elevation of the macrophage cholesterol ester content (by 35-50%) as well as an increase in the cholesterol esterification rate (by 40-70%), compared with the effect of control LDL. Macrophage cholesterol synthesis, however, was significantly decreased (by 40-50%), compared with the effect of control LDL. The effect of LDL treated by platelet-conditioned medium was similar to that of platelet-modified LDL. The effect of platelet-modified LDL on macrophage cholesterol esterification was maximal within 24 h of incubation, and it was not significantly affected by inhibition of cholesterol synthesis. The platelet-modified LDL was taken up by the macrophages in a saturable fashion and its uptake was competitively inhibited by LDL, but not by acetylated LDL. We conclude that platelet-modified LDL interacts with the LDL receptor and induces macrophage cholesterol accumulation. Since the modified lipoprotein induces in vitro foam cell formation and platelet activation, platelet-modified LDL could be considered to be pro-atherogenic.

Enhancement of platelet aggregation by low-density lipoproteins from IDDM patients.

Low-density lipoprotein (LDL) is known to enhance platelet sensitivity to some aggregating agents. In this study, we observed that LDL isolated from patients with insulin-dependent diabetes mellitus (IDDM) enhanced thrombin-induced platelet aggregation to a greater extent than LDL isolated from matched controls (P less than .01). Thromboxane B2 production during aggregation was also significantly more enhanced by LDL isolated from IDDM than by control LDL (P less than .01). There was no difference in the lipid composition (free and esterified cholesterol, total phospholipids, and triglycerides) of LDL isolated from diabetic and control subjects. In contrast, the extent of glycosylation of LDL isolated from diabetic patients was significantly greater than that observed in LDL from normal subjects (P less than .01), and a positive correlation (r = .605, P less than .01) between the degree of LDL glycosylation and the rate of platelet aggregation was observed. LDL glycosylated in vitro enhanced thrombin-, collagen-, and adenosine 5'-diphosphate-induced platelet aggregation to a greater extent than control LDL (P less than .01). Although LDL glycosylated in vitro was taken up by platelets to a greater extent than control LDL (P less than .05), the lipid composition (free cholesterol and phospholipid) of platelets was not significantly changed. We postulate that an increased degree of glycosylation of LDL may enhance its uptake by platelets and lead to increased platelet reactivity to aggregating agents, probably by altering the structure of the platelet membrane. The enhancement of platelet aggregation by LDL may contribute to the accelerated development of atherosclerosis in diabetes mellitus.

Oxidized low density lipoprotein stimulates prostacyclin production by adult human vascular endothelial cells.

Interactions between vascular endothelium and low density lipoprotein (LDL) have been implicated in the development of atherosclerosis. The effect of normal and oxidized LDL (Ox-LDL) on prostaglandin release by cultured adult human saphenous vein endothelial cells was investigated. Ox-LDL induced a rapid release of prostacyclin (PGI2) to levels which were several-fold higher than those observed with control LDL. PGI2 release was concentration-dependent and was biphasic, with a first peak occurring within 30 minutes (followed by a decrease), and a second peak occurring after several hours of incubation. PGI2 production was inhibited by lipoprotein-depleted serum and by indomethacin, an antagonist of cyclooxygenase activity. These cells produced mainly PGF2 alpha, with some PGE2 and PGI2 when stimulated by the ionophore A23187 at confluency. However, among these prostanoids, mainly PGI2 was produced in response to Ox-LDL. The data indicate that Ox-LDL induces the production of PGI2 by human vascular endothelial cells. Since Ox-LDL is cytotoxic, this phenomenon may be a manifestation of an early response to injury.

Defective receptor binding of low density lipoprotein from pigs possessing mutant apolipoprotein B alleles.

We previously identified a defect in the in vivo catabolism of low density lipoprotein (LDL) from hypercholesterolemic pigs carrying a mutant apolipoprotein B allele. In the present studies, we examined the in vitro metabolism of mutant LDL in cultured pig fibroblasts. A 3-fold higher concentration of mutant LDL (compared to control) was needed to displace 50% of control 125I-LDL binding. Mutant LDL had a 6-fold higher dissociation constant than control LDL. Scatchard plots of the binding data were concave upward, suggesting multiple classes of binding sites or negative cooperativity. The mutant LDL degradation rate was reduced by 40%; this decrease could be attributed to a dense LDL subspecies. Mutant and control buoyant LDL subspecies were degraded more slowly than the corresponding dense LDL subspecies. Together, these studies show that diminished LDL receptor binding can result from mutations in apolipoprotein B and from changes in the lipid composition of LDL particles.

11 MEMBRANE TOXICITY OF DUCHENNE MUSCULAR DYSTROPHY (DMD) SERUM IS LIPOPROTEIN‐BOUND

A toxic serum factor isolated from sera of DMD patients could be shown to increase the membrane fluidity of DMD and control lymphocytes [Hubner C et al. (1987) Pediatr Res 22:488-492]. We have recently found that isolated DMD lipoproteins have the same effect. Fluorescence anisotropy of diphenylhexatriene in peripheral blood lymphocytes (PBL) was significantly decreased when PBL were incubated for 4 h in DMD lipoproteins as compared to 4 h incubation in control lipoproteins (r=0.168±0.007, n=10 vs. r=0.182 ±0.012, n=9; p<0.02, Mann-Whitney test). Similar results could be obtained after incubation of PBL in the isolated LDL fraction of DMD lipoproteins versus control LDL (r=0.190±0.011, n=9 vs. r=0.206±0.007, n=7; p<0.02). Therefore, the LDL fraction seems to be mainly responsible for the increase of PBL membrane fluidity in DMD patients.

Lipolysis of LDL with phospholipase A2 alters the expression of selected apoB-100 epitopes and the interaction of LDL with cells

To assess the effects of perturbing the surface of low density lipoprotein (LDL) on the conformation of apoB-100, LDL (d 1.030-1.050 g/ml) isolated from normal subjects were treated with phospholipase A2 (PL-A2) for 0.5 to 15 min. The resulting P-LDL and concurrent control LDL (C-LDL) incubated without PL-A2 were isolated by gel permeation chromatography. Approximately 50% of LDL-phosphatidylcholine was hydrolyzed in 2 min and approximately 85% in 5 min. Lysophosphatidylcholine compounds (LPC) and free fatty acids (FFA) accumulated during lipolysis but most of the LPC and all of FFA could be removed by adding FFA-free albumin to the lipolysis mixtures. Immunoreactivities of P-LDL and C-LDL were evaluated in competitive radioimmunoassays, using a library of anti-human LDL monoclonal antibodies directed against the major regions of apoB-100 (the T4, T3, and T2 thrombin fragments). One epitope defined by monoclonal antibody 465B6C3 and localized near the carboxyl end of the apoB-100 molecule became less immunoreactive (ED 50s increased); three other epitopes on the T2 fragment near the LDL receptor recognition site and four epitopes localized towards the middle (T3) and amino terminal (T4) regions did not change. Altered immunoreactivities were not related to LPC and FFA contents. Thus, the conformation of apoB-100 was selectively altered by phospholipolysis. The interactions of P-LDL with cultured fibroblasts were grossly altered: P-LDL were bound nonspecifically to fibroblasts of both normal and homozygous familial hypercholesterolemic subjects and P-LDL were not degraded. LPC and FFA retained in LDL did not explain these alterations, nor did changes of epitope expression near the LDL receptor recognition site. It is likely that the apoB-100 aberrant cell interaction is due to loss of surface phospholipids and "uncovering" of core lipids that react nonspecifically with cell surface components.

Glycosylation of low-density lipoprotein enhances cholesteryl ester synthesis in human monocyte-derived macrophages.

Glucose can react with the lysine residues of low-density lipoproteins (LDLs) and convert the lipoprotein to a form with a receptor-mediated uptake by cultured cells that is impaired. However, in contrast to other modified lipoproteins taken up by both murine and human macrophages via the scavenger-receptor pathway that may induce the formation of foam cells, glycosylated LDL is not recognized by murine macrophages, and thus far, it has not been shown to lead to marked intracellular accumulation of cholesterol in human macrophages. This study illustrates that glycosylated LDL incubated with human monocyte-derived macrophages, at a concentration of 100 micrograms LDL/ml medium, stimulates significantly more cholesteryl ester (CE) synthesis than does control LDL (10.65 +/- 1.5 vs. 4.8 +/- 0.13 nmol.mg-1 cell protein.20 h-1; P less than .05). At LDL concentrations similar to those of plasma, the rate of CE synthesis in macrophages incubated with glycosylated LDL is more markedly enhanced than that observed in cells incubated with control LDL (3-fold increase). The marked stimulation of CE synthesis in human macrophages exposed to glycosylated LDL is paralleled by a significant increase in CE accumulation in these cells (P less than .001). The increase in CE synthesis and accumulation seem to be mediated by an increase in the degradation of glycosylated LDL by human macrophages. Glycosylated LDL enters the macrophages and is degraded by the classic LDL-receptor pathway in slightly smaller amounts than control LDL, but its degradation by pathways other than the classic LDL receptor or scavenger receptor is markedly enhanced.(ABSTRACT TRUNCATED AT 250 WORDS)

Defective catabolism and abnormal composition of low-density lipoproteins from mutant pigs with hypercholesterolemia.

Metabolic and chemical properties of low-density lipoproteins (LDLs) were studied in a strain of pigs carrying a specific apo-B allele associated with hypercholesterolemia and premature atherosclerosis. LDL mass was significantly greater in mutant than in control pigs (400 +/- 55 mg/dL vs 103 +/- 26 mg/dL), as was LDL cholesterol. When normal and mutant LDLs were injected into the bloodstream of normal pigs, the fractional catabolic rate (FCR) of mutant LDL was about 30% lower than that of control LDL. In mutant pigs, the mean FCRs of mutant and control LDL were similar, although they were much lower than the corresponding FCRs observed in normal pigs. The density profile of LDL particles differed in control and mutant pigs; the peak LDL flotation rate was shifted from S0f = 5.3 +/- 1.9 in controls to a more buoyant 7.4 +/- 0.5 in mutants. The elevation of LDL in the mutants was restricted to the most buoyant LDL subspecies. This subpopulation of mutant LDL was enriched with cholesteryl ester (47% vs 37%) and depleted of triglyceride, relative to LDL of similar density and size in controls. The lipid compositions of the denser LDL subpopulations (rho greater than 1.043 g/mL) were similar in mutants and controls. We conclude that the hypercholesterolemia of these mutant pigs is accounted for by defective catabolism of LDL. The buoyant cholesterol ester enriched LDL subspecies that accumulate in plasma may contribute to the accelerated atherogenesis that occurs in these animals.