Increased production of apolipoprotein B and its lipoproteins by oleic acid in Caco-2 cells

The production of lipids, apolipoproteins (apo), and lipoproteins induced by oleic acid has been examined in Caco-2 cells. The rates of accumulation in the control medium of 15-day-old Caco-2 cells of triglycerides, unesterified cholesterol, and cholesteryl esters were 102 +/- 8, 73 +/- 5, and 11 +/- 1 ng/mg cell protein/h, respectively; the accumulation rates for apolipoproteins A-I, B, C-III, and E were 111 +/- 9, 53 +/- 4, 13 +/- 1, and 63 +/- 4 ng/mg cell protein/h, respectively. Whereas apolipoproteins A-IV and C-II were detected by immunoblotting, apoA-II was absent in most culture media. In contrast to an early production of apolipoproteins A-I and E occurring 2 days after plating, the apoB expression appeared to be differentiation-dependent and was not measurable in the medium until the sixth day post-confluency. In the control medium, very low density lipoproteins (VLDL), low density lipoproteins (LDL), high density lipoproteins (HDL), and lipid-poor very high density lipoproteins (VHDL) accounted for 12%, 46%, 18%, and 24% of the total lipid and apolipoprotein contents, respectively. The triglyceride-rich VLDL contained mainly apoE (75%) and apoB (23%), while the protein moiety of LDL was composed of apoB (59%), apoE (20%), apoA-I (15%), and apoC-III (6%). The cholesterol-rich HDL contained mainly apoA-I (69%) and apoE (27%). In the control medium, major portions of apolipoproteins B and C-III (93-97%) were present in LDL, whereas the main parts of apoA-I (92%) and apoE (76%) were associated with HDL and VHDL. Oleate increased the production of triglycerides 10-fold, cholesteryl esters 7-fold, and apoB 2- to 4-fold. There was also a moderate increase (39%) in the production of apoC-III but no significant changes in those of apolipoproteins A-I and E. These increases were reflected mainly in a 55-fold elevation in the concentration of VLDL, and a 2-fold increase in the level of LDL; there were no significant changes in HDL and VHDL. VLDL contained the major parts of total neutral lipids (74-86%), apoB (65%), apoC-III (81%) and apoE (58%). In the presence of oleate, the VLDL, LDL, HDL, and VHDL accounted for 76%, 15%, 3%, and 6% of the total lipoproteins, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)     

Influence of dietary lipids on hepatic mRNA levels of proteins regulating plasma lipoproteins in baboons with high and low levels of large high density lipoproteins.

Selective breeding of baboons has produced families with increased plasma levels of large high density lipoproteins (HDL1) and very low (VLDL) and low (LDL) density lipoproteins when the animals consume a diet enriched in cholesterol and saturated fat. High HDL1 baboons have a slower cholesteryl ester transfer, which may account for the accumulation of HDL1, but not of VLDL and LDL. To investigate the mechanism of accumulation of VLDL + LDL in plasma of the high HDL1 phenotype, we selected eight half-sib pairs of baboons, one member of each pair with high HDL1, the other member with little or no HDL1 on the same high cholesterol, saturated fat diet. Baboons were fed a chow diet and four experimental diets consisting of high and low cholesterol with corn oil, and high and low cholesterol with lard, each for 6 weeks, in a crossover design. Plasma lipids and lipoproteins and hepatic mRNA levels were measured on each diet. HDL1 phenotype, type of dietary fat, and dietary cholesterol affected plasma cholesterol and apolipoprotein (apo) B concentrations, whereas dietary fat alone affected plasma triglyceride and apoA-I concentrations. HDL1 phenotype and dietary cholesterol alone did not influence hepatic mRNA levels, whereas dietary lard, compared to corn oil, significantly increased hepatic apoE mRNA levels and decreased hepatic LDL receptor and HMG-CoA synthase mRNA levels. Hepatic apoA-I message was associated with cholesterol concentration in HDL fractions as well as with apoA-I concentrations in the plasma or HDL. However, hepatic apoB message level was not associated with plasma or LDL apoB levels. Total plasma cholesterol, including HDL, was negatively associated with hepatic LDL receptor and HMG-CoA synthase mRNA levels. However, compared with low HDL1 baboons, high HDL1 baboons had higher concentrations of LDL and HDL cholesterol at the same hepatic mRNA levels. These studies suggest that neither overproduction of apoB from the liver nor decreased hepatic LDL receptor levels cause the accumulation of VLDL and LDL in the plasma of high HDL1 baboons. These studies also show that, in spite of high levels of VLDL + LDL and HDL1, the high HDL1 baboons had higher levels of mRNA for LDL receptor and HMG-CoA synthase. This paradoxical relationship needs further study to understand the pathophysiology of VLDL and LDL accumulation in the plasma of animals with the high HDL1 phenotype.     

ELISA quantitation of apolipoproteins in plasma lipoprotein fractions: ApoE in apoB-containing lipoproteins (Lp B:E) and apoB in apoE-containing lipoproteins (Lp E:B)

Growing clinical evidence suggests that metabolic behavior and atherogenic potential vary within lipoprotein subclasses that can be defined by apolipoprotein variation. Variant constituency of apolipoproteins B and E (apoB and apoE) may be particularly important because of the central roles of these apolipoproteins in the endogeneous lipid delivery cascade. ApoB is the sole protein of low-density lipoprotein (LDL), and like LDL cholesterol, the plasma apoB level has been positively correlated with risk for atherosclerotic disease. ApoE is a major functional lipoprotein in the triglyceride-rich lipoproteins, and may be crucial in the conversion of very low density lipoprotein (VLDL) to LDL. Based on work by others that enabled the quantititation of apoB-containing particles by content of up to two other types of apolipoprotein, we have developed a method for determining the amount of apoE in apoB-containing lipoproteins (Lp B:E) and the amount of apoB in apoE-containing lipoproteins (Lp E:B). From the Lp B:E and Lp E:B concentrations, the molar ratio of apoE to apoB in lipoproteins containing apoB and/or apoE in plasma can be determined. The methodology is fast, specific, and sensitive and should prove extremely useful in further categorizing lipoproteins and characterizing their behavior. In applying this method to clinical groupings of normo- and hyperlipidemia, we found that the plasma triglyceride level correlated with the apoE and Lp B:E concentrations in plasma, while the total cholesterol level correlated with the apoB and Lp E:B levels.     

Secretion of lipids, apolipoproteins, and lipoproteins by human hepatoma cell line, HepG2: effects of oleic acid and insulin

The aim of this study was to determine the effect of oleic acid and insulin on the secretion of lipoproteins by HepG2 cells grown in minimum essential medium. Triglycerides were the major neutral lipid (57% of total) and apoB was the predominant apolipoprotein (56% of total) secreted by these cells. The addition of oleate resulted in a two-fold increase in the concentration of neutral lipids but only a slight to moderate increase in the apolipoprotein (A-I, A-II, B, and E) levels. The secretion of very low density lipoproteins (VLDL) was stimulated by 425%, low density lipoproteins (LDL) by 77%, and high density lipoproteins (HDL) by 68%. Whereas neutral lipid composition of LDL was unchanged, the VLDL particles contained a significantly higher percentage of triglyceride and lower percentages of cholesterol and cholesteryl esters compared with VLDL secreted in the absence of oleate. Oleate had no significant effect on the composition of apolipoproteins in VLDL, LDL and HDL. In basal medium, insulin caused a significant decrease in the secretion of neutral lipids and apolipoproteins, particularly triglycerides and apoB. In addition to a 60-68% reduction in the total concentration of VLDL and LDL, insulin altered their composition by producing particles that had a significantly lower content of triglycerides, contained less apoB, and were deficient in apoE. There were no major changes in the concentration or composition of HDL particles. Insulin had a similar but less pronounced effect on the concentration and composition of lipoproteins secreted in the presence of oleate. The increased accumulation of triglycerides in the HepG2 cells concomitant with their reduced levels in the medium suggests that insulin may affect the secretion rather than synthesis of triglyceride-rich lipoproteins.     

Effects of dietary fats and cholesterol on liver lipid content and hepatic apolipoprotein A-I, B, and E and LDL receptor mRNA levels in cebus monkeys.

The effects of the long-term administration of the dietary fats coconut oil and corn oil at 31% of calories with or without 0.1% (wt/wt) dietary cholesterol on plasma lipoproteins, apolipoproteins (apo), hepatic lipid content, and hepatic apoA-I, apoB, apoE, and low density lipoprotein (LDL) receptor mRNA abundance were examined in 27 cebus monkeys. Relative to the corn oil-fed animals, no significant differences were noted in any of the parameters of the corn oil plus cholesterol-fed group. In animals fed coconut oil without cholesterol, significantly higher (P less than 0.05) plasma total cholesterol (145%), very low density lipoprotein (VLDL) + LDL (201%) and high density lipoprotein (HDL) (123%) cholesterol, apoA-I (103%), apoB (61%), and liver cholesteryl ester (263%) and triglyceride (325%) levels were noted, with no significant differences in mRNA levels relative to the corn oil only group. In animals fed coconut oil plus cholesterol, all plasma parameters were significantly higher (P less than 0.05), as were hepatic triglyceride (563%) and liver apoA-I (123%) and apoB (87%) mRNA levels relative to the corn oil only group, while hepatic LDL receptor mRNA (-29%) levels were significantly lower (P less than 0.05). Correlation coefficient analyses performed on pooled data demonstrated that liver triglyceride content was positively associated (P less than 0.05) with liver apoA-I and apoB mRNA levels and negatively associated (P less than 0.01) with hepatic LDL receptor mRNA levels. Liver free and esterified cholesterol levels were positively correlated (P less than 0.05) with liver apoE mRNA levels and negatively correlated (P less than 0.025) with liver LDL receptor mRNA levels. Interestingly, while a significant correlation (P less than 0.01) was noted between hepatic apoA-I mRNA abundance and plasma apoA-I levels, no such relationship was observed between liver apoB mRNA and plasma apoB levels, suggesting that the hepatic mRNA of apoA-I, but not that of apoB, is a major determinant of the circulating levels of the respective apolipoprotein. Our data indicate that a diet high in saturated fat and cholesterol may increase the accumulation of triglyceride and cholesterol in the liver, each resulting in the suppression of hepatic LDL receptor mRNA levels. We hypothesize that such elevations in hepatic lipid content differentially alter hepatic apoprotein mRNA levels, with triglyceride increasing hepatic mRNA concentrations for apoA-I and B and cholesterol elevating hepatic apoE mRNA abundance.     

Linkage analysis of the genetic determinants of high density lipoprotein concentrations and composition: evidence for involvement of the apolipoprotein A-II and cholesteryl ester transfer protein loci

We have tested for evidence of linkage between the genetic loci determining concentrations and composition of plasma high density lipoproteins (HDL) with the genes for the major apolipoproteins and enzymes participating in lipoprotein metabolism. These genes include those encoding various apolipoproteins (apo), including apoA-I, apoA-II, apoA-IV, apoB, apoC-I, apoC-II, apoC-III, apoE, and apo(a), cholesteryl ester transfer protein (CETP), HDL-binding protein, lipoprotein lipase, and the low density lipoprotein (LDL) receptor. Polymorphisms of these genes, and nearby highly polymorphic simple sequence repeat markers, were examined by quantitative sib-pair linkage analysis in 30 coronary artery disease families consisting of a total of 366 individuals. Evidence for linkage was observed between a marker locus D16S313 linked to the CETP locus and a locus determining plasma HDL-cholesterol concentration (P = 0.002), and the genetic locus for apoA-II and a locus determining the levels of the major apolipoproteins of HDL, apoA-I and apoA-II (P = 0.009 and 0.02, respectively). HDL level was also influenced by the variation at the apo(a) locus on chromosome 6 (P = 0.02). Thus, these data indicate the simultaneous involvement of at least two different genetic loci in the determination of the levels of HDL and its associated lipoproteins.     

PPARalpha deficiency increases secretion and serum levels of apolipoprotein B-containing lipoproteins.

This study investigates the importance of peroxisome proliferator activated receptor alpha (PPARalpha) for serum apolipoprotein B (apoB) levels and hepatic secretion of apoB-containing lipoproteins. Total serum apoB and VLDL-apoB levels were higher in female PPARalpha-null mice compared with female wild-type mice, but no difference was seen in male mice. Furthermore, hepatic triglyceride secretion rate, determined in vivo after Triton WR1339 injection, was 2.4-fold higher in female PPARalpha-null mice compared with female wild-type mice, but no difference was observed in male mice. However, when fed a high fat diet, male PPARalpha-null mice displayed 2-fold higher serum levels of apoB and LDL cholesterol compared with male wild-type mice, but triglyceride levels were not affected. Hepatic LDL receptor protein levels were not influenced by PPARalpha deficiency, gender, or the fat diet. Hepatocyte cultures from female PPARalpha-null mice (cultured for 4 days in serum free medium) showed 2-fold higher total apoB secretion and increased secretion of apoB-48 VLDL, as well as 2.7-fold larger accumulation of VLDL-triglycerides in the medium compared with wild-type cultures. In conclusion, PPARalpha-deficient female mice, but not males, display high serum apoB associated with VLDL and increased hepatic triglyceride secretion. Moreover, male PPARalpha-null mice show increased susceptibility to high fat diet in terms of serum apoB levels.     

Metabolism of apoprotein B in selectively bred baboons with low and high levels of low density lipoproteins

Very low density lipoprotein (VLDL) and low density lipoprotein (LDL) apoprotein (apo)-B turnover rates were measured simultaneously by injecting 131I-labeled VLDL and 125I-labeled LDL into fasting baboons (Papio sp.) selectively bred for high serum cholesterol levels and having either low or high LDL levels. The radioactivities in VLDL, intermediate density lipoprotein (IDL), LDL apoB, and urine were measured at intervals between 5 min and 6 days. Kinetic parameters for apoB were calculated in each baboon fed a chow diet or a high cholesterol, high fat diet (HCHF). VLDL apoB residence times were similar in the two groups of animals fed chow; they were increased by HCHF feeding in high LDL animals, but not in low LDL animals. Production rates of VLDL apoB were decreased by the HCHF diet in both high and low LDL animals. Most of the radioactivity from VLDL apoB was transferred to IDL. However, a greater proportion of radioactivity was removed directly from IDL apoB in low LDL animals than in high LDL animals, and only about one-third appeared in LDL. In high LDL animals, a greater proportion of this radioactivity was converted to LDL (61.4 +/- 7.2% in chow-fed animals and 49.2 +/- 10.9% in animals fed the HCHF diet; mean +/- SEM, n = 5). Production rates for LDL apoB were higher in high LDL animals than those in low LDL animals on both diets. The HCHF diet increased residence times of LDL apoB without changing production rates in both groups. VLDL apoB production was not sufficient to account for LDL apoB production in high LDL animals, a finding that suggested that a large amount of LDL apoB was derived from a source other than VLDL apoB in these animals.     

ApoE is necessary and sufficient for the binding of large triglyceride-rich lipoproteins to the LDL receptor; apoB is unnecessary

Large triglyceride-rich very low density lipoproteins (VLDL) Sf 60-400 from hypertriglyceridemic (HTG) patients, but not VLDL from normal subjects, bind to the LDL receptor of human skin fibroblasts because they contain apolipoprotein E (apoE) of the correct conformation, accessible both to the LDL receptor and to specific proteolysis by alpha-thrombin. Trypsin treatment of HTG-VLDL Sf 60-400 causes extensive apoB hydrolysis (fragments less than 100,000 mol wt), total degradation of apoE, and thus complete loss of LDL receptor binding. The reincorporation of apoE (1 mol/mol VLDL) into trypsin-treated HTG-VLDL completely restored the ability of HTG-VLDL to interact with the LDL receptor, suggesting that apoE probably does not induce a conformational change in apoB which results in receptor recognition, nor is intact apoB necessary to maintain the appropriate conformation of apoE for LDL receptor binding. As a model of large triglyceride-rich VLDL Sf greater than 60, we fractionated Intralipid by the Lindgren method of cumulative flotation and prepared apoE-Intralipid complexes. Competitive binding studies demonstrated that apoE-Intralipid is at least as effective as LDL for uptake and degradation of 125I-labeled LDL. Control Intralipid complexes containing apoA-I instead of apoE do not compete with iodinated LDL. Since these TG-rich complexes contain no apoB, apoB is, therefore, not only not sufficient for receptor-mediated uptake of large particles, it is not necessary. ApoE of the correct conformation is not only necessary but is sufficient to mediate receptor binding of large triglyceride-rich particles to the LDL receptor.     

Distinct patterns of lipoproteins with apoB defined by presence of apoE or apoC-III in hypercholesterolemia and hypertriglyceridemia

Apolipoprotein (apo) E and apoC-III concentrations in VLDL and LDL are associated with coronary heart disease. We studied the relationship between apoE and apoC-III and the abnormal concentrations and distribution of apoB lipoproteins in 10 hypercholesterolemic and 13 hypertriglyceridemic patients compared with 12 normolipidemic subjects (mean age, 45 years). Sixteen distinct types of apoB lipoprotein particles were separated by first using anti-apoE and anti-apoC-III immunoaffinity chromatography in sequence and then ultracentrifugation [light VLDL, dense VLDL, IDL, and LDL, with apoE with or without apoC-III (E(+)C-III(+), E(+)C-III(-)) or without apoE with or without apoC-III (E(-)C-III(+), E(-)C-III(-))]. The concentrations of VLDL particles with apoC-III (E(+)C-III(+), E(-)C-III(+)) were increased in the hypertriglyceridemic group compared with the hypercholesterolemic and normolipidemic groups. These particles were the most triglyceride rich of the particle types, and their triglyceride content was twice as high in hypertriglyceridemics compared with the other two groups. Hypertriglyceridemics had a similar concentration of total E(-)C-III(-) particles compared with normolipidemics, but the E(-)C-III(-) particles were distributed more to VLDL and IDL than to LDL. Hypercholesterolemics, in contrast, were distinguished from the normolipidemic group by 2-fold higher concentrations of apoB lipoproteins without apoE or apoC-III (E(-)C-III(-)), mainly LDL, which had high cholesterol content. Nonetheless, both normolipidemics and hypercholesterolemics had apoC-III-containing VLDL, which comprised 68% and 43% of their total VLDL particles. E(+)C-III(-) particles were a minor type, comprising <10% of particles in all lipoproteins and patient groups. Therefore, VLDL particles with apoC-III may play a central role in identifying the high risk of coronary heart disease in hypertriglyceridemia, but their substantial prevalence in normolipidemics may be of clinical significance as well.     

Properties of an apolipoprotein E-enriched fraction of triglyceride-rich lipoproteins isolated from human blood plasma with a monoclonal antibody to apolipoprotein B-100.

A monoclonal antibody to apolipoprotein (apo) B-100 (JI-H) with unique binding properties has been used to separate a population of triglyceride-rich lipoproteins from blood plasma of normotriglyceridemic individuals and patients with various forms of hypertriglyceridemia. This antibody fails to recognize an apoE-rich population of very low density lipoproteins (VLDL) containing apoB-100 as well as all triglyceride-rich lipoproteins containing apoB-48, but it binds other VLDL that contain apoE and almost all lipoproteins that contain apoB-100, but no apoE. The unbound triglyceride-rich lipoproteins separated by ultracentrifugation after separation from plasma by immunoaffinity chromatography contained 10-13% of the apoB of triglyceride-rich lipoproteins from three normotriglyceridemic individuals, 10-29% of that from five patients with endogenous hypertriglyceridemia, 40-48% of that from three patients with familial dysbetablipoproteinemia, and 65% of that from a patient with lipoprotein lipase deficiency. In all cases, the unbound triglyceride-rich lipoproteins contained more molecules of apoE and cholesteryl esters per particle than those that were bound to monoclonal antibody JI-H, and they were generally depleted of C apolipoproteins. These properties resemble those described for partially catabolized remnants of chylomicrons and VLDL. The affinity of the unbound lipoproteins for the low density lipoprotein (LDL) receptor varied widely, and closely resembled that of the total triglyceride-rich lipoproteins from individual subjects. Our results demonstrate that remnant-like chylomicrons and a population of remnant-like VLDL can be isolated and quantified in blood plasma obtained in the postabsorptive state from normotriglyceridemic and hypertriglyceridemic individuals alike.     

Heterogeneity of apolipoprotein E epitope expression on human lipoproteins: importance for apolipoprotein E function

The conformations of apolipoproteins on the surfaces of lipoprotein particles affect their physiologic functions. The conformations of apoE on plasma lipoproteins were examined using a panel of eight anti-apoE monoclonal antibodies (MAbs). The antibodies, which reacted with the major isoforms of apoE (E2, E3, and E4), defined at least five epitopes on apoE. Proteolytic fragments and synthetic peptides of apoE were used in binding assays to assign antibody epitopes; the epitopes were all localized to the middle third of the apoE molecule. The expression of apoE epitopes on isolated apoE and on lipoproteins was probed in competitive microtiter plate immunoassays using the anti-apoE MAbs, 125I-labeled apoE as tracer, and isolated apoE, intermediate density (IDL), very low density (VLDL1-3), and high density (HDL2 and HDL3) lipoproteins as competitors. The antibodies determined the patterns of competition exhibited by the lipoprotein preparations. Antibodies of the IgM class (WU E-1, WU E-2, WU E-3) defined two sets of conformation-dependent epitopes that were assigned towards the middle and the carboxyl terminal of the middle third of apoE. Competition curves using these antibodies, apoE, and lipoproteins showed a large variability in ED50 values. MAbs WU E-4, WU E-7, and WU E-10 defined epitopes near the receptor recognition site on apoE. Competition curves demonstrated small ranges of ED50 values. MAbs WU E-11 and WU E-12, which defined epitopes toward the amino-terminal region of apoE, exhibited competition curves for apoE and lipoproteins that had consistent, but wider ranges of ED50 values. There was no strict relationship between lipoprotein flotation rates and epitope expression for any of the MAbs. Immunoaffinity chromatography of VLDL subfractions on four different MAb columns indicated that the differences in the competitive abilities of VLDL subfractions were partly due to heterogeneity of apoE epitope expression within any population of particles. VLDL particles specifically retained on two different anti-apoE MAb columns were better competitors than unretained fractions for 125I-labeled LDL binding to the apoB, E-receptor of cultured human fibroblasts, suggesting that increased accessibility of apoE on the surface of VLDL is associated with increased receptor recognition. These data suggest that individual epitopes of apoE can be modulated; epitope expressions are not determined solely by the sizes and/or densities of lipoprotein particles; and differences in apoE conformation have significant metabolic consequences.     

Interactions between human and carp (Cyprimus carpio) low density lipoproteins (LDL) and LDL receptors

1. We have compared the concentration and chemical composition of carp and human plasma lipoproteins and studied their interaction with human fibroblast LDL receptors. 2. The main lipoproteins in carp are of high density (HDL) in contrast to low density lipoproteins (LDL) in human. 3. Carp lipoproteins are devoid of apolipoprotein (apo) E, a major ligand for interaction with LDL receptors in mammals. 4. Carp very low density lipoproteins (VLDL) and LDL but not HDL nor apoA-I cross react with human LDL in their interaction with LDL receptors on human cultured fibroblasts. 5. Carp liver membranes possess high affinity receptors that are saturable and have calcium dependent ligand specificity (apoB and apoE) similar to human LDL receptor. Carp VLDL and LDL but not HDL nor its major apolipoprotein complexed to L-alpha-phosphatidylcholine dimyristoyl (apoA-I-DMPC) competed with the specific binding of human LDL to this receptor.     

Apolipoprotein E Structure and Substrate and Receptor-Binding Activities of Triglyceride-Rich Human Plasma Lipoproteins in Normo- and Hypertriglyceridemia

Cysteine-arginine interchanges along the primary sequence of human plasma apolipoprotein E (apoE) play an important role in determining its biological functions due to a high mutation frequency of cytosine in CGX triplet that codes 33 of 34 apolipoprotein arginine residues. The contribution of apoE secondary structure to apolipoprotein-lipid interaction is described. The significance of apolipoprotein in triglyceride synthesis, lipoprotein lipolysis, and receptor-mediated clearance of lipolytic remnants of triglyceride-rich lipoproteins is discussed as well. The metabolic flow of lipoproteins in normo- and hypertriglyceridemia can be described by separate compartments that contribute to lipoprotein interaction with at least six different receptors: 1) low density lipoprotein (LDL) receptor; 2) LDL receptor-related protein (LRP); 3) apoB(48) macrophage receptor for hypertriglyceridemic very low density lipoproteins (VLDL); 4) scavenger receptors; 5) VLDL receptor; 6) lipolysis-stimulated receptor. The contribution of the exposure of apoE molecules on the surface of triglyceride-rich particles sensitive both to lipolysis and plasma triglyceride content to the interaction with LDL receptor and LRP is emphasized.     

Rapid turnover of apolipoprotein C-III-containing triglyceride-rich lipoproteins contributing to the formation of LDL subfractions

The atherogenicity theory for triglyceride-rich lipoproteins (TRLs; VLDL + intermediate density lipoprotein) generally cites the action of apolipoprotein C-III (apoC-III), a component of some TRLs, to retard their metabolism in plasma. We studied the kinetics of multiple TRL and LDL subfractions according to the content of apoC-III and apoE in 11 hypertriglyceridemic and normolipidemic persons. The liver secretes mainly two types of apoB lipoproteins: TRL with apoC-III and LDL without apoC-III. Approximately 45% of TRLs with apoC-III are secreted together with apoE. Contrary to expectation, TRLs with apoC-III but not apoE have fast catabolism, losing some or all of their apoC-III and becoming LDL. In contrast, apoE directs TRL flux toward rapid clearance, limiting LDL formation. Direct clearance of TRL with apoC-III is suppressed among particles also containing apoE. TRLs without apoC-III or apoE are a minor, slow-metabolizing precursor of LDL with little direct removal. Increased VLDL apoC-III levels are correlated with increased VLDL production rather than with slow particle turnover. Finally, hypertriglyceridemic subjects have significantly greater production of apoC-III-containing VLDL and global prolongation in residence time of all particle types. ApoE may be the key determinant of the metabolic fate of atherogenic apoC-III-containing TRLs in plasma, channeling them toward removal from the circulation and reducing the formation of LDLs, both those with apoC-III and the main type without apoC-III.     

Effect of the ingestion of thermically treated fat on blood lipids and blood lipoproteins in the rat

The effect on rats fed on a diet with 15% solid frying fat (diet B) is compared to the effect of a diet with 15% of the same fat but in the raw state (diet A). After 10 weeks being fed on these diets serum triglycerides, phospholipids, total cholesterol, free cholesterol, esterified cholesterol, high density lipoprotein-cholesterol and free fatty acid levels were checked. Percentage of very low density lipoproteins (VLDL), low density lipoproteins (LDL) and high density lipoproteins as well as the composition of these lipoproteins was determined in parallel. Rats fed on diet B showed a significant increase in phospholipids and a significant decrease in VLDL when compared to those fed on diet A. Phospholipids on LDL decreased significantly in diet B fed rats. The data obtained seem to indicate that the hypercholesterolemic tendency induced by frying fat is neutralized by a decrease in VLDL levels.     

Apolipoproteins of the orotic acid fatty liver: implications for the biogenesis of plasma lipoproteins

Rats fed orotic acid develop fatty livers characterized by triglyceride-laden, membrane-bounded vesicles designated "liposomes." We have measured the levels of apolipoproteins in isolated liposomes and other subcellular fractions by SDS-polyacrylamide gel electrophoresis, electrotransfer, and immunodecoration. Apolipoproteins Bh, Bl, E, and C appear to cofractionate; for these proteins, the liposomal pool represents a large portion of their total intracellular mass. However, liposomes are deficient in both variants of apoB relative to apoE and apoC when compared with rat plasma very low density lipoprotein (VLDL). Albumin and apolipoproteins A-I and A-IV are also found in liposomes, but this organelle represents a minor fraction of their total intracellular mass. The liposomal apolipoproteins show varying degrees of association with cisternal lipid and with organelle membranes. Orotic acid may selectively block VLDL production at the level of particle assembly or transorganellar movement. We conclude that liposomal contents probably represent exaggerated accumulations of VLDL assembly intermediates, and that the intracellular partitioning of high density lipoprotein-destined from VLDL-destined components occurs at an early stage in particle biogenesis. Moreover, some unique structural feature of apoB may effect movement of VLDL assembly intermediates through secretory organelles.     

Effects of atherogenic diet consumption on lipoproteins in mouse strains C57BL/6 and C3H

Inbred mouse strains C57BL/6J (B6) (susceptible) and C3H/HeJ (C3H) (resistant) differ in atherosclerosis susceptibility due to a single gene, Ath-1. Plasma lipoproteins from female mice fed chow or an atherogenic diet displayed strain differences in lipoprotein particle sizes and apolipoprotein (apo) composition. High density lipoprotein (HDL) particle sizes were 9.5 +/- 0.1 nm for B6 and 10.2 +/- 0.1 nm for C3H. No major HDL particle size subclasses were observed. Plasma HDL level in the B6 strain was reduced by the atherogenic diet consumption while the HDL level in the resistant C3H mice was unaffected. The reduction in HDL in the B6 strain was associated with decreases in HDL apolipoproteins A-I(-34%) and A-II(-60%). The HDL apoC content in mice fed chow was two-fold higher in C3H than B6. Lipoproteins containing apolipoprotein B (VLDL, IDL, LDL) shifted from a preponderance of the B-100 (chow diet) to a preponderance of the B-48 (atherogenic diet). The LDL-particle size distribution was strain-specific with the chow diet but not genetically associated with the Ath-1 gene. In both strains on each diet, apolipoprotein E was largely distributed in the VLDL, LDL, and HDL fractions. The B6 strain became sixfold elevated in total lipoprotein E content which in the C3H strain was not significantly affected by diet. However, the C3H LDL apoE content was reduced. On both diets, the C3H strain exhibited apolipoprotein E levels comparable to the atherogenic diet-induced levels of the B6 mice.