Canine lipoproteins and atherosclerosis. II. Characterization of the plasma lipoproteins associated with atherogenic and nonatherogenic hyperlipidemia

In vivo clearance of low-density lipoproteins and beta-very-low-density lipoproteins in normal and hypercholesterolemic White Carneau pigeons

Low-density lipoproteins (hLDL) and beta-migrating-very-low-density lipoproteins (beta-VLDL) were isolated from the plasma of cholesterol-fed White Carneau (WC) pigeons and low-density lipoproteins (nLDL) were isolated from the plasma of grain-fed WC pigeons. The lipoproteins were radiolabeled with 125I or 131I and injected into normocholesterolemic or hypercholesterolemic WC pigeons to determine their rate of clearance from the plasma. The fractional catabolic rate (FCR) of nLDL and hLDL in normocholesterolemic pigeons averaged 0.202 and 0.206 pools/h.respectively. beta-VLDL was cleared at a significantly slower rate of 0.155 pools/h. The FCR of the same lipoproteins injected into hypercholesterolemic pigeons was reduced by 17% for nLDL, 50% for hLDL and 57% for beta-VLDL, indicating that the effect of hypercholesterolemia on clearance in vivo was different for the three lipoproteins. The FCR of reductively methylated pigeon LDL (MeLDL), which gives a measure of receptor-independent clearance of LDL, was shown previously to be 0.037 pools/h. These studies suggest therefore that LDL and beta-VLDL are cleared from the plasma of normocholesterolemic and hypercholesterolemic pigeons at a rate substantially greater than that predicted for non-specific processes. Despite the reduction in the clearance rate of hLDL and beta-VLDL due to cholesterol feeding, the absolute amount of cholesterol that was cleared from the plasma by these lipoproteins was increased from approx. 200 mg/kg body weight per day in the normocholesterolemic pigeons to greater than 1000 mg/kg body weight per day in the hypercholesterolemic pigeons. This is due principally to the enrichment in cholesterol relative to protein of the lipoproteins isolated from cholesterol-fed pigeons and the failure of hypercholesterolemia to completely inhibit receptor-dependent clearance of LDL and beta-VLDL. The lower rate of clearance of beta-VLDL relative to LDL is in marked contrast to mammalian beta-VLDL, which is cleared much faster than LDL, but is consistent with the lack of apo E on pigeon lipoproteins. Apo E is the apoprotein that is thought to be responsible for the rapid clearance of beta-VLDL in normocholesterolemic mammals. The low rate of beta-VLDL clearance in pigeons also suggests that pigeons lack an apolipoprotein that function like mammalian apo E.

Isolation and characterization of plasma lipoproteins of common marmoset monkey. Comparison of effects of control and atherogenic diets

This study examines the potential of the common marmoset monkey (Callithrix jacchus) to serve as a model for human lipoprotein metabolism and atherosclerosis. The lipoproteins of animals fed a low-fat, low-cholesterol diet and a high-fat (12% wt/wt lard), high-cholesterol (0.34% wt/wt) diet were characterized by the combination of sequential ultracentrifugation and Pevikon block electrophoresis. Based on chemical and physical properties, equivalents of human very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL), including and HDL-with apolipoprotein E subclass, were demonstrated. In control animals, whose plasma cholesterol concentration was 140.1 +/- 20.2 mg/dl (means +/- SD), approximately 40% of the plasma cholesterol was transported by LDL as compared with approximately 70% in humans. The cholesterol-fed marmosets segregated into two groups: hypo- and hyper-responders. The hyper-responders had plasma cholesterol levels of 450 to 970 mg/ml. The hypercholesterolemia was associated with elevated concentrations of VLDL, intermediate density lipoproteins, and LDL; in addition, these lipoproteins were enriched in cholesteryl esters relative to lipoproteins isolated from control animals. The HDL (d greater than 1.09 g/ml) levels did not change in response to cholesterol feeding, although the HDL-with apolipoprotein E found in the d = 1.02 to 1.09 g/ml fraction increased approximately fivefold. Based on immunological characteristics and sialic acid content, the common marmoset appeared to lack a lipoprotein(a) equivalent. The results of a short-term feeding study (11 months) suggest that this monkey was susceptible to the development of diet-induced atherosclerosis. The hyper-responsive animals developed foam cell lesions and moderately proliferative intimal lesions, predominantly within the thoracic aorta. In summary, the results of our studies indicate that the common marmoset monkey potentially is a useful model for the study of both lipoprotein metabolism and diet-induced atherosclerosis.

Discoidal complexes containing apolipoprotein E and their transformation by lecithin-cholesterol acyltransferase

The primary objectives of this study were to determine whether analogs to native discoidal apolipoprotein (apo)E-containing high-density lipoproteins (HDL) could be prepared in vitro, and if so, whether their conversion by lecithin-cholesterol acyltransferase (LCAT; EC 2.3.1.43) produced particles with properties comparable to those of core-containing, spherical, apoE-containing HDL in human plasma. Complexes composed of apoE and POPC, without and with incorporated unesterified cholesterol, were prepared by the cholate-dialysis technique. Gradient gel electrophoresis showed that these preparations contain discrete species both within (14-40 nm) and outside (10.8-14 nm) the size range of discoidal apoE-containing HDL reported in LCAT deficiency. The isolated complexes were discoidal particles whose size directly correlated with their POPC:apoE molar ratio: increasing this ratio resulted in an increase in larger complexes and a reduction in smaller ones. At all POPC:apoE molar ratios, size profiles included a major peak corresponding to a discoidal complex 14.4 nm long. Preparations with POPC:apoE molar ratios greater than 150:1 contained two distinct groups of complexes, also in the size range of discoidal apoE-containing HDL from patients with LCAT deficiency. Incorporation of unesterified cholesterol into preparations (molar ratio of 0.5:1, unesterified cholesterol:POPC) resulted in component profiles exhibiting a major peak corresponding to a discoidal complex 10.9 nm long. An increase of unesterified cholesterol and POPC (at the 0.5:1 molar ratio) in the initial mixture, increased the proportion of larger complexes in the profile. Incubation of isolated POPC-apoE discoidal complexes (mean sizes, 14.4 and 23.9 nm) with purified LCAT and a source of unesterified cholesterol converted the complexes to spherical, cholesteryl ester-containing products with mean diameters of 11.1 nm and 14.0 nm, corresponding to apoE-containing HDL found in normal plasma. Conversion of smaller cholesterol-containing discoidal complexes (mean size, 10.9 nm) under identical conditions resulted in spherical products 11.3, 13.3, and 14.7 nm across. The mean sizes of these conversion products compared favorably with those (mean diameter, 12.3 nm) of apoE-containing HDL of human plasma. This conversion of cholesterol-containing complexes is accompanied by a shift of some apoE to the LDL particle size interval. Our study indicates that apoE-containing complexes formed by the cholate-dialysis method include species similar to discoidal apoE-containing HDL and that incubation with LCAT converts most of them to spherical core-containing particles in the size range of plasma apoE-containing HDL. Plasma HDL particles containing apoE may arise in part from direct conversion of discoidal apoE-containing HDL by LCAT.

Serum lipoprotein and apoprotein concentrations in 4-(4-chlorophenyl)-2-hydroxytetronic acid and clofibrate-treated cholesterol and cholic acid-fed rats

Influence of clofibrate and an aci-reductone, 4-(4-chlorophenyl)-2-hydroxytetronic acid (CHTA) on lipoproteins and apoproteins was studied in cholesterol- plus cholic acid-fed rats. CHTA (0.4 mmol/kg body wt, twice daily) significantly lowered serum total cholesterol and triglyceride concentrations at both 10 and 16 days, whereas clofibrate at the same dose did not alter serum cholesterol levels, but elevated serum triglyceride concentrations at 16 days. The abnormal cholesterol-rich very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL) and low density lipoproteins (LDL) produced by cholesterol plus cholic acid were significantly reduced in their cholesterol content by treatment with CHTA, a compound having an oxidation reduction potential. Conversely, clofibrate administration increased VLDL-cholesterol with concomitant decreases in IDL- and LDL-cholesterol concentrations. Administration of CHTA to cholesterol- plus cholic acid-fed rats significantly increased concentrations of VLDL and IDL, but had no effect on HDL protein. Both CHTA and clofibrate administration to cholesterol- plus cholic acid-fed rats significantly lowered IDL protein concentrations. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) studies of apoproteins revealed that clofibrate treatment significantly reduced apoC-III and C-II in VLDL, C-II in IDL, and apoA-IV and A-I in HDL. Rats treated with CHTA significantly raised apoC-II and C-III in HDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Dependence of metabolic and structural heterogeneity of cholesterol ester-rich very low density lipoproteins on the duration of cholesterol feeding in rabbits

Cholesterol ester-rich (CER) VLDL accumulate rapidly in the plasma of rabbits fed cholesterol-enriched diets. However, the major loci of enhanced synthesis of subfractions of CER-VLDL, their interaction with macrophages, and their relative contribution to atherogenesis have not yet been elucidated. To determine whether anabolism is hepatic or intestinal, subfractions of CER-VLDL were characterized at selected intervals from day 0 to 60 of cholesterol feeding. Rate zonal ultracentrifugation of CER-VLDL from rabbits fed cholesterol for 4 and 60 d demonstrated an early increase of the proportion of cholesterol carried in the intestinally-derived fraction (designated as Fx-I) of VLDL compared with that in hepatically-derived particles (Fx-H). Quantification by size exclusion HPLC also demonstrated that Fx-I was a prominent CER-VLDL component at day 4, while Fx-H particles became increasingly prominent with further cholesterol feeding. At both 4 and 60 d Fx-I stimulated cholesterol esterification and intracellular cholesterol content in macrophages more than the corresponding Fx-H did. In fact, Fx-H harvested at 4 d produced no cholesterol ester deposition. In contrast, Fx-H harvested at 60 d markedly stimulated cholesterol esterification and intracellular cholesterol content. Thus, both compositional and metabolic characteristics of CER-VLDL changed as a function of the duration cholesterol feeding.

Apolipoprotein E: cholesterol transport protein with expanding role in cell biology

Apolipoprotein E is a plasma protein that serves as a ligand for low density lipoprotein receptors and, through its interaction with these receptors, participates in the transport of cholesterol and other lipids among various cells of the body. A mutant form of apolipoprotein E that is defective in binding to low density lipoprotein receptors is associated with familial type III hyperlipoproteinemia, a genetic disorder characterized by elevated plasma cholesterol levels and accelerated coronary artery disease. Apolipoprotein E is synthesized in various organs, including liver, brain, spleen, and kidney, and is present in high concentrations in interstitial fluid, where it appears to participate in cholesterol redistribution from cells with excess cholesterol to those requiring cholesterol. Apolipo-protein E also appears to be involved in the repair response to tissue injury; for example, markedly increased amounts of apolipoprotein E are found at sites of peripheral nerve injury and regeneration. Other functions of apolipoprotein E, unrelated to lipid transport, are becoming known, including immunoregulation and modulation of cell growth and differentiation.

Novel aspects of vitamin A metabolism in the dog: distribution of lipoprotein retinyl esters in vitamin A-deprived and cholesterol-fed animals

Retinyl ester concentrations in plasma from fasting humans, rabbits and rats are usually negligible. In contrast, plasma from fasting dogs contains appreciable amounts of retinyl esters, associated almost entirely with the low-density lipoproteins. This study was undertaken to gather additional information about the nature and origin of canine retinyl ester-containing lipoproteins. We examined the metabolism of endogenous lipoprotein retinyl esters in adult mongrel dogs with moderate vitamin A deficiency. Four animals were fed a diet of oatmeal and tuna fish that provided only 4% of the vitamin A contained in their control rations (15 vs. 367% of the canine recommended daily intake). There was an initial rapid decline in plasma retinyl esters. However, measurable concentrations persisted in plasma for up to 1 year of restricted vitamin A intake. Total plasma retinyl ester concentrations after 6 months of vitamin A deprivation, extrapolated from best-fit monoexponential decay curves for each animal, ranged from 11 to 89% of control, suggesting that there was sustained secretion of retinyl esters from endogenous stores. Density gradient ultracentrifugation of plasma from fasting vitamin A-deprived dogs showed retinyl esters in the very-low- and low-density lipoproteins. After fat and vitamin A feeding retinyl esters appeared among the very-low-, intermediate- and low-density lipoproteins, consistent with the suggestion that chylomicron retinyl esters are first taken up by the liver, and then resecreted as density less than 1.006-1.063 g/ml lipoproteins. Maximal incorporation of dietary retinyl esters into low-density lipoproteins was not reached until 24-48 h. Intermediate-density and beta-migrating low-density lipoprotein retinyl esters were increased markedly in fasting animals maintained on cholesterol- and saturated fat-enriched diets. These observations provide further evidence for the proposal that the canine liver secretes retinyl ester-containing particles, in amounts governed by dietary composition and vitamin A content. What selective advantage this unusual transport pathway might provide is not apparent.

Lipoproteins and atherosclerosis

The plasma lipoproteins are the primary means of transport of cholesterol among tissues. In particular, the apo B-containing lipoproteins (VLDL, IDL and LDL) are important for the delivery of cholesterol from the liver to peripheral tissues, while HDL appear to mediate the reverse process of movement of cholesterol from tissues back to the liver. Both of these transport processes are necessary for efficient whole body cholesterol homeostasis, because the liver is the major site of both the production and excretion of cholesterol. However, deviations from a proper balance of transport of cholesterol, either increases in LDL levels or decreases in HDL cholesterol flux, may result in accumulation of cholesterol in extrahepatic tissues. Increased risk of atherosclerosis and CHD may be associated with elevation in the number of LDL particles, increase or decrease in LDL particle size, or changes in the composition of plasma LDL. These modifications of plasma LDL may be brought about following perturbation of one of several aspects of LDL metabolism. These include decreased LDL receptor activity, increased VLDL production and cholesterol enrichment of the liver-derived VLDL. The events in the arterial wall that make some LDL particles apparently atherogenic are not well understood. In the case of nonhuman primates, large-size LDL are associated with an increased risk of CHD. One characteristic of these LDL is that their core lipids are rich in saturated cholesteryl esters and their transition temperatures are frequently above body temperature. The liquid crystalline cholesteryl ester cores of such LDL may modulate the conformation of apo B on the surface and thereby affect the interaction of these LDL with cellular receptors or connective tissue matrix proteoglycans. It is likely, though, that changes in LDL particle number, LDL particle size and LDL particle composition may each contribute to progression of atherosclerosis. The presumed metabolic events that make HDL protective against atherosclerosis have been termed reverse cholesterol transport, and suggest that small HDL that are deficient in free cholesterol acquire this lipid from cell membranes. The HDL cholesterol is esterified by LCAT in the circulation, forming large HDL that can then deliver the cholesteryl ester to the liver by both direct and indirect means. In most circumstances, it is assumed that an increase in plasma HDL cholesterol concentration reflects an increase in the rate at which HDL is removing cholesterol from tissues and, consequently, a decrease in atherosclerosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Serum lipids and lipoproteins in the atherosclerosis prone LA/N corpulent rat

The LA/N rat is one of two congenic strains bred from the original obese, hyperphagic and hypertensive rats of Koletsky. With the exception of hypertension the LA/N strain, when homozygous for the corpulent gene, is phenotypically similar to the parent Koletsky strain and prone to the development of vascular and myocardial lesions. Here we report a detailed analysis of the serum lipids, lipoproteins and apolipoproteins B, E and A-I levels in young adult homozygous corpulent (cp/cp) rats of both sexes and in lean males of the same age which were demonstrable non-carriers (+/+) of the cp gene. Both male and female cp/cp rats were hypertriglyceridemic (282-512 mg/100 ml) and moderately hypercholesterolemic (74-84 mg/100 ml). Elevations in these lipids reflected the presence of large (622 A), triacylglycerol-rich and apoprotein-poor VLDL containing both apolipoproteins Bh and B1 and increased phospholipid-rich HDL. Similar, but less pronounced, elevations in serum apolipoproteins B and E in the cp/cp rats when compared to the +/+ animals were also noted. Apolipoproteins A-I levels were 2.7-3-fold higher in cp/cp rats. The levels of VLDL were significantly higher in female cp/cp rats; however, the levels of IDL (intermediate-density lipoproteins), LDL and HDL were significantly lower than in the more atherosclerosis prone male cp/cp rats. Similarly, apolipoprotein A-I was higher and apolipoprotein B lower in the male cp/cp than in the female cp/cp rats. The LDL (d = 1.030-1.063 g/ml) in cp/cp rats, like that in normal animals, was heterogeneous and contained apolipoproteins Bh, E, A-I and C. This fraction was significantly elevated in male cp/cp rats when compared to females but still represented less than 13% of the total serum cholesterol and less than 6% of the total serum lipids in 3-month-old cp/cp animals. The ratio of cholesterol to phospholipids was significantly lower for all lipoproteins in cp/cp rats when compared to +/+ males and these ratios for female cp/cp rats were in all cases lower than those of male cp/cp animals.

The effect of acute hypercholesterolemia on myocardial infarct size and the no-reflow phenomenon during coronary occlusion-reperfusion

The goal of this study was to determine the effects of acute hypercholesterolemia on the evolution of myocardial infarction in a preparation of coronary occlusion-reperfusion. New Zealand white rabbits were fed a 2% cholesterol-enriched diet for 3 days (plasma cholesterol 329 +/- 70 mg/dl), or maintained on the control diet (plasma cholesterol 67 +/- 12 mg/dl). Temporary (30 min) coronary artery occlusion was performed in open-chest rabbits with a suture snare. The snare was released to permit reperfusion. When the animals were killed 5.5 hr later, left ventricles were cut into 3 mm slices. Infarct size was determined by planimetry of tetrazolium-stained slices while the area at risk of infarction (hypoperfused zone) was determined by planimetry of the "cold spots" on autoradiograms of the slices that contained 99m Tc-labeled microspheres that had been injected 1 min after occlusion. Infarct size, expressed as percent of the hypoperfused zone, was 42.8 +/- 1.3% (n = 10) in the control group and was increased by approximately 100% in cholesterol-fed animals to 83.7 +/- 2.0% (n = 10, p less than .001). To test the hypothesis that vascular obstruction (no reflow) might account for the larger infarct size, thioflavin S was injected immediately before the animals were killed to demarcate perfused myocardium in three additional groups of animals: standard chow-fed rabbits (n = 5), cholesterol-fed rabbits (n = 5), and standard chow-fed rabbits that, in addition, received an infusion of isoproterenol (0.1 microgram/kg/min, n = 6), an intervention believed to increase infarct size through a mechanism not dependent on the no-reflow phenomenon.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism of very low density lipoproteins after cessation of cholesterol feeding in rabbits. A factor potentially contributing to the slow regression of atheromatous plaques

Aortic atheromatous plaques regress slowly in cholesterol-fed rabbits that have been returned to normal laboratory diet. To delineate metabolic factors potentially responsible for persistence of atherosclerosis under these conditions, the physical, chemical, and metabolic characteristics were determined for lipoproteins of d less than 1.006 g/ml; such lipoproteins are thought to be the major determinant of progression of atherosclerotic lesions in cholesterol-fed rabbits. At the time of return to a normal laboratory diet regimen after 3 mo of feeding with cholesterol-enriched laboratory diet, plasma cholesterol concentrations were 2,275 +/- 252 mg/dl, mostly attributable to cholesteryl ester-rich very low density lipoproteins (VLDL). On the hypercholesterolemic diet, fractional catabolic rates of plasma clearance of 125I-labeled VLDL were reduced (0.011 +/- 0.002 vs. 0.151 +/- 0.015 h-1), but the total VLDL catabolic rate was increased considerably (17.1 +/- 2.2 vs. less than 1.2 +/- 0.4 mg of protein/kg X d), because of the expansion of the endogenous pool of cholesteryl ester-rich VLDL. The total catabolic rate of VLDL was maintained above estimated control values (5.8 +/- 0.7 mg protein/kg X d) even 10 wk after return of the rabbits to a normal chow regimen, an effect attributable to continued high rates of cholesteryl ester-rich VLDL synthesis in liver. Accumulation of cholesteryl ester-rich VLDL into aortic tissue persisted at a high rate. Thus the persistence of aortic atheromatous lesions after cessation of cholesterol feeding was attributable in part to continued high rates of hepatic production of cholesteryl ester-rich VLDL and its persistent delivery into the aortic wall.

A comparison of the cellulose acetate electrophoretic serum lipoprotein-cholesterol profile of the adult male rat with other species

A comparison was made of the serum lipoprotein-cholesterol profile, obtained by cellulose acetate electrophoresis coupled with an enzymatic stain for total cholesterol, of the adult male rat, mouse, rabbit, dog, monkey and human. Four cholesterol-staining lipoprotein bands were detected in rat serum, while only three cholesterol-staining lipoprotein bands were present in the other species studied. The apparently unique lipoprotein-cholesterol band in the rat was found to electrophoretically migrate in the prealbumin region of rat serum, has been named prealbumin lipoprotein-cholesterol (PAL-C) and was shown to be a high density lipoprotein (HDL). Of the species studied those more susceptible to experimentally induced atherosclerosis had higher low density lipoprotein-cholesterol to HDL-cholesterol ratios compared to those species least susceptible to experimentally induced atherosclerosis.

Metabolism of human plasma triacylglycerol-rich lipoproteins in rodent macrophages: capacity for interaction at beta-VLDL receptor

The capacity of human plasma triacylglycerol-rich lipoproteins to be metabolized by rat macrophages was studied with plasma triacylglycerol-rich lipoproteins obtained from subjects with fasting chylomicronemia or from normal subjects after a fat meal. Triacylglycerol-rich lipoproteins were separated by chromatography into two fractions designated TRL1 and TRL2; from their composition and changing concentration during alimentary lipemia, TRL1 contained a higher proportion of chylomicron remnants than TRL2. Degradation of 125I-labeled TRL1 was greater than that of 125I-labeled TRL2. In competition studies with 125I-labeled beta-VLDL from cholesterol-fed rabbits, unlabeled TRL1 displaced beta-VLDL as completely as did unlabeled beta-VLDL, being slightly more potent than TRL2, which contained less apolipoprotein E than TRL1. This reflected common interaction at receptors that probably included both beta-VLDL and B/E receptors, since: (1) in fresh macrophages, VLDL from hypertriglyceridemic subjects partially displaced beta-VLDL; (2) in B/E receptor-repressed macrophages, TRL1 maintained capacity to totally displace beta-VLDL. This was confirmed in experiments with J774 murine macrophages in which triacylglycerol-rich lipoproteins and beta-VLDL displaced each other equally, whereas LDL was ineffective in displacing beta-VLDL. Furthermore, monoclonal antibodies raised against apolipoprotein B48 and reacting strongly with LDL, failed to inhibit the binding of triacylglycerol-rich lipoprotein to the macrophages. This indicates an interaction through apolipoprotein E which is present in high concentration in triacylglycerol-rich lipoprotein as well as in beta-VLDL. It applies to triacylglycerol-rich particles derived from either the intestine (chylomicron remnants) or the liver (VLDL remnants from hypertriglyceridemic subjects).

Apo E levels and distribution in rhesus monkeys consuming an atherogenic diet

The effect of an atherogenic diet on serum apo E levels and distribution among the lipoproteins of rhesus monkeys was studied. Animals able to maintain their serum cholesterol levels below 250 mg/dl (hyporesponders) showed no significant change in their serum apo E levels; however, in monkeys whose serum cholesterol concentrations ranged from 250 to 850 mg/dl, serum apo E levels appeared to have increased in direct proportion to plasma cholesterol concentration (r2 = 0.92) such that in monkeys whose serum cholesterol concentration exceeded 650 mg/dl (hyperresponders), the apo E levels had increased 5-6-fold. The majority of the apo E (60%) in hyporesponders consuming the atherogenic diet was associated with HDL, whereas only 10% of the serum apo E was associated with HDL in hypercholesterolemic hyperresponders. Nonetheless, the absolute amount of HDL-associated apo E was the same in both phenotypes. Thus, essentially all of the increase in serum apo E levels in hyperresponders was due to an increase in non-HDL-associated apo E. The mean density of the fraction showing the greatest increase in apo E, and accounting for the majority of the d less than 1.063 g/ml apo E in hyperresponders, was 1.010 g/ml. That fraction was distinct from the lipoproteins principally responsible for the increase in apo B and cholesterol levels in those animals. The latter were smaller in size and higher in density than the major apo E-rich fraction. Nonetheless, the d less than 1.063 g/ml apo E apparently circulates on apo B-containing particles, since it was retained on an anti-apo B immunoaffinity column. These data show that a diet-induced hypercholesterolemia is accompanied by a marked increase in serum apo E levels in rhesus monkeys, but that the lipoproteins principally responsible for the increase in apo E levels are distinct from those mainly responsible for the hypercholesterolemia. They also suggest that the levels of apo E-containing HDL in hypercholesterolemic hyperresponders are not significantly lowered by the diet, even though those animals' apo A-I levels were severely reduced. Thus, both the LDL and the HDL of the hypercholesterolemic primate contain apo E-rich subfractions which are metabolically distinct from the principal lipoprotein family in each fraction.

Immunoaffinity isolation of apolipoprotein E-containing lipoproteins

Discrete apolipoprotein E-containing lipoproteins can be identified when EDTA plasma is fractionated on columns of 4% agarose. The present study has demonstrated, by physical and metabolic criteria, that these apolipoprotein E-containing lipoprotein subclasses may be further isolated by immunoaffinity chromatography. Whole plasma was first bound to an anti-apolipoprotein E immunoadsorbent prior to gel filtration on 4% agarose. After elution from the affinity column and dialysis, the bound fraction was chromatographed on 4% agarose. Discrete subfractions of apolipoprotein E could be demonstrated within elution volumes similar to those observed in the original plasma. When whole plasma was first submitted to gel filtration and the apolipoprotein E-containing lipoproteins of either intermediate- or of high-density lipoprotein (HDL) size were subsequently bound to anti-apolipoprotein E columns, the bound eluted fractions maintained their size and physical properties as shown by electron microscopy and by rechromatography on columns of 4% agarose. The metabolic integrity of apolipoprotein E-containing very-low-density lipoproteins (VLDL) was examined by coinjection into a cynomolgus monkey of 125I-labeled apolipoprotein E-rich and 131I-labeled apolipoprotein E-deficient human VLDL which had been separated by immunoaffinity chromatography. The plasma specific activity time curves of the apolipoprotein B in VLDL, intermediate-density (IDL) and low-density (LDL) lipoproteins demonstrated rates of decay and precursor-product relationships similar to those obtained after injection of whole labeled VLDL, supporting the metabolic integrity of VLDL isolated by immunoaffinity chromatography.

The cellular pathobiology of atherosclerosis in 1983

Atherosclerosis is a dynamic process in which a number of stimuli to arterial lipid deposition, cell proliferation and cell death, and the synthesis of fiber and matrix proteins by modulated smooth muscle cells are the major factors which can vary to control to process. The circulating levels and interaction of lipoproteins, the integrity of the endothelium, the heart rate and the height of the blood pressure and many incompletely understood arterial wall factors all appear to affect these components. Although there are many unanswered questions, it is clear that immense progress has been made in the past 25 years in developing a reasonable understanding of the cellular pathobiology of the atherosclerotic process. The momentum of this progress is such as to project a very promising future in which the molecular pathology of atherogenesis will be understood well enough to permit prevention, retardation and more effective regression of the dangerous advanced plaque.

Changes in plasma apo B, apo E, apo A-I, and apo A-IV concentrations in dogs consuming different atherogenic diets

To define the relationship between apoprotein levels and plasma cholesterol concentration in dogs, we measured the cholesterol, apo B, apo E, apo A-IV, and apo A-I levels in 6 dogs fed a synthetic diet (Diet I), and in 5 dogs fed dog chow supplemented with lard, cholesterol, bile salts, and propylthiouracil (Diet II). The diet-induced hypercholesterolemia exceeded 900 mg/dl in dogs fed Diet I and was accompanied by a 12-fold increase in apo B, a 30-fold increase in apo E, an 8-fold increase in apo A-IV, and a 1 1/2-fold increase in apo A-I. By contrast, the hypercholesterolemia averaged 1300 mg/dl in dogs fed Diet II and was accompanied by a 12-fold increase in apo B, an 11-fold increase in apo E, a 3-fold increase in apo A-IV, and a 5-fold decrease in apo A-I levels. When 3 of the Diet I dogs were switched to dog chow, their plasma cholesterol, apo B, and apo E levels dropped to 30% of their peak value within 7 days. The change in apo B and apo E levels was found to be highly correlated with the change in plasma cholesterol concentrations in each of the Diet I animals (r2 ranged from 0.92 to 0.99 for both apoproteins). A strong linear relationship was also observed between apo E and apo B (r2 ranged from 0.94 to 0.98), indicating that the plasma apo E to apo B ratio remained constant in these animals as the hypercholesterolemia progressed or regressed.

Characterization of HDL and lipoproteins intermediate to LDL and HDL in the serum of pedigreed baboons fed an atherogenic diet

A lipoprotein species with ultracentrifugal flotation rates (F0(1.20) 9-28) intermediate to high density lipoproteins (HDL, F0(1.20) 0-9) and low density lipoproteins (LDL, F0(1.20) 28-56) found in the plasma of certain pedigreed baboons fed an atherogenic diet was studied by gradient gel electrophoresis (GGE) and ultracentrifugal techniques. These lipoproteins were found to be heterogeneous in size (125-220 A) and hydrated density (1.028-1.080 g/ml). The major apolipoprotein in all density subfractions of the F0(1.20) 9-28 lipoproteins exhibited the molecular weight (2.8 X 10(4) daltons) and immunochemical properties of apolipoprotein A-I (apoA-I). Protein corresponding to apolipoprotein E (apoE, 3.5 X 10(4) daltons) was observed primarily in the less dense subspecies of F0(1.20) 9-28 lipoproteins. Some low molecular weight (1.8 X 10(4), 1.3 X 10(4), and 1.1 X 10(4) daltons) apolipoproteins were also detected. At low serum F0(1.20) 9-28 lipoprotein concentrations, only the smaller, more dense, protein-rich species were present; at higher F0(1.20) 9-28 concentrations, the larger, less dense species were observed in addition to the small species. The HDL of pedigreed baboons in families with and without serum F0(1.20) 9-28 lipoproteins were also characterized. The HDL of both groups of progeny consisted of a similar set of 5 subpopulations designated HDL-I through HDL-V determined by GGE. HDL-I, consisting of material 100-125 A in size, was the major HDL subpopulation. ApoA-I was the major protein moiety in all HDL subpopulations; none contained apoE. Baboons in families with F0(1.20) 9-28 lipoproteins had more HLD-I (292 +/- 80 mg/dl vs. 235 +/- 55 mg/dl) and less HDL-II (86 +/- 22 mg/dl vs. 135 +/- 34 mg/dl) than baboons in families without F0(1.20) 9-28 lipoproteins; both groups showed identical total HDL concentrations (446 +/- 90 mg/dl and 444 +/- 49 mg/dl, respectively). Among those baboons in families with F0(1.20) 9-28 lipoproteins, there was an inverse correlation between F0(1.20) 9-28 concentration and total HDL, HDL-I and HDL-II concentrations, indicating a possible metabolic relationship between these HDL subpopulations and the F0(1.20) 9-28 species.

Lipoproteins and apolipoproteins in peripheral lymph of normal and cholesterol-fed dogs

Prenodal peripheral lymph was used as a model of interstitial fluid to obtain information on the composition of lipoproteins and apolipoproteins which are in direct contact with peripheral cells. Lipoproteins resembling plasma lipoproteins in size and electrophoretic mobility were present in the prenodal peripheral lymph of control as well as cholesterol-fed dogs. Most of the lipoproteins in control dogs were high density lipoproteins, both in the plasma and in the lymph. Cholesterol feeding resulted in an increased concentration of lipoprotein particles with decreased electrophoretic mobility (beta-VLDL and, in plasma, HDLc) and decreased concentration of HDL, both in plasma and in lymph. Size distribution of lipoproteins was also markedly altered by cholesterol feeding; most of the lipoproteins were present as IDL and VLDL both in plasma and in lymph. Judged by agarose gel chromatography, the size of the lymph HDL as consistently larger than plasma HDL in both groups of dogs. Furthermore, it appears that cholesterol feeding increased the size of an HDL subfraction, partially resolved by agarose chromatography, both in lymph and plasma. All apolipoproteins present in plasma were also present in lymph. Cholesterol feeding resulted in 3-10-fold increases in plasma apo B, E, and A-IV while apo A-I was drastically decreased. These changes were reflected in lymph to different degrees depending on the size of the lipoprotein fraction containing the individual apolipoproteins. Our findings provide direct evidence that the large, cholesterol-rich, 'atherogenic' lipoproteins found in the plasma of cholesterol-fed dogs (beta-VLDL) are also present in the interstitial fluid and presumably interact with peripheral cells. Our studies furthermore suggest modification of plasma HDL by peripheral cells and/or de novo assembly of an HDL subfraction. The utilization of this animal model may thus provide a direct approach to the study of the interaction of lipoproteins with peripheral cells.

Identification of a new structural variant of human apolipoprotein E, E2(Lys146 leads to Gln), in a type III hyperlipoproteinemic subject with the E3/2 phenotype

A type III hyperlipoproteinemic subject having the apolipoprotein E (apo E) phenotype E3/2 was identified. From isoelectric focusing experiments in conjunction with cysteamine treatment (a method that measures cysteine content in apo E), the E2 isoform of this subject was determined to have only one cysteine residue, in contrast to all previously studied E2 apoproteins, which had two cysteines. This single cysteine was shown to be at residue 112, the same site at which it occurs in apo E3. From amino acid and sequence analyses, it was determined that this apo E2 differed from apo E3 by the occurrence of glutamine rather than lysine at residue 146. When phospholipid X protein recombinants of the subject's isolated E3 and E2 isoforms were tested for their ability to bind to the human fibroblast apo-B,E receptor, it was found that the E3 bound normally (compared with an apo E3 control) but that the E2 had defective binding (approximately 40% of normal). Although they contained E3 as well as E2, the beta-very low density lipoproteins (beta-VLDL) from this subject were very similar in character to the beta-VLDL from an E2/2 type III hyperlipoproteinemic subject; similar subfractions could be obtained from each subject and were shown to have a similar ability to stimulate cholesteryl ester accumulation in mouse peritoneal macrophages. The new apo E2 variant has also been detected in a second type III hyperlipoproteinemic subject.

Cholesteryl ester accumulation in mouse peritoneal macrophages induced by beta-migrating very low density lipoproteins from patients with atypical dysbetalipoproteinemia

The d < 1.006 lipoproteins of patients in a kindred with atypical dysbetalipoproteinemia induced marked cholesteryl ester accumulation in mouse peritoneal macrophages. The affected family members had severe hypercholesterolemia and hypertriglyceridemia, xanthomatosis, premature vascular disease, the apo-E3/3 phenotype, and a predominance of cholesterol-rich beta-very low density lipoproteins (beta-VLDL) in the d < 1.006 fraction. When incubated with mouse peritoneal macrophages, the d < 1.006 lipoproteins or beta-VLDL from the affected family members stimulated cholesteryl [(14)C]oleate synthesis 15- to 30-fold above that caused by normal, control d < 1.006 lipoproteins (VLDL). The ability of the beta-VLDL to stimulate macrophage cholesteryl ester accumulation was greatly reduced as a consequence of treatment with hypolipidemic agents, which specifically reduced the concentration of beta-VLDL. Two important differences were noted in a comparison of the beta-VLDL from these atypical dysbetalipoproteinemic subjects with that of classic E2/2 dysbetalipoproteinemics: (a) the beta-VLDL from the atypical subjects were severalfold more active in stimulating cholesteryl ester accumulation in macrophages, and (b) both the intestinal and hepatic beta-VLDL from the atypical subjects were active. The triglyceriderich, alpha(2)-migrating VLDL from the affected family members constituted <10% of the d < 1.006 fraction and were similar to normal VLDL in that they did not stimulate cholesteryl ester synthesis in the macrophages. Several lines of evidence indicate that the macrophage accumulation of cholesteryl esters was induced by a receptor-mediated uptake process and that the beta-VLDL were bound by a specific beta-VLDL receptor. First, the uptake and degradation of the lipoproteins and the induction of cholesteryl ester formation displayed qualities of high affinity, saturable kinetics. Second, the uptake and degradation process was inhibited when the lysyl residues of the beta-VLDL apoproteins were modified by reductive methylation. Third, the beta-VLDL from the affected subjects competed with diet-induced canine (125)I-beta-VLDL for the same cell surface receptors, but did not compete with chemically modified low density lipoproteins. Finally, the receptor-mediated uptake of these beta-VLDL resulted in lysosomal degradation of the lipoproteins, which could be prevented by incubating the cells with chloroquine. Normal, triglyceride-rich VLDL were also degraded when incubated with the macrophages, but they were not degraded by the same receptor-mediated process responsible for the degradation of the beta-VLDL of the patients. The degradation of the VLDL was not abolished by reductive methylation of the lipoproteins or by treatment of the cells with choloroquine. These studies demonstrate that the beta-VLDL from subjects with atypical dysbetalipoproteinemia are taken up by macrophages via the same receptor-mediated process responsible for the uptake of diet induced beta-VLDL. The accelerated vascular disease seen in these patients may be the result of high concentrations of beta-VLDL capable of binding to and delivering large quantities of cholesterol to macrophages and converting them into cells resembling the foam cells of atherosclerotic lesions.

Apolipoprotein E polymorphism and coronary artery disease

Lipid status and apolipoprotein E phenotypes were tested in 1000 patients who underwent coronary angiography. The same number of factory employees was chosen as a control group. We distinguished between six different apolipoprotein E phenotypes and determined their frequencies in all groups. For the three homozygous phenotypes E3/3, E4/4, and E2/2, the percentage distribution in the group of factory employees was 62.7%, 2.3%, and 0.8%, respectively; for the three heterozygous phenotypes E4/3, E3/2, and E4/2, we determined frequencies of 20.3%, 11.0%, and 3.0%, respectively. In the group of patients with and without signs of coronary atherosclerosis, we observed almost the same frequencies except that heterozygotes (E3/2) occurred significantly more frequently in the group of coronary angiography patients unaffected by coronary sclerosis. Cholesterol and triglyceride values were significantly elevated in patients with coronary artery disease, whereas high density lipoprotein cholesterol levels were not significantly different. The data further suggest that apolipoprotein E2/2 homozygosity, despite the presence of beta-very low density lipoproteins in the plasma of these patients, cannot be considered a biochemical indicator of an increased risk of coronary atherosclerosis. On the other hand, apolipoprotein E3/2 heterozygosity may have a protective effect on the development of early atherosclerosis.