Cholesterol-fed Dogs Research Articles

Thermography basket catheter: in vivo measurement of the temperature of atherosclerotic plaques for detection of vulnerable plaques.

We have developed an intravascular thermography basket catheter to measure the temperature of the vessel wall to locate foci of inflammation. Our 3 Fr thermography basket catheter is a thermocouple-based catheter made of a nitinol expandable and externally controllable basket system loaded with nine small and flexible built-in thermosensors. It is equipped with real-time data acquisition software with a thermal resolution of 0.0001 degrees C and a sampling rate of 20 readings per second. In 10 inbred cholesterol-fed dogs with femoral (but not carotid) atherosclerosis, we found foci of warmth on the surface of atherosclerotic but not disease-free regions (P < 0.05). Marked temperature heterogeneity was also observed in the aortas of atherosclerotic Watanabe rabbits but not in normal rabbits. The catheters showed satisfactory accuracy, reproducibility, and safety. If confirmed in further studies, it has the potential to be utilized in detection of vulnerable plaques.

Ezetimibe: a selective inhibitor of cholesterol absorption

Ezetimibe is a novel selective cholesterol absorption inhibitor that effectively blocks intestinal absorption of dietary and biliary cholesterol. Ezetimibe undergoes glucuronidation to a single metabolite and is localized in the intestinal wall, where it prevents cholesterol absorption. Enterohepatic recirculation of ezetimibe and the glucoronide ensures repeated delivery to the site of action and limits peripheral exposure. Ezetimibe does not affect the absorption of fat-soluble vitamins or triglycerides. Results from pre-clinical studies in various animal models have shown the lipid-lowering and anti-atherosclerotic properties of ezetimibe as a single agent, and a synergistic effect when combined with a statin. In cholesterol-fed rhesus monkeys, ezetimibe reduced both plasma cholesterol (ED50 = 0·0005 mg.kg−1.day−1) and low-density lipoprotein cholesterol levels in a dose-dependent manner. In apo F knockout mice, ezetimibe reduced serum cholesterol more than 50% and decreased carotid (97%) and aortic (47–87%) atherosclerosis. Ezetimibe inhibited the rise of plasma cholesterol in cholesterol-fed dogs (ED50 = 0·007 mg.kg−1.day−1). Co-administration of ezetimibe and lovastatin in chow-fed dogs synergistically reduced plasma cholesterol to levels lower than those achieved with either agent alone (⁠ \(P{<}0{\cdot}05\) ⁠). These results suggest that ezetimibe combined with a statin may similarly reduce plasma cholesterol levels in patients with hypercholesterolaemia.

Heterocyclic amides: inhibitors of acyl-CoA:cholesterol O-acyl transferase with hypocholesterolemic activity in several species and antiatherosclerotic activity in the rabbit.

A series of heterocyclic amides were synthesized and evaluated as inhibitors of acyl-CoA: cholesterol O-acyltransferase (ACAT) in vitro and for cholesterol lowering in cholesterol-fed rats. Compounds were evaluated for cell-based macrophage ACAT inhibition, bioactivity, and adrenal toxicity. Candidates were selected for evaluation in cholesterol-fed dogs and, ultimately, the injured cholesterol-fed rabbit model of atherosclerosis. The heterocyclic amides potently inhibited rabbit liver ACAT (IC50's = 0.014-0.11 microM), and the majority of compounds significantly lowered plasma cholesterol (42-68%) in an acute cholesterol-fed rat model at 3 mg/kg. The most efficacious compounds in the rat were evaluated for bioactivity in vivo and arterial ACAT inhibition in a cell-based macrophage ACAT assay. Two highly bioactive analogs, (+/-)-2-(3-dodecylisoxazol-5-yl)-2-phenyl-N-(2,4,6-trimethoxypheny l) acetamide (13a) and (+/-)-2-(5-dodecylisoxazol-3-yl)-2-phenyl-N-(2,4,6-trimethoxypheny l) acetamide (16a), were selected for further study and were found to be nontoxic in a guinea pig model of adrenal toxicity. Compounds 13a and 16a lowered total cholesterol in the cholesterol-fed rat, rabbit, and dog models of pre-established hypercholesterolemia. Compound 13a in the injured cholesterol-fed rabbit model of atherosclerosis was effective in slowing the development of cholesteryl ester-rich thoracic aortic lesions, reducing lesion coverage by 53% at a dose of 1 mg/kg.

Comparison of the lipid and apolipoprotein composition of skeletal muscle and peripheral lymph in control dogs and in dogs fed a high fat, high cholesterol, hypothyroid-inducing diet

Most studies of peripheral interstitial fluid lipoprotein composition have been made on interstitial fluid-derived from skin and connective tissue. We developed techniques which allowed simultaneous comparison of lymph (a model of interstitial fluid) from skeletal muscle and skin in control (C) and cholesterol-fed (CF) dogs. Lipoprotein fractions were separated by ultracentrifugation. Skeletal muscle interstitial fluid HDL concentrations were approximately twice those of skin. However, the concentration of VLDL-LDL particles was similar in both interstitial spaces. HDL particles from both microvascular beds showed evidence of extensive remodelling when compared to plasma HDL from the same animal. Relative to apo A-I, skeletal muscle HDL was enriched in free cholesterol and apo E (C and CF dogs) and apo A-IV (CF dogs). Skin-derived HDL was consistently enriched in free cholesterol, apo E and A-IV in both C and CF dogs. These studies indicate that similar remodeling of plasma HDL occurs in widely different tissues which together constitute approximately 70% of the total interstitial space. The relatively high concentration of plasma-derived and remodeled HDL within the interstitial space of skeletal muscle is consistent with that tissue's importance in reverse cholesterol transport.

Metabolism of canine beta-very low density lipoproteins in normal and cholesterol-fed dogs.

Cholesteryl ester-rich beta-very low density lipoproteins (beta-VLDL) are beta-migrating lipoproteins that accumulate in the plasma of cholesterol-fed animals and of patients with type III hyperlipoproteinemia. There are two distinct fractions: fraction I beta-VLDL are chylomicron remnants of intestinal origin, and fraction II beta-VLDL are cholesterol-rich VLDL of hepatic origin. The liver rapidly clears fraction I beta-VLDL from the plasma of both normal and cholesterol-fed dogs. The liver also clears fraction II beta-VLDL rapidly and efficiently from the plasma of normal dogs by receptor-mediated uptake. In cholesterol-fed dogs the clearance is biphasic: an initial rapid die-away of about 30% to 40% of the injected dose within 5 minutes, followed by a slow clearance of plasma radioactivity (a half-life of more than 20 hours). The rapid, initial phase of fraction II beta-VLDL clearance appears to be related to sequestration of the lipoproteins presumably on endothelial cells and is apparently associated with lipolytic processing. Treatment of the fraction II beta-VLDL with lipoprotein lipase abolishes this rapid phase. In the cholesterol-fed dog, the slow, late phase of clearance corresponds to the conversion of fraction II beta-VLDL to the smaller, denser intermediate and low density lipoproteins (IDL and LDL), which are slowly cleared from the plasma. It is concluded that fraction II beta-VLDL are catabolized in the normal dog by rapid uptake mediated at least in part by the apo B,E(LDL) receptor of hepatic parenchymal cells. In cholesterol-fed dogs, in which these receptors are markedly down-regulated, fraction II beta-VLDL are apparently initially bound to endothelial cells and converted to IDL and LDL by lipolytic processing.

Characterization of lipoprotein receptors on rat Fu5AH hepatoma cells.

The rat hepatoma cell line Fu5AH has the unusual property of accumulating massive amounts of cholesteryl ester upon incubation with hypercholesterolemic serum, and especially when incubated with beta-very low density lipoproteins (beta-VLDL) from cholesterol-fed dogs. The present study was designed to identify and characterize the lipoprotein receptors that mediate the cholesteryl ester accumulation. The beta-VLDL and cholesterol-induced apolipoprotein (apo) E-containing high density lipoproteins (apoE HDLc) bound to Fu5AH cells with very high affinity (Kd approximately equal to 10(-10) M), whereas low density lipoproteins (LDL) bound with unusually low affinity (Kd approximately equal to 10(-8) M). Receptor binding activity of 125I-labeled beta-VLDL, 125I-labeled apoE HDLc, and 125I-labeled LDL was abolished by incubation in the presence of an excess of unlabeled LDL or of a polyclonal antibody to the bovine adrenal apoB,E(LDL) receptor. The receptors were completely down-regulated by preincubating Fu5AH cells with beta-VLDL, but much higher levels of beta-VLDL were required than for down-regulation of fibroblast apoB,E(LDL) receptors. Receptor binding was abolished by reductive methylation of the lysyl residues of the apolipoprotein of the beta-VLDL and by an apoE monoclonal antibody (1D7) that blocks receptor binding. The Fu5AH receptor was further characterized by using the bovine adrenal apoB,E(LDL) receptor antibody. A single protein (Mr approximately equal to 130,000) was identified in Triton extracts of whole cells, and two proteins (Mr approximately equal to 130,000 and 115,000) were found in Fu5AH cell membranes disrupted by homogenization. The Mr approximately equal to 115,000 protein was released from the membranes and did not react with an antibody to the carboxyl-terminal (cytoplasmic) domain of the apoB,E(LDL) receptors. These studies indicate that Fu5AH cells express apoB,E(LDL) receptors that have unusually low affinity for apoB-continuing lipoproteins, require large amounts of cholesterol to induce down-regulation, and are susceptible to specific proteolysis in cell homogenates. These apoB,E(LDL) receptors are responsible for the receptor-mediated uptake of beta-VLDL and chylomicron remnants by Fu5AH cells.

Peripheral synthesis and isoform distribution of dog apoprotein E. An in vivo approach.

Purified dog plasma apo-E is composed of four major isoforms with the following pI values: 5.40, 5.31, 5.26, and 5.22. Treatment with neuraminidase suggests that the multiple forms are due to progressive sialation. The acidic isoforms (pI = 5.22 and less), rarely detectable in plasma lipoprotein samples (except in the d less than 1.05 g/ml fraction of cholesterol-fed dogs), are present in high concentrations in the interstitial fluid high density lipoprotein fraction I (HDLI) of cholesterol-fed dogs, a lipoprotein recently described (Dory, L., Boquet, L.M., Hamilton, R.L., Sloop, C.H., and Roheim, P.S. (1985) J. Lipid Res. 26, 519-527). Apo-Es0 (the most basic isoform) is a major constituent of the plasma d less than 1.05 g/ml fraction, but it is usually only a minor component of other plasma or interstitial fluid lipoproteins. This is likely a reflection of the prolonged residence time of a specific lipoprotein species in this density range, resulting in more extensive desialation. Peripheral apo-E synthesis has been measured under in vivo conditions using a hindlimb cannulation of the lymphatics and collection of prenodal peripheral lymph. Following injection of [3H]leucine or [35S]methionine into the dorsal skin of the toes, the specific activity of the interstitial fluid apo-E far exceeded that of plasma apo-E at all time points examined. Incorporation was measurable 30 min after the isotope injection and peaked at 150 min in control dogs and between 120-150 min in cholesterol-fed dogs. The rate of peripheral synthesis of apo-E in cholesterol-fed dogs appeared to be twice that of control dogs. The newly synthesized and secreted apo-E preferentially associated with the disc-shaped HDLI of the interstitial fluid; less than 15% of the apo-E-associated radioactivity was recovered in the d less than 1.05 g/ml fraction, despite the fact that this fraction contains well over 50% of the total interstitial fluid apo-E mass. The newly secreted, HDLI-associated apo-E can be converted by neuraminidase into apo-Es0.

Heterogeneity of dog interstitial fluid (peripheral lymph) high density lipoproteins: implications for a role in reverse cholesterol transport.

The heterogeneity of dog interstitial fluid (peripheral lymph) high density lipoprotein (HDL) was investigated and compared to plasma HDL. Interstitial fluid and plasma HDL of normal and cholesterol-fed dogs was subfractionated by ultracentrifugation and affinity and molecular weight sieving chromatography. Both plasma (P) and interstitial fluid (L) HDL can be subfractionated into a larger fraction (P-I and L-I) and a smaller one (P-II and L-II). Cholesterol feeding induces a large increase in the P-I and L-I component of HDL, but the increase in L-I is far greater in proportion than that of P-I. Furthermore, L-I of cholesterol-fed dogs appears to be almost exclusively discoid in shape, while only approximately 15% of particles in P-I are discoidal. The discoid HDL of L-I is reflected in its chemical composition: 28% unesterified cholesterol, 6% cholesteryl ester, 45% phospholipid, and 21% protein. It contains large amounts of apoE in addition to apoA-I and apoA-IV. We found that the association of apoE with discoid particles is frequent, but not necessary. Calculations based on known protein mass and quantitation of discoid particles on electron micrographs suggest that the concentration of discoid particles in the peripheral lymph of cholesterol-fed dogs is about fourfold that of the plasma of the same animal. These findings provide strong circumstantial evidence for the peripheral formation of discoid HDL, perhaps as an early event in reverse cholesterol transport.

Binding of chylomicron remnants and beta-very low density lipoproteins to hepatic and extrahepatic lipoprotein receptors. A process independent of apolipoprotein B48.

The ability of apolipoprotein (apo-) B48 to interact with lipoprotein receptors was investigated using three different types of lipoproteins. First, canine chylomicron remnants, which contained apo-B48 as their primary apoprotein constituent, were generated by the hydrolysis of chylomicrons with milk lipoprotein lipase. These apo-B48-containing chylomicron remnants are deficient in apo-E and reacted very poorly with apo-E receptors on adult dog liver membranes and the low density lipoprotein (apo-B,E) receptors on human fibroblasts. Addition of normal human apo-E3 restored the receptor binding activity of these lipoproteins. Second, beta-very low density lipoproteins (beta-VLDL) from cholesterol-fed dogs were subfractionated into distinct classes containing apo-E along with either apo-B48 or apo-B100. Both classes bound to the apo-B,E and apo-E receptors. Their binding was almost completely mediated by apo-E, as evidenced by the ability of the anti-apo-E to inhibit the receptor interaction. Third, beta-VLDL from type III hyperlipoproteinemic patients were subfractionated by immunoaffinity chromatography into lipoproteins containing apo-E plus either apo-B48 or apo-B100. Both subfractions bound poorly to apo-B,E and apo-E receptors due to the presence of defective apo-E2. However, the residual binding of the apo-B48-containing and apo-B100-containing human beta-VLDL was inhibited by the anti-apo-E. After lipase hydrolysis, apo-B100 became a more prominant determinant responsible for mediating receptor binding to the apo-B,E receptor. By contrast, lipase hydrolysis did not increase the binding activity of the apo-B48-containing beta-VLDL. These results indicate that apo-B48 does not play a direct role in mediating the interaction of lipoproteins with receptors on fibroblasts or liver membranes.

Uptake of apolipoprotein E-containing high density lipoproteins by hepatic parenchymal cells.

Cholesterol-enriched, apolipoprotein E-containing high density lipoproteins (apo E HDLc), which were isolated from the plasma of cholesterol-fed dogs by using agarose column chromatography or ultracentrifugation, possessed essentially identical biochemical and metabolic characteristics. Radioiodinated (125I) apo E HDLc isolated by either method gave identical rates of clearance from the plasma, i.e., greater than 50% of the injected dose was cleared from the plasma within 5 to 10 minutes, principally by the liver. Detailed studies localizing apo E HDLc uptake to specific cell types within the liver were performed in both normal and cholesterol-fed rats. The validity of using the canine apo E HDLc in the rat was supported by observations of marked similarities in plasma clearance, i.e., a rapid acute phase of disappearance, and a near-quantitative hepatic uptake of lipoproteins in both species. Canine apo E HDLc (fluorescently labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine), which were injected into normal rats, appeared to be taken up primarily by parenchymal cells, as determined by fluorescence microscopy. Light microscopic autoradiography also revealed that the uptake of 125I apo E HDLc was principally carried out by parenchymal cells in rat liver. Likewise, the uptake of apo E HDLc by the liver of cholesterol-fed rats was extensively localized to parenchymal cells. An in situ, single-pass perfusion of a lobule of the liver of a normal dog with iodinated and fluorescently labeled apo E HDLc confirmed that the uptake of the lipoproteins was principally carried out by parenchymal cells. The plasma clearance of apo E HDLc by hepatic parenchymal cells, even in cholesterol-fed animals in which the apo B,E (LDL) receptors were markedly down-regulated (undetectable), suggests that the apo E receptor, presumably the remnant receptor, is localized in the parenchymal cells.

Cardiac and peripheral lymph lipoproteins in dogs fed cholesterol and saturated fat.

In normal dogs fed diets containing 1% cholesterol and 30% saturated fat, cholesterol concentration increased in plasma, cardiac lymph, and peripheral lymph. The increase in cardiac lymph cholesterol always exceeded that of peripheral lymph. At plateau, LDL cholesterol increased in plasma and peripheral lymph but not in cardiac lymph; HDL2 cholesterol increased in all compartments and HDLc appeared in plasma (86.6 +/- 22.2 mg cholesterol/dl) with cholesterol feeding. In cardiac and peripheral lymph, a lipoprotein fraction with ultracentrifugal and pevikon electrophoretic characteristics of plasma HDLc (designated HDL 1.063) was isolated and contained 9.4 +/- 1.3 and 11.9 +/- 6.8 mg cholesterol/dl respectively. The lymph/plasma (L/P) ratios of lipoproteins were generally higher in cardiac lymph compared to peripheral lymph. In cholesterol-fed dogs, the L/P ratios of LDL were reduced compared to control dogs while HDL2 was unaffected, indicating selective interstitial distribution of lipoproteins. On agarose electrophoresis, lymph lipoproteins migrated faster than plasma lipoproteins, suggesting subtle changes in the structure and/or composition, findings consistent with the suggestion that lipoproteins undergo modification in the interstitial space. Thus, each interstitial space, as reflected by lymph, has a characteristic mixture of lipoproteins that can be altered by a high cholesterol diet in a manner that is unique to each particular tissue compartment.

Characterization of dog peripheral lymph lipoproteins: the presence of a disc-shaped "nascent" high density lipoprotein.

The distribution, chemical, and apoprotein composition of plasma and peripheral lymph lipoproteins were compared in control and cholesterol-fed dogs. In both groups of animals, the agarose electrophoretic patterns of plasma and lymph lipoproteins were similar. In hypercholesterolemic dogs, beta-very low density lipoprotein, beta-migrating intermediate density lipoprotein, and HDLc were major components both in plasma and lymph, providing evidence for a potential interaction of these atherogenic particles with macrophages and other peripheral cells. The chemical composition and physical appearance of peripheral lymph HDL was markedly different from that of plasma HDL (high density lipoprotein), especially in the cholesterol-fed animals. Lymph HDL had a higher cholesterol to protein ratio and a markedly increased free cholesterol content (free cholesterol to cholesteryl ester ratio of 1.7 as opposed to 0.2 in plasma HDL in cholesterol-fed animals). The phospholipid content of lymph HDL was higher than that of plasma HDL, while the protein content was lower. A significant proportion of lymph HDL obtained from cholesterol-fed dogs was in the form of disc-shaped particles stacked in rouleau structures. Changes in plasma apolipoprotein concentrations due to cholesterol feeding were reflected in peripheral lymph to different degrees, depending largely on the relative size of the lipoproteins containing the individual lipoproteins. A considerable enrichment of lymph HDL with apoE and apoA-IV was observed by both immunochemical and electrophoretic methods. In lymph HDL from control and cholesterol-fed dogs, the apoE/apoA-I and apoA-IV/apoA-I ratios were several-fold elevated, compared to those of plasma HDL. It is concluded, therefore, that during cholesterol feeding a substantial portion of interstitial HDL is assembled de novo in the periphery as a crucial stage of reverse cholesterol transport to the liver. It is likely that further modification occurs upon entry to plasma and exposure to lecithin:cholesterol acyltransferase, possibly leading to generation of HDLc. Alternatively, these particles may be directly and rapidly removed by the liver.

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 (β-VLDL) and, in plasma, HDL c) 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

Lecithin:cholesterol acyltransferase-mediated modification of discoidal peripheral lymph high density lipoproteins: possible mechanism of formation of cholesterol-induced high density lipoproteins (HDLc) in cholesterol-fed dogs.

Peripheral lymph high density lipoproteins (HDL) of the cholesterol-fed dog differ in a number of characteristics from plasma HDL of the same animal. Their high content of free cholesterol, phospholipid, apoprotein E, and apoprotein A-IV, their greater heterogeneity in size, and the presence of many discoidal particles suggest that a portion of lymph HDL is assembled within the interstitial fluid. The present experiments demonstrate that the endogenous lecithin:cholesterol acyltransferase (LCAT) activity of whole peripheral lymph of the cholesterol-fed dog is far less (less than 1%) than that found in the plasma of the same animal (0.3 nmol/hr per ml versus 40.6 nmol/hr per ml). Addition of partially purified LCAT to whole lymph induced many changes in the chemical composition of peripheral lymph lipoproteins. After incubation with LCAT, the free cholesterol and phospholipid contents of lymph HDL decreased, from 17% to 12% and from 46% to 33%, respectively, whereas cholesteryl ester content increased from 7% to 13%. These changes were accompanied by a mass transfer of apoprotein E and cholesterol to the p less than 1.05 g/ml fraction, the complete disappearance of the discoidal particles, and a decrease in size heterogeneity of lymph HDL. These results suggest that, in the cholesterol-fed dog, cholesterol efflux into the interstitial spaces may occur in the absence of significant LCAT activity. Furthermore, our studies suggest that the subsequent reaction between lymph HDL and LCAT within the vascular compartment leads to the generation of apoprotein E and cholesteryl ester-rich cholesterol-induced HDL.

Structural and metabolic heterogeneity of beta-very low density lipoproteins from cholesterol-fed dogs and from humans with type III hyperlipoproteinemia.

Cholesteryl ester-rich beta-very low density lipoproteins (beta-VLDL) are beta-migrating lipoproteins that accumulate in the d < 1.006 g/ml fraction of plasma from cholesterol-fed animals and from patients with Type III hyperlipoproteinemia. They can be separated from pre-beta-migrating very low density lipoproteins in the d 1.006 g/ml fraction by Geon-Pevikon block electrophoresis. The beta-VLDL have a general property of stimulating cholesteryl ester synthesis and accumulation in macrophages. In the present study, we demonstrated that beta-VLDL obtained from cholesterol-fed dogs fasted for 16 hr were heterogeneous and that two subpopulations of particles, referred to as Fractions I and II, could be isolated from the whole beta-VLDL fraction using gel filtration chromatography. These fractions of beta-VLDL were similar in that both were cholesteryl ester rich, had beta-electrophoretic mobility on Geon-Pevikon electrophoresis, and possessed the B and E apoproteins as major constituents. However, Fractions I and II differed in size, shape, electrophoretic mobility, chemical composition, and apoprotein B type. (Fraction I vs. Fraction II: size: 90 to 300 nm vs. 20 to 70 nm; shape: irregular with redundant surface vs. spherical; electrophoretic mobility on paper: origin vs. beta; chemical composition: rich in phospholipid and poor in protein vs. rich in protein and poor in triglycerides; apoprotein B types: equal amounts of the high and low molecular weight forms vs. predominantly the high molecular weight form.) Furthermore, Fraction I was 3- to 15-fold more active than Fraction II in stimulating cholesteryl ester formation in mouse peritoneal macrophages. The concentration of Fraction I, but not Fraction II, was diminished in plasma by prolonged fasting, and Fraction I transported more intestinal-absorbed retinol than Fraction II. In addition, the plasma clearance of Fraction I injected into cholesterol-fed dogs was distinctly different from the clearance of Fraction II, and the in vivo dieaway of Fraction I resembled that of chylomicrons and chylomicron remnants. These findings suggest that beta-VLDL in dogs are composed of cholesteryl ester-rich chylomicron remnants (Fraction I) and cholesteryl ester-rich lipoproteins, probably of liver origin (Fraction II). Finally, in studies of two patients with Type III hyperlipoproteinemia, we also identified the existence of two fractions in the beta-VLDL with characteristics similar to Fractions I and II of cholesterol-fed dogs.-Fainaru, M., R. W. Mahley, R. L. Hamilton, and T. L. Innerarity. Structural and metabolic heterogeneity of beta-very low density lipoproteins from cholesterol-fed dogs and from humans with Type III hyperlipoproteinemia.

Chylomicron metabolism during dietary-induced hypercholesterolemia in dogs.

The metabolism of [1-3H]retinol- and [4-14C]cholesterol-labeled chylomicrons was studied in normal and cholesterol-fed dogs in order to estimate the relative contribution of chylomicron remnant cholesterol to diet-induced hypercholesterolemia. The plasma t 1/2 of intravenously administered Sf greater than 400 chylomicrons, Sf 20-400 chylomicrons, and whole lymph doubly labeled with [1-3H]retinol and [4-14C]cholesterol was not significantly prolonged in hypercholesterolemic recipients. When Sf greater than 400 chylomicrons were administered intravenously, 90% of the radioactivity was cleared from the plasma of both normal and cholesterol-fed dogs within 1 hr and 68 +/- 18% appeared in the liver within approximately 2 hr in normal dogs and 4 hr in hypercholesterolemic dogs. The use of the retinol-labeling technique for intestinal lipoproteins provided evidence that some LDL, but essentially none of the HDLc, was derived from d greater than 1.006 g/ml lymph lipoproteins. The failure of significant radioactivity to accumulate in the plasma compartment of hypercholesterolemic dogs after intravenous administration of doubly labeled chylomicrons and the relatively efficient uptake of radioactivity by the liver indicate that the dietary-induced hypercholesterolemia in dogs is not the result of impaired hepatic removal of chylomicron remnants.

Immunoregulatory plasma lipoproteins. Role of apoprotein E and apoprotein B.

Plasma lipoproteins containing either the B (apo-B) or E (apo-E) apoproteins, e.g. apo-B low density lipoproteins (LDL) isolated from normolipemic humans and apo-E-containing cholesterol-induced high density lipoproteins (apo-E HDLc) isolated from cholesterol-fed dogs, suppress phytohemagglutinin (PHA)-induced lymphocyte activation and inhibit early events such as mitogen-enhanced 45Ca2+ accumulation and late events such as enhanced DNA synthesis. On a molar basis, apo-E HDLc are 3.5 times more effective than apo-B LDL in inhibiting 45Ca2+ accumulation by 50% and 3.8 times more effective than apo-B LDL in suppressing DNA synthesis by 50%. Both lipoproteins bind to the lymphocyte surface, and in competitive binding assays apo-E HDLc and apo-B LDL bind to the same receptors. These receptors, termed immunoregulatory receptors, comprise a homogeneous class of binding sites which do not act cooperatively. The equilibrium dissociation constant (Kd) for apo-B LDL is 2.0 X 10(-7) M at 4 degrees C and 37 degrees C; the Kd for apo-E HDLc is 9.3 X 10(-8) M at 4 degrees C and 7.3 X 10(-8) M at 37 degrees C. At saturation, 16,000 and 20,000 LDL particles are bound/cell at 4 degrees C and 37 degrees C, respectively. The corresponding values for apo-E HDLc are 6,700 at 4 degrees C and 5,500 at 37 degrees C. The increased effectiveness of apo-E HDLc in supp]ressing the PHA-induced Ca2+ accumulation and DNA synthesis is due to the multiple receptor binding of apo-E HDLc. At 37 degrees C, each apo-E HDLc particle occupies multiple receptors at a ratio of 3.6:1 relative to apo-B LDL. The enhanced affinity of apo-E HDLc most likely results from the multiple interactions of this lipoprotein with the receptors.

Cholesteryl ester synthesis in macrophages: stimulation by beta-very low density lipoproteins from cholesterol-fed animals of several species

Animals fed cholesterol accumulate several types of cholesterol-rich lipoproteins in their plasma and ultimately develop cholesteryl ester deposition in tissue macrophages. Previous studies in the cholesterol-fed dog have shown that one class of cholesterol-rich lipoproteins. beta-migrating very low density lipoproteins (beta-VLDL, density < 1.006 g/ml), possesses a unique ability to produce cellular cholesteryl ester accumulation when incubated with mouse peritoneal macrophages in vitro. This accumulation results from the receptor-mediated uptake of beta-VLDL with subsequent lysosomal hydrolysis of the lipoprotein and re-esterification of the liberated cholesterol. In the current studies, we demonstrate that beta-VLDL obtained from cholesterol-fed animals of several other species, including monkeys, rabbits, and rats, also causes cholesteryl ester accumulation in monolayers of mouse peritoneal macrophages, as monitored by an increase in the rate at which the cells incorporate exogenous [14C]oleate into cholesteryl [14C]oleate. Like canine beta-VLDL, the beta-VLDL from these three other species were effective at low concentrations and exhibited saturation kinetics, suggesting that they, too, entered macrophages by receptor-mediated endocytosis. Very low density lipoprotein (VLDL) from normal animals and low density lipoprotein (LDL) from normal and cholesterol-fed monkeys, rats, and rabbits did not stimulate cholesteryl ester synthesis in mouse peritoneal macrophages. In addition to their effects on mouse macrophages, the beta-VLDL from cholesterol-fed dogs and rabbits stimulated cholesteryl ester synthesis in cultured human monocytes. The current findings suggest that beta-VLDL from cholesterol-fed animals has the general property of stimulating cholesteryl ester synthesis and accumulation in macrophages.

Red cell cholesterol enrichment and spur cell anemia in dogs fed a cholesterol-enriched atherogenic diet

A diet supplemented with cholesterol and coconut oil is atherogenic in dogs. The purpose of the present study was to examine the effects of this diet on red cells in pure-bred beagles and greyhounds. Within 3 days after the initiation of this diet red cell cholesterol/phospholipid increased and membrane fluidity decreased, with maximum changes attained by 12 weeks. Serum lipoprotein cholesterol/phospholipid also increased, and serum from cholesterol-fed dogs transferred cholesterol to normal red cells. Significant abnormalities of liver function developed in all cholesterol-fed dogs. Hematocrit declined beginning at 6 weeks, with a parallel increase in osmotic fragility. Reticulocytes were elevated in beagles but normal in greyhounds. Red cell morphology resembled acanthocytes or spur cells. All red cell parameters returned to normal within 4 weeks after stopping the diet. These studies demonstrate that a cholestrol-enriched, atherogenic diet causes profound and reversible changes in the lipid composition, membrane fluidity, and morphology of red cells in dogs.

Cholesteryl ester accumulation in macrophages resulting from receptor-mediated uptake and degradation of hypercholesterolemic canine beta-very low density lipoproteins.

The synthesis and accumulation of cholesteryl esters by monolayers of mouse peritoneal macrophages was stimulated 20- to 160-fold by incubation with beta-migrating very low density lipoproteins (beta-VLDL, density less than 1.006 g/ml) isolated from the plasma of cholesterol-fed dogs. Three other cholesterol-rich lipoprotein fractions obtained from the plasma of the same hypercholesterolemic dogs, including low density lipoprotein (LDL), cholesterol-induced high density lipoprotein (HDLc), and apo-E HDLc, had little to no stimulatory effect. Plasma VLDL (density less than 1.006 g/ml) from normal dogs did not increase cholesteryl ester formation in macrophages. The enhancement in cholesteryl ester synthesis and accumulation by hypercholesterolemic canine beta-VLDL was due to the presence of a high affinity binding site on the macrophage cell surface that mediated the uptake and lysosomal degradation of the beta-VLDL. Competition studies with fucoidin and dextran sulfate indicated that the receptor for canine beta-VLDL was different from that previously described for human acetylated low density lipoprotein (acetyl-LDL). Prior incubation of macrophage monolayers with either unlabeled canine beta-VLDL or human acetyl-LDL, both of which raised the cellular content of cholesteryl esters, reduced the ability of the cells to degrade 125I-labeled beta-VLDL, suggesting that the receptor for beta-VLDL is subject to regulation. The current findings indicate: 1) that macrophages possess a high affinity receptor that recognizes one of the four cholesterol-rich lipoproteins present in the plasma of cholesterol-fed dogs, beta-VLDL, and 2) that the receptor-mediated ingestion of beta-VLDL leads to cholesteryl ester deposition in these cells.