High-density Lipoprotein Catabolism Research Articles

Phenotypic characterization of the Ath-1 gene controlling high density lipoprotein levels and susceptibility to atherosclerosis.

The Ath-1 gene determines the levels of high density lipoprotein (HDL) lipid in response to a high fat diet challenge as well as susceptibility to diet-induced atherosclerosis in mice (Paigen et al. 1987. Proc. Natl. Acad. Sci. USA. 84: 3763-3767). As yet, the identity of the Ath-1 gene and how it acts to affect HDL levels are completely unknown. In an effort to clarify the nature of the gene, we have examined HDL phenotypes in strains carrying either the susceptible or resistant alleles. When challenged with a high fat diet, the susceptible strain C57BL/6 exhibited a marked decrease in the levels of HDL cholesterol and apolipoprotein A-I (apoA-I), the major protein of HDL, whereas the resistant strains C3H and BALB/c maintained high levels of both. Separation of HDL subfractions by polyacrylamide gradient gel electrophoresis revealed that the decrease was particularly striking among the larger HDL species. The rates of synthesis of apoA-I in liver and intestine were similar in the strains and were unaffected by the high fat diet. Although the rates of synthesis of apoA-II and the levels of apoA-II mRNA were decreased in response to the high fat diet, similar decreases were observed in both the susceptible and resistant strains. We conclude that the Ath-1 gene results in a rapid decrease in both HDL lipid and HDL apolipoprotein levels in the susceptible strain in response to the high fat diet and that this is mediated primarily at the level of HDL catabolism.

Abnormalities in lipoprotein metabolism in Gaucher type 1 disease

We have previously described an association between Gaucher type 1 disease and reduced levels of total, low density lipoprotein (LDL), and high density lipoprotein (HDL) cholesterol. Plasma concentrations of apolipoprotein B and apolipoprotein AI were reduced in these subjects, while plasma apolipoprotein E (apoE), which can be synthesized and secreted by macrophages, was increased. To study the pathophysiologic basis for these changes in lipoprotein and apolipoprotein levels, we studied very low density lipoprotein (VLDL), LDL, and HDL metabolism in-depth in four subjects with Gaucher disease. Gel filtration of their plasma revealed that apoE was present in essentially a single population of lipoproteins in the large HLD range. In subject no. 4, studied presplenectomy and post-splenectomy, plasma apoE levels fell after surgery in association with a redistribution of apoE among the plasma lipoproteins to a pattern seen in normal subjects. Determination of the rates of secretion and catabolism of VLDL apoB and triglyceride were within normal limits. The reduced plasma levels of LDL and HDL cholesterol, and of both plasma apoB and apoAI, were associated with increased fractional catabolic rates of these apolipoproteins in LDL and HDL. These results indicate that the hypocholesterolemia present in subjects with Gaucher type 1 disease is associated with increased fractional catabolism of LDL and HDL. These findings, together with the evidence for alternations in plasma apoE metabolism in this disorder, suggest a role for the macrophage as the basis for these abnormalities.

High-density lipoprotein apolipoprotein AI and AII turnover in moderate and severe proteinuria.

The kinetic parameters of high-density lipoprotein (HDL) and its major apolipoproteins (Apo) AI and Apo AII were studied in 2 patients with moderate and severe proteinuria and 2 normal controls after intravenous injection of autologous 125I-HDL. The fractional catabolic rates (FCR) of HDL estimated by urine/plasma radioactivity ratio, and FCR of Apo AI and Apo AII calculated from the radioactivity decay curves were higher in the patients. These results support the concept that high-density lipoproteinuria and renal parenchymal sequestration of HDL found in the nephrotic syndrome contribute to accelerated HDL catabolism.

Significance of hepatic triglyceride lipase activity in the regulation of serum high density lipoproteins in type II diabetes mellitus.

We investigated the regulation of serum high density lipoprotein (HDL) cholesterol metabolism in patients with type II diabetes mellitus by determining the activities of the two lipolytic enzymes that play major roles in the production and degradation of HDL. The activity of lipoprotein lipase (LPL), the enzyme responsible for HDL cholesterol production, and the activity of hepatic triglyceride lipase (HTGL), the enzyme that facilitates the catabolism of HDL, were measured in plasma obtained after iv injection of heparin. Thirty patients were selected to represent a wide range of serum HDL cholesterol concentrations (low, normal, and high HDL cholesterol groups). Mean lipoprotein lipase activity was similar in all three groups [122 +/- 10 (SEM) U/mL in the low HDL, 141 +/- 11 U/mL in the normal HDL, and 148 +/- 30 U/mL in the high HDL group]. Mean HTGL activity was markedly decreased in the high HDL group; the mean values were 346 +/- 28 U/mL in the low HDL, 320 +/- 25 U/mL in the normal HDL, and 191 +/- 23 U/mL in the high HDL groups, respectively. Body weight and insulin requirement correlated directly with HTGL activity and inversely with serum HDL cholesterol levels. These findings suggest that in type II diabetes mellitus low serum HDL cholesterol levels may be due to an increased rate of clearance by HTGL.

Metabolism of high-density lipoprotein in the hyperlipidemic, diabetic [formula omitted] rat

The SHR N -corpulent rat is a new genetically obese strain that is both hyperlipidemic and diabetic. The high density lipoprotein (HDL) fraction from 12-week-old obese males contained significantly greater amounts of protein (+83%), free (+72%) and esterified (+76%) cholesterol, phospholipid (+94%), and triglyceride (+78%). HDL from obese rats were also enriched in C apolipoproteins (apo C-III 0 and apo C-III 3) but had similar relative amounts of both apo A-I and apo E compared to HDL from their lean littermates. HDL protein turnover, measured with 125I-labeled HDL, showed that obese rats had a smaller fractional catabolic rate (FCR) than lean rats, but due to their much larger HDL pool size, they had a significantly higher rate of HDL protein catabolism (obese, 1.98 ± 0.07 mg/whole animal/h v lean, 1.32 ± 0.05 mg/whole animal/h). Therefore, under steady-state conditions, HDL protein production must also have been increased in the obese animals. To determine whether the increased catabolism of HDL protein was associated with increased catabolism of cholesteryl ester (CE), tissue uptake of HDL CE was measured using the nonhydrolyzable ether analogue [ 3H]cholesteryl linoleyl ether. After four hours 41.6 ± 1.6% of the injected dose was cleared from the plasma of lean rats compared with 37.0 ± 1.1% from the plasma of obese rats. In both groups almost one third of the injected dose appeared in the liver ( (lean, 31.01 ± 1.95% v obese, 30.07 ± 1.17%). However, taking into account that the HDL CE pool in the obese rats was more than 80% larger than in the lean rats, total hepatic catabolism of HDL CE was much greater in the obese animals. Adipose tissue and the small intestine also catabolized more HDL CE in the obese v lean rats. We conclude that the increased levels of HDL protein and HDL CE in the obese rats are due predominantly to overproduction rather than to impaired catabolism of HDL, and that the liver remains quantitatively the most important site for HDL CE catabolism in these animals.

High-density lipoprotein turnover

High-density lipoprotein (HDL) metabolism has been reviewed from information derived from turnover studies in humans. The two major HDL apoproteins AI and AII have different removal rates, reflecting the faster catabolism of HDL 2 than of HDL 3. This is caused by the continual cycle of formation of HDL 2 from HDL 3 and its reversion to HDL 3, in response to the need to transport cholesterol and other lipids from extrahepatic cells and catabolized triglyceride-rich lipoproteins. The conversion of HDL 2 to HDL 3 is mediated through a hepatic lipase. Because this lipase is inhibited by estrogen and stimulated by androgens, women have higher HDL 2 levels than men. The synthesis of apoproteins AI and AII is also higher in women than in men. Nutrition also influences HDL turnover. Carbohydrates increase AI and HDL 2 removal, whereas polyunsaturated fatty acids inhibit synthesis. Vegeterians show high HDL removal rates. Thus low-fat, low-cholesterol diets generally lead to lower HDL levels. Disorders that alter HDL composition (such as alcoholic liver disease or Tangier disease) accelerate HDL removal. Other HDL proteins such as apoproteins C and E show faster turnover rates than AI and AII, since the former exchange with triglyceride-rich lipoproteins and participate in their catabolism. Diminished exchange of apoprotein C from HDL to chylomicrons may be responsible for the diminished catabolism of these particles in type V hyperlipoproteinemia. The unusual turnover characteristics of HDL apoprotein AIV are reviewed, suggesting a dual role for this protein in both triglyceride and cholesterol transport. The striking relationship between very low-density lipoprotein (VLDL) and HDL metabolism is expressed in an inverse association between their respective removal rates. That is, when the VLDL removal rate is slow, as in hypertriglyceridemia, HDL removal is fast. This leads to low HDL levels in hypertriglyceridemia, a combination that is favorable for atherosclerosis. The hypothesis is that in hypertriglyceridemia a high rate of transfer of triglyceride to HDL 3 occurs; this produces triglyceride-rich HDL 2, which is highly susceptible to catabolism by hepatic lipase. This not only depletes the HDL 2 concentration but reduces the capacity of available HDL 3 to participate in reverse cholesterol transport

Specific high affinity binding and degradation of high density lipoproteins by isolated epithelial cells of human small intestine

The interaction of high density lipoproteins (HDL) with isolated epithelial cells of human small intestine (enterocytes) was studied. 125I-HDL 3 (density = 1,125 to 1,126 g/cm 3) exhibit a high-affinity (Kd = 8.3 × 10 −8. Bmax = 886 ng/mg cell protein), saturable and reversible binding to isolated enterocytes. In the presence of excess unlabeled HDL 3, the cell surface-bound 125I-HDL 3 are released into the medium nondegraded. Treatment of cells with pronase does not affect 125I-HDL 3 binding. The binding is accompanied with internalization and degradation of 125I-HDL 3. Chloroquine inhibits the degradation and increases 125I-HDL 3 uptake. A threefold excess of HDL 3 and HDL 2 inhibits the binding and degradation of 125I-HDL 3 by 60%, whereas a 20-fold excess of low density lipoproteins (LDL), only by 20%. HDL 3 (20 to 1,000 μg/mL) stimulates the synthesis of sterols and inhibits sterol ester synthesis in enterocytes. The obtained results make it possible to assume that epithelial cells of the small intestine may participate in the catabolism of HDL in human organism.

Metabolism of high density lipoproteins by the perfused rabbit liver.

The role of the liver in the catabolism of high density lipoproteins (HDL) was examined in isolated perfused rabbit livers. Using 125I-labeled rabbit HDL the disappearance of labeled apolipoproteins from the perfusate was biphasic with 7% of the label removed after 20 min and a further 6% between 20 and 90 min. In contrast, with HDL labeled with [3H]cholesteryl esters 35% of label had been removed after 90 min. The effect of liver perfusion on HDL size and composition was further studied by recirculating rabbit HDL for 120 min. In control experiments HDL was incubated at 37 degrees C for 120 min with nonperfused media and with media that had been liver perfused. The added HDL was predominantly particles of 4.8-4.9-mm radius, and incubation with nonperfused and preperfused media produced no significant change in size. However, liver perfusion resulted in particles predominantly 4.2-4.3-mm radius. Hepatic perfusion also significantly reduced HDL cholesteryl ester composition as a percentage of lipoproteins mass from 13.3 +/- 2.2% in control incubations to 10.7 +/- 3.1% (p less than 0.001), and cholesteryl ester:protein mass ratio was reduced from 0.31 +/- 0.06 in control to 0.24 +/- 0.10 (p less than 0.001) after 120 min of liver perfusion. Thus interaction of rabbit HDL with rabbit liver results in smaller HDL particles significantly depleted of core cholesteryl esters.

ラット肝のHDL<sub>3</sub>レセプターの検討

The liver plays an important role in the catabolism of High Density Lipoprotein (HDL). It has been difficult to identify HDL3 receptor because of HDL3 containing apo E. The present study is to elucidate the characteristics of HDL3 receptor by studying the binding of HDL3 with rat liver plasma membrane.In HDL3 isolated by sequential ultracentrifugation, the protein moiety consists more than 90 of apo A-I and A-II, and 0.1% of apo E. In isolated perfused rat liver, rat liver uptook 4% of HDL3 within the first pass and its ratio did not increase by the treatment of Ethynil-Estradiol.HDL3 bound to the rat liver plasma membrane in the fashion of time, temperature, and pH dependence. At 4°C, the specific binding of HDL3 was equilibrated and saturated after 60 min. The Kd was 11.0μg/ml. The binding sites showed heterogeneity by Scatchard analysis and the high affinity binding sites was 1.15μg/mg·membrane protein. The specific binding was Calcium independent and competed with HDL3, but not with LDL. This suggest that rat liver has HDL3 receptor different from apo B and E receptor and also apo E receptor. The HDL3 receptor binding assay may be useful to estimate the physiological and pathological circumstances.

Catabolism of Newly Formed Triglyceride-Rich Lipoproteins and Serum High Density Lipoproteins in Rats Fed Soybean Phospholipid and Soybean Oil

Previous research has shown that serum cholesterol and apolipoprotein (apo) A-I were lower and serum apoB was higher in rats fed soybean phospholipid (PL) than in rats fed soybean oil. Secretion of cholesterol and apoA-I, but not apoB, from the liver and intestine was lower in rats fed soybean PL. In the present study catabolism of newly formed triglyceride (TG)-rich lipoproteins, from the liver and intestine, and of serum high density lipoproteins (HDL) were compared in rats fed soybean PL and in rats fed soybean oil. The following results were seen: Feeding of soybean PL was related to more TG and less cholesterol in intestinal lymph chylomicrons (CM) and hepatic very low density lipoproteins (VLDL) than was feeding of soybean oil. A lesser amount of the TG, labeled with [3H]oleate in CM and hepatic VLDL, was incorporated into the adipose tissue and muscle in vivo. The clearance of CM-TG from plasma was higher, but that of VLDL-TG and apoB labeled with [3H]lysine was lower. The catabolism of 125I-labeled HDL from the plasma was lower while hepatic uptake of 125I-labeled HDL, in vivo and in vitro, was higher. These results indicate that soybean PL modifies the catabolism of the respective serum lipoproteins. Therefore, we suggest that changes in both the catabolism and secretion of lipoproteins are responsible for the altered serum lipid and apolipoprotein patterns found in rats fed soybean PL.

Apolipoprotein AI and AII metabolism in patients with primary high-density lipoprotein deficiency associated with familial hypertriglyceridemia

Plasma high-density lipoproteins (HDL) and their major proteins—apolipoprotein (apo) AI and apo AII—are subnormal in most patients with familial hypertriglyceridemia. However, the pathophysiology of low-plasma apo AI and apo AII is unclear. The kinetic parameters (turnover) of HDL apo AI and apo AII were studied in six lean patients with primary HDL deficiency associated with familial hypertriglyceridemia and five normolipidemic controls. Autologous 125I labeled HDL were injected intravenously (IV; 25 μCi) and blood samples drawn ten minutes after the injection and periodically thereafter for 12 days. Urine samples were collected daily and their radioactivity measured. Kinetic parameters were calculated from the area under the decay curve using three exponentials. Mean plasma apo AI and apo AII were significantly lower ( P < 0.001) in patients than normals (70.4 ± 2.7 v 106.9 ± 7.0; 24.2 ± 1.6 v 39.2 ± 0.9 mg/dL, respectively). The mean fractional catabolic rates (FCR) obtained from plasma 125I-HDL, apo AI, apo AII radioactivity decay curves and by Berson and Yalow's method (urine/plasma radioactivity ratios) were significantly greater ( P < 0.05) in patients than in controls (0.387 v 0.299; 0.391 v 0.309; 0.361 v 0.275; 0.272 v 0.207/d; respectively). The mean synthetic rates (SR) of apo AI and apo AII were significantly lower in patients than in controls (11.12 v 14.17 mg/kg body weight/d, P < 0.05; 3.53 v 4.68 mg/kg body weight/d, P < 0.05, respectively). In vitro lipolysis of triglyceride (TG) rich lipoproteins by bovine lipoprotein lipase, and measurement of hepatic TG lipase and lipoprotein lipase in postheparin plasma were similar in patients and controls, indicating no abnormality in these factors that are linked to HDL and TG catabolism. However, a significant positive correlation between hepatic TG lipase and the FCR of apo AI and apo AII was found. The data suggest that in this series of patients with HDL deficiency the low plasma HDL-cholesterol, apo AI, and apo AII levels resulted from decreased synthesis and an increased fractional catabolic rate of apo AI and apo AII, the major proteins of HDL.

Preliminary report: Kinetic studies on the modulation of high-density lipoprotein, apolipoprotein, and subfraction metabolism by sex steroids in a postmenopausal woman

To investigate the effects of estrogens and androgens on the metabolism of high density lipoproteins (HDL) and low density lipoproteins (LDL), a normolipidemic postmenopausal woman was studied under the following conditions: (1) during supplementation with ethinyl estradiol (0.06 mg/d); (2) without sex steroid therapy; and (3) during treatment with stanozolol, an androgenic, anabolic steroid (6 mg/d). During these manipulations HDL and LDL cholesterol levels fluctuated widely but reciprocally: during estrogen supplementation HDL increased while LDL decreased; during stanozolol HDL-C decreased while LDL-C increased. Simultaneous changes in post-heparin plasma hepatic triglyceride lipase activity paralleled those of LDL (and opposed those of HDL), decreasing with estrogen and increasing with stanozolol. During all three phases, autologous 125I-HDL turnover studies disclosed similarities between HDL 2 and apolipoprotein A-I metabolism and between HDL 3 and apolipoprotein A-II metabolism. In the untreated state the residence times of HDL 2 and apo A-I were only half those of HDL 3 and apo A-II. During estrogen treatment HDL 2 and apo A-I, residence times were selectively prolonged, coming to resemble those of HDL 3 and apo A-II, which remained unchanged. By contrast, during stanozolol treatment HDL 3 and apo A-II residence times were selectively reduced, coming to resemble those of HDL 2 and apo A-I, which remained unchanged. Apo A-I levels increased on estrogen and decreased on stanozolol, while apo A-II remained stable. Hence, estrogen increased HDL primarily by retarding the catabolism of the HDL 2 subfraction rich in apo A-I, whereas stanozolol decreased HDL by accelerating the catabolism of HDL 3, relatively rich in apo A-II. These effects may have been mediated via the concomitant sex steroid-induced changes in the activity of hepatic triglyceride lipase, hypothesized to be a central enzyme in the regulation of HDL catabolism.

Renal handling of high-density lipoproteins by isolated perfused kidneys

Recent studies show that the normal and diseased kidney is an important organ in the catabolism of high-density lipoproteins (HDL). However, little is known about the renal handling of HDL. To investigate this aspect, kidneys were isolated from normal rats and rats made nephrotic with puromycin aminonucleoside. They were perfused in a chamber at 37°C with a modified Krebs-Hensleit Bicarbonate Buffer containing 1%, 3%, 6%, and 10% Bovine Serum Albumin (BSA). Presence of 10% BSA in the perfusate prevented glomerular filtration and urine formation. Thus, the filtering and the nonfiltering kidney perfusion models distinguish the renal parenchymal function independently of luminal events that follow filtration. 125I-labeled rat HDL was injected into the perfusate and radioactivity in perfusate, urine, and kidney was examined. At the end of perfusion (30 minutes or four hours), each kidney was flushed with 125I-HDL-free perfusate and kidney radioactivity was measured. At four hours, 1.9% ± 0.5% of injected radioactivity was present in urine from kidneys perfused with 6% BSA. Kidneys with intact glomerular filtration sequestered significantly more radioactivity (1.1% ± 0.2% of injected radioactivity) than nonfiltering kidneys (0.7% ± 0.2%); P < 0.05. Radioactivity in filtering kidneys was significantly higher than in nonfiltering kidneys (33.9 ± 7.8 v 15.6 ± 2.6 cpm/mg kidney tissue protein, respectively; P < 0.001). Nephrotic kidneys (filtering and nonfiltering) sequestered two to four times more 125I-HDL than normal kidneys. These data support the hypothesis that prior to urinary excretion, partial reabsorption of filtered HDL (or subfractions) occurs in the normal kidney. In the nephrotic kidney, increased filtration (and reabsorption) may account for the enhanced uptake of HDL by the diseased kidney.

Influence of bezafibrate, fenofibrate, nicotinic acid and etofibrate on plasma high-density lipoprotein levels

Increased levels of plasma high-density lipoprotein (HDL) cholesterol have been described after exposure to different dietary (such as ethanol) and environmental (such as chlorinated pesticides) factors as well as intense physical training. Several lipid-lowering drugs have also been shown to elevate HDL cholesterol. These findings are of particular interest, because adult levels of the putatively protective lipoprotein fraction appear not to be regulated by genetic factors. Four hypolipidemic drugs are herein considered, all of which reportedly elevate HDL cholesterolemia. Two of them, bezafibrate and fenofibrate, belong to the class of “fibrate” derivatives; nicotinic acid (NA) and etofibrate (E) show some similarities, E being the chemical association between nicotinic acid and clofibrate. In most studies, bezafibrate (B), with a very short half-life (2.1 hours), has induced an HDL cholesterol elevation of about 20%, both in types II and IV hyperlipidemic patients; it is controversial whether HDL 2 rather than HDL 3 levels are elevated by B. Fenofibrate (F) shows prolonged plasma half-life in man (>21 hours) and markedly increases HDL cholesterol levels, particularly in type IV patients; data are less homogeneous in type IIA and IIB cohorts. It appears that F may increase apoprotein C-II levels in very low density lipoproteins, possibly an index of stimulated lipoprotein lipase activity. Both B and F may behave as “catabolic agents,” stimulating the breakdown of triglyceride-rich lipoproteins and the transfer of surface components (free cholesterol) to HDL. A different case may be that of NA, which prevents the decrease of HDL cholesterol induced by carbohydrate-rich diets in volunteers. NA reduces the catabolism of HDL and markedly increases (300% or more) the HDL 2 HDL 3 ratio in both volunteers and type IV patients, but not in type IIB patients. Whether, in addition to a “sparing” action on HDL, NA may also increase HDL levels by stimulating lipoprotein lipase activity is being disrupted. Finally, E may increase HDL cholesterol by 10% or more while exerting significant hypocholesterolemic activity. This compound is of interest because it may stimulate the release of prostacyclin from the arterial wall; prostacyclin may be derived from HDL phospholipids, unusually rich in arachidonic acid. This observation may offer another clue to the understanding of the mode of arterial protection exerted by HDL.

Different kinetic fates of apolipoproteins A-I and A-II from lymph chylomicra of nonhuman primates. Effect of saturated versus polyunsaturated dietary fat.

Monkeys fed polyunsaturated fat had significantly lower plasma cholesterol (186 +/- 18 vs. 276 +/- 31 mg/dl) and high density lipoprotein (HDL) mass concentrations (466 +/- 28 vs. 518 +/- 34 mg/dl) than did animals fed saturated fat. Plasma apoA-I concentrations also were significantly lower and apoA-II levels were generally, though not significantly, lower in the group fed polyunsaturated fat. In vivo reinjection studies, using thoracic duct lymph chylomicra labeled with (131)I and HDL labeled with (125)I, were done in order to study the mechanism of plasma HDL-lowering by polyunsaturated dietary fat. The peak specific activity (SA) of HDL apoA-I derived from (131)I-labeled chylomicra occurred at 3 hr after injection (172 +/- 11% of 1 min S.A.) and then an exponential decay occurred indicative of a precursor-product relationship between chylomicron apoA-I and HDL apoA-I. In contrast, HDL apoA-II derived from (131)I-labeled chylomicra had no early S.A. increase and began to die away immediately after injection. Labeled apoA-I from chylomicron and HDL origin had similar plasma fractional catabolic rates (FCR = 0.34-0.38 vs. 0.32-0.38 d(-1), respectively); apoA-II from chylomicron or HDL origin also had similar FCR (0.46-0.51 vs. 0.42-0.51 d(-1), respectively), which were significantly shorter than those for HDL apoA-I. There was a consistent trend toward a higher FCR for HDL apoA-I or A-II of polyunsaturated fat-fed recipients. Chylomicron apoA-I/triglyceride and apoA-II/triglyceride mass ratios were lower in polyunsaturated fat-fed animals (A-I/TG = 1.56 x 10(-3); A-II/TG = 1.47 x 10(-3)) vs. saturated fat-fed animals (A-I/TG = 2.58 x 10(-3); A-II/TG = 2.77 x 10(-3)). It was concluded that: (1) dietary polyunsaturated fat significantly lowered plasma cholesterol, HDL, and apoA-I concentrations relative to saturated fat; (2) the HDL-lowering effect of the dietary polyunsaturated fat may be due to the combined effects of decreased apoprotein production by the intestine and increased HDL catabolism; and (3) in the blood, chylomicron apoA-I and A-II differ in their metabolic fates.-Parks, J. S., and L. L. Rudel. Different kinetic fates of apolipoproteins A-I and A-II from lymph chylomicra of nonhuman primates. Effect of saturated versus polyunsaturated dietary fat.

Increased catabolism of high density lipoprotein in alcoholic hepatitis

The kinetics of high density lipoprotein apoprotein A-I were determined in three subjects with severe alcoholic hepatitis. Lipoprotein was obtained from three healthy subjects, radioiodinated, and injected into the patients and the donors. The catabolic rate of A-I was two to four times higher in the patients compared to a group of 18 control subjects studied previously. In the patients only, about 20% of the radiolabeled apoproteins rapidly appeared in the lipoprotein-poor fraction of plasma, which also contained substantial amounts of unlabeled A-I. The findings show that the reduced concentration of A-I in alcoholic hepatitis (less than half-normal), is due not to diminished synthesis (synthesis was normal) but to rapid degradation. This may reflect disruption of abnormal high density lipoproteins due to lack of cofactors.

Quantification of the hepatic contribution to the catabolism of high density lipoproteins in rats.

Isolated rat livers were perfused for four hours in a recirculating system containing washed rat erythrocytes. Biologically screened radioiodinated rat high density lipoproteins (1.090 < d < 1.21 g/ml) were added to the perfusate with different amounts of whole serum to supply unlabeled rat high density lipoproteins. The protein moiety of the lipoprotein contained more than 95% of the radioiodine. The fraction of apolipoprotein mass degraded during the perfusion was quantified by the linear increment of non-protein-bound radioiodine in the perfusate, corrected for the increment observed during recirculation of the perfusate in the absence of a liver. The small amount of (131)I secreted into bile was added to calculate the fractional catabolic rate. The fractional catabolic rate ranged from 0.22 to 0.63% per hour in 12 experiments and was inversely related to the size of the perfusate pool of high density apolipoprotein. The absolute catabolic rate of high density apolipoprotein (fractional catabolic rate x pool size) in three livers in which the concentration of rat HDL in the perfusate approximated that in intact rats was 69.5 +/- 10.4 micro g hr(-1) (mean +/- SD). The rate of disappearance of cholesteryl esters of rat high density lipoproteins (labeled biologically by injecting donor rats with [5-(3)H]mevalonic acid) from the liver perfusate did not exceed that of the apoprotein component. These rates were compared with catabolic rates for rat high density lipoproteins in intact rats. Fractional catabolic rate in vivo, obtained by multicompartmental analysis of the disappearance curve of (131)I-high density apolipoprotein from blood plasma, was 11.9 +/- 1.3% hr(-1) (mean +/- SD). Total catabolic rate in vivo (fractional catabolic rate x intravascular pool of high density apolipoprotein) was 986 +/- 145 micro g hr(-1) (mean +/- SD). The results suggest that only a small fraction of high density lipoproteins in blood plasma of rats is degraded directly by the liver.-Sigurdsson, G., S-P. Noel, and R. J. Havel. Quantification of the hepatic contribution to the catabolism of high density lipoproteins in rats.

High density lipoprotein infusion and partial plasma exchange in Tangier disease

High density lipoprotein (HDL) infusion and partial plasma exchange were undertaken in two patients homozygous for Tangier disease. Serum samples and ultracentrifugally isolated serum fractions were analysed over a period of 7 days post infusion by agarose electrophoresis, two-dimensional immunoelectrophoresis (employing antibodies to HDL, HDL3, Apoprotein A-I, and Apoprotein A-II), Apoprotein A radioimmunoassay, and analytical polyacrylamide electrophoresis. The following observations were made: (a) immediately after HDL substitution the broad-beta band, normally visible upon agarose electrophoresis of Tangier plasma, resolved into a distinct beta and pre-beta band; (b)as HDL was catabolized, an abnormal alpha-migrating lipoprotein was generated which contained Apoprotein A-II as protein constituent; and (c) there was a proferential loss of Apoprotein A-I from HDL and the plasma compartment in the course of HDL catabolism. The results suggest that the defect in Tangier disease resides with enhanced catabolism or defective synthesis of Apoprotein A-I.

High density lipoprotein catabolism before and after partial hepatectomy

The serum decay and tissue distribution of iodine-labeled high density lipoprotein HDL)-apoproteins were measured in rats 2 2 h after partial hepatectomy or sham-operation. A method was developed allowing continuous bloodsampling without using anticoagulantia or anaesthetics. The serum decay of HDL-apoproteins was biexponential. Neither the initial rapid phase ( t sol1 2 0.3 ± 0.1 h), nor the slow phase ( t 1 2 6.2 ± 0.3 h) were influenced by the removal of sol2 3 of the liver and consequently there was no effect on the fractional catabolic rate (F.C.R.: 2.9 ± 0.2/day). The level of circulating HDL was decreased by partial hepatectomy but the chemical composition of HDL was unchanged. Total tissue HDL radioactivity in the control rats was 5.7, 2.8, 2.7, 1.0, 0.7, 0.2, 0.4 and 0.1% of the injected dose for skeletal muscle, adipose tissue, liver, jejunum, kidneys, spleen, lungs and heart, respectively. Only the value for liver was affected significantly by partial hepatectomy (0.6%). It is concluded that the in vivo degradation rate of HDL-apoproteins is not influenced by the removal of sol2 3 of the liver and that the decrease in serum HDL concentration is due to an impaired rate of hepatic synthesis. These results indicate the possibility of extrahepatic HDL-apoprotein catabolism or a stimulation of HDL-apoprotein degradation, induced by partial hepatectomy, in the remaining liver lobes.

Binding, internalization, and degradation of high density lipoprotein by cultured normal human fibroblasts

Comparative studies were made of the metabolism of plasma high density lipoprotein (HDL) and low density lipoprotein (LDL) by cultured normal human fibroblasts. On a molar basis, the surface binding of (125)I-HDL was only slightly less than that of (125)I-LDL, whereas the rates of internalization and degradation of (125)I-HDL were very low relative to those of (125)I-LDL. The relationships of internalization and degradation to binding suggested the presence of a saturable uptake mechanism for LDL functionally related to high-affinity binding. This was confirmed by the finding that the total uptake of (125)I-LDL (internalized plus degraded) at 5 micro g LDL protein/ml was 100-fold greater than that attributable to fluid or bulk pinocytosis, quantified with [(14)C]sucrose, and 10-fold greater than that attributable to the sum of fluid endocytosis and adsorptive endocytosis. In contrast, (125)I-HDL uptake could be almost completely accounted for by the uptake of medium during pinocytosis and by invagination of surface membrane (bearing bound lipoprotein) during pinocytosis. These findings imply that, at most, only a small fraction of bound HDL binds to the high-affinity LDL receptor and/or that HDL binding there is internalized very slowly. The rate of (125)I-HDL degradation by cultured fibroblasts (per unit cell mass) exceeded an estimate of the turnover rate of HDL in vivo, suggesting that peripheral tissues may contribute to HDL catabolism. In accordance with their differing rates of uptake and cholesterol content, LDL increased the cholesterol content of fibroblasts and selectively inhibited sterol biosynthesis, whereas HDL had neither effect.