High density lipoprotein metabolism

Regulation of apolipoprotein A-I synthesis in lymph-drained rats

The effect of lymph diversion on plasma apolipoprotein A-I levels was studied. In lymph fistula rats apolipoprotein A-I levels in plasma stayed constant in spite of a loss of an equivalent of one-half plasma pool of apolipoprotein A-I per day through the lymph fistula. This indicates that synthesis of apolipoprotein A-I increases or that catabolism of apolipoprotein A-I decreases in a compensatory manner as intestinal apolipoprotein A-I is diverted. By using incorporation of [3H]leucine into newly synthesized apolipoprotein A-I it was shown that 2.6-times as much [3H]leucine was incorporated into apolipoprotein A-I in thoracic duct drained animals compared to controls. In experiments in which 125I-labeled HDL was injected intravenously into rats, it was shown that catabolism of HDL and apolipoprotein A-I was not decreased in lymph-drained rats. These data thus suggest that an increased synthesis of apolipoprotein A-I occurs when the intestinal contribution of apolipoprotein A-I diminishes. This is probably due to an increase in liver protein synthesis.

Metabolism of high-density lipoprotein in the hyperlipidemic, diabetic SHR/N-corpulent 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-III0 and apo C-III3) 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Nutrition and biogenesis of plasma lipoproteins in nonhuman primates

In this article we examine the production of very low-density lipoprotein (VLDL) by perfused livers obtained from chow- and cholesterol-fed nonhuman primates. These data illustrate two important features of VLDL production. First, VLDL is secreted from the liver in a form very close to that of its plasma counterpart. Thus for chow-fed animals, plasma VLDL and liver perfusate VLDL have similar lipid compositions. Second, the composition of VLDL can be modified significantly by diet in each of two primate species, the Rhesus monkey and the baboon. Rhesus monkey livers uniformly secrete larger quantities of VLDL and show more dramatic dietary effects than do baboon livers. Nevertheless, perfused livers from both species reveal qualitatively similar responses to dietary peanut oil and to lard fed in combination with cholesterol. Both fat-containing diets induce the livers to secrete VLDL enriched in cholesteryl ester compared with control perfusates yet still cholesteryl ester deficient compared with the animals' plasma VLDL. Peanut oil diet reduces the hepatic output of VLDL-associated apoprotein B and triglyceride, whereas lard increases hepatic secretion of VLDL-associated lipids and apoprotein E. We conclude that the nature of dietary fat plays an important role in determining the profile and composition of lipoproteins formed and secreted by the primate liver. We have also briefly reviewed the production of high-density lipoprotein, which is probably formed in the plasma from many sources, with special emphasis on the possible role of newly secreted lipid-poor apolipoprotein A-I and A-II.

Proapolipoprotein-converting enzymes and high-density lipoprotein early events in biogenesis

The early events in high-density lipoprotein biogenesis involve the extracellular action of two converting enzymes affecting the cleavage of the prosegment of either proapolipoprotein A-I or proapolipoprotein A-II and the generation of mature apolipoprotein (apo) A-I and apo A-II, the main apolipoprotein of high-density lipoproteins. These two converting enzymes differ from each other in mechanism of action and specificity. The observation that they can be secreted by human hepatocarcinoma G2 cells in culture provides an experimental basis for examining the possible coordination between the synthesis and secretion of these two converting enzymes and the events attending the production and cellular export of apo A-I and apo A-II.

Regulation of hepatic lipase and serum lipoproteins by sex steroids

The influence of sex steroids on the serum lipoprotein pattern was recognized more than 30 years ago, and it still remains among the areas of major interest. This is because of the compatible sex difference in plasma lipoprotein pattern and in coronary heart disease risk. Recent discoveries of the role of hepatic lipase in lipoprotein metabolism have elucidated mechanisms behind sex steroid-induced changes in lipoproteins. These steroids regulate the activity of hepatic lipase, an enzyme bound to the endothelial cells of liver sinusoids. Hepatic lipase has a central role in the removal of phospholipids and triglycerides from subfractions of high-density lipoprotein (HDL2) particles, but it may also function in the lipolysis of triglyceride-rich particles. Some older and more recent developments in this area will be reviewed.

Distribution of high-density lipoprotein 2 and 3 constituents during in vitro phospholipid hydrolysis

Human high-density lipoproteins HDL2 (d = 1.068-1.125) and HDL3 (d = 1.125-1.210) doubly labelled with [3H]cholesterol/cholesteryl ester and with [acyl-14C]phosphatidylcholine were further incubated with phospholipases. Highly purified phospholipase A2 from Crotalus adamanteus allowed gradual degrees of lipolysis (30-90%) on both HDL2 and HDL3. Moderate phospholipid hydrolyses were achieved using hepatic triacylglycerol lipase, partially purified from post-heparin plasma. Moreover, the latter enzyme seemed to exert a lysophospholipase activity, acting on the 2-acyl-sn-glycero-3-phosphocholine generated. A purified sphingomyelinase C from Staphylococcus aureus was also used and completely hydrolysed HDL sphingomyelin. After incubation, doubly labelled HDL2/HDL3 were reisolated in their appropriate density interval. In the presence of albumin, which bound most of the lipolysis products, phospholipolysis induced a phospholipid depletion of the particles and a heterogeneous partition of all HDL2 constituents between the HDL2 and HDL3 density intervals. Radioactivity distributions correlated with mass movements. The 'HDL3-like' particles isolated after HDL2 lipolysis were twice as rich in cholesterol as plasma HDL3. No loss of apoprotein A1 was recorded due to phospholipolysis. In the absence of albumin, the density distributions of HDL2 or HDL3 constituents were unaffected by phospholipolysis, the products of lipolysis being reisolated with the stable particles. Control and treated HDL were also reisolated by equilibrium density gradient ultracentrifugation, gel chromatography or by gradient gel electrophoresis. Phospholipase treatment in the presence of albumin induced a shift of the HDL2 or HDL3 whole distribution towards particles of higher density and lower apparent size. Lipolysed HDL2 thus showed characteristics intermediate between those of HDL2 and HDL3. So, phospholipolysis may affect the physical parameters of HDL particles, but additional pathways such as cholesterol movements and apoprotein loss must be linked to achieve the HDL2----HDL3 interconversion.

Cholesterol efflux from cultured adipose cells is mediated by LpAI particles but not by LpAI:AII particles

Cholesterol efflux was studied in cultured adipose cells after preloading with LDL cholesterol. Long-term exposure to LpAI and LpAI:AII particles isolated from the HDL fraction showed that LpAI particles only were able to promote cholesterol efflux. Liposomes containing different ApoAI/ApoAII molar ratios were tested: the larger the proportion of ApoAI, the faster the ability to remove cholesterol from Ob1771 cells. Dose-response curves showed that LpAI particles were active within a physiological range of concentrations, whereas LpAI:AII particles had no effect at all concentrations. The results are in favour of LpAI particles being the active components of the HDL fraction for the promotion of cholesterol efflux and suggest that LpAI particles and LpAI:AII particles represent distinct metabolic entities.

Glucocorticoid-induced elevation of serum high-density lipoprotein-cholesterol and its reversal by adrenocorticotropin in the rat

The effect of administration of a high dose of glucocorticoid (triamcinolone) on serum lipids and lipoproteins was studied in rats. Changes in serum lipids, especially cholesterol, were most marked when 5 mg/kg body weight of triamcinolone was injected daily for 5 days. Serum lipoproteins were separated by ultracentrifugation followed by gel-filtration chromatography. Cholesterol distribution between apolipoprotein B-containing lipoproteins (very-low-density and low-density lipoproteins), high-density lipoprotein1 (HDL1), and HDL2 was determined after administration of triamcinolone with or without additional treatment with adrenocorticotropin (ACTH; Cortrosyn, 6 IU/rat). When triamcinolone was administered, cholesterol concentrations in HDL1 and HDL2 were elevated in a dose-dependent manner, but there was no significant change in apolipoprotein B-containing lipoprotein cholesterol levels. When ACTH was administered in combination with triamcinolone, the concentrations of all serum lipids except triacylglycerol were significantly lowered compared with rats treated with triamcinolone alone. HDL1-cholesterol concentration in serum was significantly (P less than 0.001) lowered from 69 +/- 13 mg/dl (mean +/- S.D.) in triamcinolone-treated rats to 36 +/- 4 mg/dl by the administration of ACTH plus triamcinolone. The additional administration of ACTH in triamcinolone-treated rats caused a slight, but significant, decrease in cholesterol concentration in apolipoprotein B-containing lipoproteins; however, HDL2-cholesterol level was not significantly affected, although there was a tendency for it to be lowered.

Changes in plasma lipoproteins and tissue lipoprotein lipase and salt-resistant lipase activities during spawning in the rainbow trout (Salmo gairdnerii R.)

1. Lipoprotein lipase and salt-resistant lipase activities increased in the ovaries but decreased in the adipose tissue of female trout in the months leading up to spawning. 2. The activity of the plasma cholesterol esterifying enzyme increased significantly immediately prior to spawning. 3. Plasma lipoprotein concentrations decreased during the approach to spawning. 4. These studies suggest that the developing ovaries in the trout receive their nutrients by lipolysis of plasma lipoproteins as well as by vitellogenin uptake; differentiation of the roles of the lipid stores in different tissues is proposed.

High-density lipoprotein cholesterol subfractions in chronic uremia

Cholesterol content in high-density lipoprotein (HDL) subfractions has been studied in 108 patients at different evolutive stages of chronic renal failure (CRF) under conservative treatment. Results have been compared with healthy control subjects, patients receiving hemodialysis, and renal graft recipients. Significant low levels of total HDL and HDL2 cholesterol are observed in men with CRF. The more severe the CRF, the more likely that total HDL and HDL2 cholesterol will be low. Moreover, a significant inverse correlation is found between HDL2 cholesterol and serum creatinine levels. In women, although a decrease in total and HDL2 subfraction is observed, no significant differences are found across the severity of CRF. Serum HDL2 cholesterol levels are decreased in men and women receiving hemodialysis, while raised total HDL and HDL2 cholesterol levels are observed in normally functioning renal grafts. These results indicate that according to the "HDL hypothesis," despite other associated risk factors, the high cardiovascular mortality rates noted mainly in men with CRF under conservative treatment and in patients receiving hemodialysis could be explained, at least in part, by the sustained and progressive decrease in total HDL and HDL2 values. From this point of view, our study suggests the need to promote early kidney transplantation.

High-density lipoprotein cholesterol and coronary heart disease in hypercholesterolemic men: the Lipid Research Clinics Coronary Primary Prevention Trial

Plasma levels of high-density lipoprotein cholesterol (HDL-C) at entry and subsequent changes from these baseline levels were inversely predictive of coronary heart disease (CHD) end points in hypercholesterolemic men followed for 7 to 10 years in the Lipid Research Clinics Coronary Primary Prevention Trial, especially in the 1907 participants receiving cholestyramine. When the men in this cohort were compared, each 1 mg/dl increment in baseline HDL-C (mean 44.3 mg/dl) was associated with a 5.5% decrement in risk of "definite" CHD death or myocardial infarction (Z = -5.4), and each 1 mg/dl increase from baseline HDL-C levels (mean increase = 1.6 mg/dl) during the trial was associated with a 4.4% risk reduction (Z = -2.2). In the 1899 participants receiving placebo, the corresponding risk decrements were 3.4% and 1.1%. Although baseline HDL-C level (mean = 44.4 mg/dl) remained a significant risk predictor (Z = -3.8) in the placebo cohort, increases in HDL-C (mean increase 0.5 mg/dl) were not significantly predictive of CHD (Z = -0.6) unless "suspect" as well as "definite" end points were analyzed (Z = -2.0). When the associations between HDL-C (baseline plus change) and incidence of definite CHD end points within each treatment cohort were compared, their difference approached nominal significance (Z = 1.9). The results suggest a synergistic interaction, in which cholestyramine treatment reduced CHD risk most substantially in men maintaining the highest HDL-C levels.

Evidence that an extract of Staphylococcus capitis can precipitate human serum HDL2

The high density lipoprotein (HDL) fraction involved in the precipitation with Staphylococcus capitis can be quantitated by single radial diffusion in agar containing an extract of the bacteria. Both whole serum and purified HDL produce a precipitation ring with bacterial extract. When dilutions of serum are tested, the squared radius of the precipitation ring plotted against serum concentration gives a linear relationship. Cumulative density flotation of HDL, followed by quantitative analysis of precipitating activity and apo A, indicates that the precipitating activity is present in the less dense fraction of serum HDL (i.e. HDL2). This suggestion is supported by the finding that the precipitating activity correlates positively with HDL2 (but not with HDL3) in human sera. Precipitating activity was higher in sera from women than from men, in accordance with the sex difference in serum HDL. The results raise the question whether precipitation with an extract of S. capitis might serve as a simple, direct method to assess serum HDL2 levels.

Restriction-fragment-length polymorphisms in the A-I-C-III gene complex occurring in a family with hypoalphalipoproteinemia

Two restriction-fragment-length polymorphisms in the apolipoprotein A-I-C-III gene complex were defined by digestion with PstI and SacI in a family with hypoalphalipoproteinemia. These polymorphisms established a PstI + /SacI - haplotype which may constitute a linkage marker for this condition within the family.

Cholesterol efflux from cultured skin fibroblasts in 12 patients clinically heterozygous for familial hypercholesterolemia

The [3H]cholesterol efflux from cultured skin fibroblasts was investigated in 12 patients, clinically heterozygous for familial hypercholesterolemia (FH). The [3H]cholesterol efflux from fibroblasts was studied in the presence of high-density lipoproteins, isolated either from control sera or from the patient's own serum. The [3H]cholesterol efflux from fibroblasts of 3 patients was increased; it was decreased for 4 patients and similar to that of control cells for 5 patients. This study indicates that in some patients with heterozygous FH, abnormalities of [3H]cholesterol efflux from fibroblasts can be detected. The heterogeneity of the results is consistent with the heterogeneity of the low-density lipoprotein receptor activity and with the variability of the clinical picture found in this disease.

Increases in the particle size of high-density lipoproteins induced by purified lecithin: cholesterol acyltransferase: effect of low-density lipoproteins

Homogeneous subpopulations of human high-density lipoproteins subfraction-3 (HDL3) have been incubated at 37 degrees C with purified lecithin: cholesterol acyltransferase, human serum albumin and varying concentrations of human low-density lipoproteins (LDL). Changes in HDL particle size and composition during these incubations were monitored. Incubation of HDL3a (particle radius 4.3 nm) in the absence of LDL resulted in an esterification of more than 70% of the HDL free cholesterol after 24 h of incubation. This, however, was sufficient to increase the HDL cholesteryl ester by less than 10% and was not accompanied by any change in particle size. When this mixture was incubated in the presence of progressively increasing concentrations of LDL, which donated free cholesterol to the HDL, the molar rate of production of cholesteryl ester was much greater; at the highest LDL concentration HDL cholesteryl ester content was almost doubled after 24 h and there was an increase in the HDL particle size up to the HDL2 range. In the case of HDL3b (radius 3.9 nm), there were again only minimal changes in particle size in incubations not containing LDL. In the presence of the highest concentration of LDL tested, however, the particles were again enlarged into the HDL2 size range after 24 h incubation. These HDL2-like particles were markedly enriched with cholesteryl ester but depleted of phospholipid and free cholesterol when compared with native HDL2. Furthermore, the ratio of apolipoprotein A-I to apolipoprotein A-II resembled that in the parent-HDL3 and was very much lower than that in native HDL2. It has been concluded that purified lecithin: cholesterol acyltransferase is capable of increasing the size of HDL3 towards that of HDL2 but that other factors must operate in vivo to modulate the chemical composition of the enlarged particles.

Acid phosphatases bind to the main high density lipoprotein apolipoprotein A-I

The serum protein binding secretory prostatic acid phosphatase (PAP) and lysosomal placental acid phosphatase (LAP) was purified using affinity chromatography on gels containing immobilized acid phosphatases. The protein, which could be eluted from these enzyme affinity gels only with 0.05 mol/l HCl (pH 2.0), was shown to be apolipoprotein A-I (apo A-I), the main structural protein of high density lipoprotein (HDL).

Lipoprotein fractions and receptors: a role for probucol?

Cholesterol transport in the plasma involves several lipoprotein families. The process is remarkably ordered and is driven vectorily by apolipoproteins, which activate appropriate enzymes or serve as recognition sites for lipoprotein receptors. Of the lipoproteins in plasma, low density lipoprotein (LDL) contains most of the cholesterol and has the greatest atherogenic potential. Its plasma concentration is determined by LDL receptor activity, which serves to regulate intracellular cholesterol concentrations. LDL receptor activity in the body is not fixed, but can be stimulated by drugs that affect hepatic cholesterol content, such as inhibitors of HMG-CoA reductase or bile acid sequestrants. By stimulating LDL receptor activity, these drugs increase the fractional catabolic rate of apoLDL. HMG-CoA reductase inhibitors also appear to reduce the apoLDL synthetic rate. As a consequence, LDL cholesterol levels are reduced while high density lipoprotein (HDL) cholesterol levels remain stable or increase. Probucol is a drug that lowers both LDL and HDL cholesterol levels. It appears to lower LDL cholesterol levels by affecting LDL structure rather than by stimulating LDL receptor activity. It has no consistent effect on LDL synthetic rates. Probucol lowers plasma HDL cholesterol levels by decreasing the synthetic rates of the major HDL apolipoproteins. The biologic significance of these probucol-induced changes in HDL metabolism is unknown.

Lipoprotein abnormalities in insulin-dependent diabetes mellitus

Despite the increased risk of atherosclerosis in diabetes mellitus, levels of serum cholesterol, triglycerides, low-density-lipoprotein (LDL) cholesterol, and high-density-lipoprotein (HDL) cholesterol were similar in 57 men with insulin-dependent diabetes mellitus (IDDM) and 81 non-diabetic controls. However, substantially lower serum levels of apolipoprotein B, the principal apolipoprotein of LDL, and a concomitant increase in the cholesterol-loading of apolipoprotein B were found in IDDM. The likely changes in LDL density and particle size resulting from this compositional abnormality might lead to accelerated atherogenesis analogous to that seen in type III hyperlipoproteinaemia. In addition, there was an increase in the concentration of cholesterol in the smaller, denser, HDL3 subfraction of serum HDL in IDDM.

Relationship between baseline risk factors and coronary heart disease and total mortality in the Multiple Risk Factor Intervention Trial. Multiple Risk Factor Intervention Trial Research Group

The relationship between selected baseline risk factors and subsequent coronary heart disease (CHD) death and total mortality among participants in the Multiple Risk Factor Intervention Trial (MRFIT) was studied in order to determine whether the three risk factors used to identify high-risk men for the trial were associated with CHD death; whether other risk factors measured at baseline, especially lipoprotein cholesterol levels, were associated with CHD and total mortality; and whether there were any differences between special intervention (SI) and usual care (UC) participants in the relationship of the specific levels of risk factors to CHD or total mortality. The three main risk factors (blood cholesterol, cigarette smoking, and diastolic blood pressure) and age were significantly associated with CHD mortality; age, diastolic blood pressure, and cigarette smoking were associated with total mortality. The risk score based on the multiple logistic equation developed from the Framingham Study was also strongly associated with CHD mortality. When the joint associations of selected baseline risk factors with CHD and total mortality were considered, age, diastolic blood pressure, cigarette smoking, and low- and high-density lipoprotein cholesterol were significantly associated with CHD mortality; age, cigarette smoking, and low-density lipoprotein cholesterol were positively associated with total mortality. Systolic blood pressure significantly improved the prediction of CHD mortality for SI and UC men when it was added to a regression model that included age, diastolic blood pressure, cigarettes smoked per day, body mass index, and lipoprotein levels, but improved the prediction of total mortality only for SI men. In similar analyses, serum thiocyanate improved the prediction of both CHD and total mortality for UC men. Among SI men the improved prediction gained by considering serum thiocyanate was less pronounced and not significant for CHD death. This latter finding may be due in part to the changes made in smoking behavior by SI participants during the course of the study. The estimated regression coefficients for CHD and total mortality endpoints were not significantly different between the SI and UC groups.

Plasma triglycerides related decrease in high-density lipoprotein cholesterol and its association with myocardial infarction in heterozygous familial hypercholesterolemia

Recent studies suggest that decreased levels of high-density lipoprotein (HDL) may contribute to the risk of premature occlusive atherosclerosis in familial hypercholesterolemia (FH). To investigate further, we have analyzed the concentration as well as distribution of HDL cholesterol in relation to plasma triglycerides and their influence on ischaemic heart disease in FH subjects. The study was carried out in 71 men with heterozygous FH and 46 matched controls. FH subjects were relatively young with a mean age of 38 +/- 11 years. Tendon xanthomatas were observed in 57% of the subjects, whereas ischemic heart disease was identified in 33%. Compared to normals, the mean value of HDL cholesterol is significantly reduced by 21% in FH heterozygotes (42 +/- 12 v 33 +/- 9 mg/dL, P less than 0.001). The decrease in HDL cholesterol is highly correlated to the levels of plasma triglycerides (r = -0.50, P less than 0.001) and VLDL cholesterol (r = -0.53, P less than 0.001). Moreover, HDL cholesterol decrease is not associated with elevated levels of LDL cholesterol (r = -0.20, NS), which is the primary characteristic feature of FH subjects. However, HDL cholesterol decrease is weakly related to total plasma cholesterol concentration (r = -0.24, P less than 0.05). The body weight is also contributory to the reduction of HDL cholesterol (r = -0.42, P less than 0.01), probably due to its strong positive correlation to plasma triglycerides (r = +0.54, P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Apoprotein D in a healthy, male population and in male myocardial infarction patients and their male, first-degree relatives

This report examines the correlation of serum apoprotein D with other lipoprotein lipids and apoproteins in a healthy, male population and compares the levels of high density lipoprotein apoprotein D of this control population with 2 samples composed of male, acute myocardial infarction patients and their healthy, male, first-degree relatives. Highly significant correlations were observed with very low density lipoprotein lipids (negative), high density lipoprotein lipids (positive) and serum triglycerides (negative). Serum and low density lipoprotein apoprotein B was not correlated with serum apoprotein D, whereas apoprotein A-I from serum and high density lipoproteins was strongly correlated with apoprotein D. A significant reduction in high density lipoprotein apoprotein D was observed in male, myocardial infarction patients. Their male, first-degree relatives also had lower apoprotein D levels, but the difference was not significant.

Hepatic glucose-6-phosphate dehydrogenase, cholesterogenesis, and serum lipoproteins in liver regeneration after partial hepatectomy

Liver regeneration after partial hepatectomy was used as an experimental model for studying mammalian cell division and replication. The rate of cell proliferation in this hyperplastic model was correlated with hepatic de novo synthesis of cholesterol, with the hexose monophosphate shunt pathway of glucose metabolism, and with serum lipoproteins. An increase of hepatic cholesterol esters and of incorporation of tritiated water in cholesterol esters was observed at 24 hr after partial hepatectomy. Partial hepatectomy also resulted in an increase of hepatic glucose-6-phosphate dehydrogenase and in alteration of serum lipoproteins, primarily due to a selective decline in high density lipoprotein fraction.

Comparison of selectivity of LDL removal by double filtration and dextran-sulfate cellulose column plasmapheresis

The possibility of selective removal of VLDL, IDL and LDL by double filtration (DF) and dextran-sulfate cellulose (DSC) column plasmapheresis was investigated in hypercholesterolemia. Two and a half liters of plasma were treated. Sixty six percent of TC and 68% of LDL-C were removed by DF plasmapheresis. The removal rate of HDL-C was 50% which was significantly lower than that of LDL-C. The removal rate of apoprotein A-I and A-II was also significantly lower than that of apoprotein B. Sixty percent of LDL-C and 61% of apoprotein B were removed by DSC column plasmapheresis while the decrease of HDL-C, apoprotein A-I and A-II was minimal. Therefore, DSC column plasmapheresis could remove atherogenic lipoproteins more selectively than DF plasmapheresis.

Sex differences in high-density lipoprotein cholesterol and subfractions among young black adults

The high-density lipoprotein cholesterol and high-density lipoprotein subfraction (HDL2 and HDL3) concentrations were examined in 170 young black adults. The women examined had significantly higher total high-density lipoprotein cholesterol concentrations than the men, i.e., 57.0 mg/dl vs 51.2 mg/dl, P less than 0.002. The increased high-density lipoprotein cholesterol among women represented a significant increase in both HDL2 and HDL3 cholesterol concentrations. The sex differential could not be explained by body mass index, alcohol consumption, or physical activity. The determinants of high-density lipoprotein cholesterol for men appeared to be different than those for women.

Role of apolipoproteins in cellular cholesterol efflux

The effects of serum apolipoproteins, particle size and concentration on the effectiveness of phosphatidylcholine (PC)-containing acceptor particles in causing release of cholesterol from cells growing in culture have been investigated. The acceptor particles were prepared by detergent-dialysis procedures and were either egg PC small unilamellar vesicles (SUV) or discoidal complexes of egg PC with apoproteins from human high-density lipoprotein (HDL). Gel filtration chromatography was employed to isolate particles of defined composition and size. The half-times (t 1/2) for the unidirectional efflux of cholesterol from cells prelabeled with [3H]cholesterol were measured as a function of acceptor PC concentration in the extracellular medium. HDL apolipoprotein-egg PC discoidal complexes at 100 micrograms PC/ml gave the following t 1/2 values when incubated with rat Fu5AH hepatoma, human HepG2 hepatoma, human GM3468 skin fibroblast, L-cell and mouse J774 macrophage-tumor cells: 11 +/- 2, 22 +/- 5, 84 +/- 18, 17 +/- 2 and 32 +/- 6 h, respectively. Equivalent experiments using purified apolipoprotein A-I or the total apolipoprotein C fraction to form the egg PC complexes showed that the t 1/2 values for the hepatoma cells were unaltered. However, with the fibroblasts, L-cells and J774 macrophages, the apolipoprotein C complexes gave significantly longer t 1/2 than complexes of egg PC with either apolipoprotein A-I or HDL apolipoprotein which gave the same t 1/2. An analysis based on the theory of fast coagulation of colloid particles to describe collisions between desorbed cholesterol molecules and acceptor particles predicts that the dependence of t 1/2 for cholesterol efflux from a given cell to different acceptors should be normalized when the extracellular level of acceptors is expressed in terms of the product of the radius of the particle times the number concentration of acceptor particles. The decrease in t 1/2 for cholesterol efflux from fibroblasts when the egg PC acceptor was changed from an SUV to an apolipoprotein HDL discoidal complex is consistent with the above concepts. The primary effect of the apolipoproteins in promoting cellular cholesterol efflux seems to be the solubilization of PC so that the PC is present in the extracellular medium as many small particles.

Adrenergic effects on plasma lipoprotein metabolism. Speculation on mechanisms of action

Recently, the effects of alpha-adrenergic and beta-adrenergic antagonists on plasma lipoprotein concentrations have been reported. Evidence from diverse lines of research has been brought together that suggests three potential mechanisms by which these antihypertensive agents affect lipoprotein metabolism. First, the known alterations in plasma triglyceride levels caused by adrenergic antagonists may be mediated through the activity of lipoprotein lipase. This enzyme, located on the capillary endothelium of adipose tissue and skeletal muscle, catabolizes chylomicrons and very-low-density lipoproteins. Catecholamine-induced changes in precapillary sphincter tone could affect the delivery of triglyceride-rich lipoproteins to endothelial lipase, cause changes in capillary endothelial surface area for lipase-binding sites, and/or modulate the synthesis of lipoprotein lipase by adipocytes and myocytes. Since high-density lipoprotein levels increase by taking up components of chylomicrons released by lipoprotein lipase, these pathways might explain the adrenergic-induced changes in high-density lipoprotein that are reciprocal to those in plasma triglycerides. Second, hepatic production of very-low-density lipoproteins might be affected by adrenergic-induced changes in insulin release. Decreased insulin release may direct glucose metabolites from adipocytes to hepatocytes for lipogenesis. Catecholamines and other glucoregulatory hormones are known to alter hepatic cholesterol synthesis and secretion of very-low-density lipoproteins. These observations also suggest that dietary carbohydrate and fat might modulate adrenergic influences on lipoprotein metabolism by other than classic means. Third, as suggested by studies of cultured fibroblasts, alpha-adrenergic antagonists may increase receptor-mediated catabolism of low-density lipoprotein.

Influence of the fatty acid composition of high-density lipoprotein phospholipids on the cholesterol efflux from cultured fibroblasts

The purpose of this work was to determine whether the changes induced by dietary manipulations in the chemical composition of high-density lipoproteins (HDL) (particularly phospholipid fatty acid composition) modified their capacity to promote [3H]cholesterol efflux from cultured fibroblasts. Plasma HDL were obtained from subjects fed for six successive long periods on diets consisting of one predominant fat: peanut oil, corn oil, olive oil, soybean oil, low erucic acid rapeseed oil or milk fats. The [3H]cholesterol efflux from cells in the presence of plasma HDL was studied by means of normal adult human fibroblasts in culture. The [3H]cholesterol efflux from fibroblasts appeared to be independent of the overall composition of HDL and of the degree of saturation of the HDL phospholipid fatty acids, but it was correlated with the phospholipid fatty acid chain length. The [3H]cholesterol efflux from fibroblasts is highly and positively correlated with the sum of the HDL phospholipid C20, C22, C24 fatty acids, and negatively correlated with the sum of the HDL phospholipid C18 fatty acids.

Morphological evidence of lysosomal uptake of high-density lipoproteins by rat adrenocortical cells in vitro

The high-density lipoprotein (HDL) pathway in rat adrenocortical cells was studied at the electron microscopic level in vitro via colloidal gold labelling. Steroid hormone assays were performed to confirm that the cells remained intact, viable, responsive to ACTH under the applied conditions, and to reveal the steroidogenic effect of HDL. The gold-labelled HDL particles (HDL-Au) were observed on the surface of the parenchymal cells, often attached to the membranes of the microvilli, but rarely in coated pits and coated vesicles. HDL-Au was accumulated by non-coated vesicles, multivesicular bodies and lysosomes. The lysosomes were identified by means of a non-specific esterase reaction. It is concluded that HDL particles are internalized by both zona glomerulosa and zona fasciculata cells. HDL is required for the enhanced functional activity of these cells in long-term incubation, and the lysosomes are involved in the process.

Age-related changes in the rate of esterification of plasma cholesterol in Fischer-344 rats

Plasma cholesterol and triglyceride levels and selected molecular species of plasma cholesteryl esters and triglycerides were determined in 6-, 12-, 15-, 18-, 21-, and 24-month-old Fischer-344 rats. Lecithin:cholesterol acyltransferase (LCAT) activity was also determined using two independent methods utilizing endogenous and exogenous substrates. Plasma cholesterol levels increased up to 18 months of age and then plateaued. Of the plasma triglyceride molecular species investigated (C50, C52, C54 and C56), only the levels of C52 increased linearly with age. The concentration of other triglyceride molecular species did not change with age. The fractional rate of plasma cholesterol esterification showed a decreasing trend with age, whereas, the net cholesterol esterification rate showed a gradual age related increase. However, this latter parameter remained unchanged with age when the data were normalized for body weight. The cholesterol esterification rates measured using an exogenous substrate (estimating LCAT enzyme levels) showed essentially no change with age. These data indicate that changes in the levels and/or composition of lipoprotein substrate(s) for LCAT are likely causes of the observed age-related changes in the fractional rate of plasma cholesterol esterification. The net esterification rate of plasma cholesterol was significantly correlated with the plasma triglyceride levels when the animals for all age groups were treated as one experimental group.

Isolation of a cDNA clone for chick intestinal apolipoprotein AI (Apo-AI) and its use for detecting apo-AI mRNA expression in several chick tissues

Three cDNA clones for chick apolipoprotein AI (Apo-AI), the major protein component of plasma high-density lipoproteins, have been isolated. The identity of the clones has been established first by screening a cDNA library in the pEX1 expression vector with anti-Apo-AI antibodies, second by Western blot analysis of the proteins expressed by positive clones. The use of the clone containing the largest, presumably full-size, cDNA insert (apo5C12) in molecular hybridization experiments confirms that apo-AI mRNA is expressed mainly in chick small intestine and liver. Furthermore, we provide evidence that brain, heart and skeletal muscle also synthesize significant amounts of apo-AI mRNA. The Southern-blot hybridization pattern of the restriction-enzyme-digested chick DNA with the apo5C12 DNA is consistent with there being a single copy of the apo-AI gene.

Modifications and degradation of high density lipoproteins

It is evident that lipoprotein modifications, degradation and clearance from plasma and interstitial compartments involves both cellular and extracellular processing. Cellular uptake of the intact particle as a whole and/or selective removal of constituent apoproteins and lipids by various parenchymal cells goes on continuously. Regulation of these processes undoubtedly varies tissue to tissue and much remains to be clarified in human tissues in vivo. The metabolic effects of chemical, proteolytic, and lipolytic modification of lipoproteins secondary to transient cellular encounters (e.g. during transit through endothelial barriers, or reversible binding to cells) on apolipoprotein clearance remains to be defined. It is likely that multiple post-secretory modifications occur and together represent subtle regulatory events that modulate lipid shuttle functions and cellular targetting properties of HDL particles.

Plasma lipoprotein distribution, faecal cholesterol excretion, and activities of lipoprotein lipase, hepatic lipase and lecithin:cholesterol acyltransferase in rats fed diets rich in sucrose or sunflower oil

Plasma HDL2 has been suggested to carry cholesterol to the liver for subsequent excretion in the bile and faeces. The enzymes lipoprotein lipase (LPL), hepatic lipase (HL) and lecithin:cholesterol acyltransferase (LCAT) have been implicated in the centripetal cholesterol transport. Activities of these enzymes, the amount of faecal cholesterol excretion and the level of plasma lipoproteins were determined in male rats fed for 4 weeks on purified diets in which the sunflower oil:sucrose ratio was either 0.03 (group a) or 1.01 (group b). Whole plasma triacylglycerols (TG), unesterified cholesterol (UC) and phospholipids (PL) were highest in group (a). The concentration of cholesteryl esters (CE) was similar in the two groups. Protein, TG and UC of VLDL, and TG, UC, CE and PL of HDL2 were higher in group (a) than in group (b). The HDL3-protein and TG were lowest in group (a). Thus, total weight of VLDL and HDL2 were increased, and HDL3 reduced in group (a), which had also increased activities of HL and adipose tissue LPL. Activity of LCAT was lower, and faecal excretion of cholesterol was reduced by about 50% in group (a) compared to group (b). Accordingly, in the rat increased plasma levels of HDL2 are not necessarily indicative of increased faecal cholesterol excretion.