The lecithin-cholesterol acyl transferase activity of rat intestinal lymph.

The lecithin-cholesterol acyl transferase (LCAT) activity in rat mesenteric lymph was examined as a possible source of chylomicron cholesteryl ester. Lymph activity was only 2-3% of rat serum activity. Removal of d less than 1.006 lipoproteins increased lymph LCAT activity, but only to 6-8% of that of serum. Relative to total cholesterol in the d greater than 1.08 g/ml fractions, lymph LCAT activity in lymph from fasting rats was less than serum, but in lymph from nonfasting rats the ratio LCAT/HDL-cholesterol reached levels greater than serum, suggesting a contribution of enzyme from the gut. Both LCAT activity and HDL concentration in mesenteric lymph increased during feeding. Subfractions of lymph that inhibited serum LCAT were: chylomicrons, VLDL, chylomicron lipid, VLDL apoprotein, and HDL apoprotein. In the rat, the low LCAT activity of mesenteric lymph was in part due to the low enzyme concentration present, and the activity was apparently lowered further by lipid-rich lipoproteins that inhibited the reaction. Enzyme inhibition due to the apoprotein fractions of lipoproteins is probably minor in the rat in vivo.     

Lipoprotein metabolism in the suckling rat: characterization of plasma and lymphatic lipoproteins

Suckling rat plasma contains (in mg/dl): chylomicrons (85 +/- 12); VLDL (50 +/- 6); LDL (200 +/- 23); HDL1 (125 +/- 20); and HDL2 (220 +/- 10), while lymph contains (in mg/dl): chylomicrons (9650 +/- 850) and VLDL (4570 +/- 435) and smaller amounts of LDL and HDL. The lipid composition of plasma and lymph lipoproteins are similar to those reported for adults, except that LDL and HDL1 have a somewhat higher lipid content. The apoprotein compositions of plasma lipoproteins are similar to those of adult lipoproteins except for the LDL fraction, which contains appreciable quantities of apoproteins other than apoB. Although the LDL fraction was homogeneous by analytical ultracentrifugation and electrophoresis, the apoprotein composition suggests the presence of another class of lipoproteins, perhaps a lipid-rich HDL1. The lipoproteins of lymph showed low levels of apoproteins E and C. The triacylglycerols in chylomicrons and VLDL of both lymph and plasma are rich in medium-chain-length fatty acids, whereas those in LDL and HDL have little or none. Phospholipids in all lipoproteins lack medium-chain-length fatty acids. The cholesteryl esters of the high density lipoproteins are enriched in arachidonic acid, whereas those in chylomicrons, VLDL, and LDL are enriched in linoleic acid, suggesting little or no exchange of cholesteryl esters between these classes of lipoproteins. The fatty acid composition of phosphatidylcholine, sphingomyelin, and lysophosphatidylcholine were relatively constant in all lipoprotein fractions, suggesting ready exchange of these phospholipids. However, the fatty acid composition of phosphatidylethanolamine in plasma chylomicrons and VLDL differed from that in plasma LDL, HDL1, and HDL2. LDL, HDL1, and HDL2 were characterized by analytical ultracentrifugation and shown to have properties similar to that reported for adult lipoproteins. The much higher concentration of triacylglycerol-rich lipoproteins in lymph, compared to plasma, suggests rapid clearance of these lipoproteins from the circulation.     

Study of the atherogenic dyslipoproteinemia induced by dietary cholesterol in rhesus monekys (Macaca mulatta).

Hypercholesterolemia was induced in adult male rhesus monkeys with a high-fat diet containing an elevated cholesterol level (0.5%). Plasma lipoproteins were chromatographically separated into four size populations (regions) that were subdivided by density until fractions with single electrophoretic mobilities were obtained. The region III lipoproteins (LDL) contained 80% of plasma cholesterol and were present in the highest concentration of all fractions. Their molecular weight was increased over that of controls so that each particle averaged 1.8 times the number of cholesteryl ester molecules as did control LDL. Region II lipoproteins, a heterogeneous group, were present in next highest concentration. Most were cholesteryl ester-rich, beta-migrating lipoproteins that overlapped the VLDL and LDL density ranges; apoB was the predominant apoprotein. One region II subfraction had pre beta 2 migration and the density range. 1.050 less than d less than 1.10. Another subfraction, cholesteryl ester-rich VLDL including only about 1% of plasma cholesterol, had pre beta 1 migration and apoB and apoC as the predominant apoproteins with no apoprotein E. Region I lipoproteins were larger sized, slow beta-migrating cholesteryl ester-rich VLDL that included 5% of plasma cholesterol. ApoB and apoE were the predominant apoproteins. Region IV lipoproteins (HDL) contained 4% of the plasma cholesterol; their concentration was decreased to about 1/3 of the control level. Atherogenic features of the diet-induced dyslipoproteinemia included the increased plasma concentrations and cholesteryl ester contents of the region I, II, and III lipoproteins in addition to the decreased HDL concentration.     

Influence of dietary fatty acid composition on the relationship between CETP activity and plasma lipoproteins in monkeys.

CETP activity, measured as transfer of cholesteryl ester from exogenous HDL to exogenous VLDL and LDL, reflecting CETP mass as determined by ELISA, was documented in three groups of St. Kitts vervet monkeys fed diets enriched in saturated (Sat), monounsaturated (Mono), or n-6 polyunsaturated (Poly) fatty acids. CETP activity was not different when comparing the three dietary fats. However, CETP activity was significantly higher when cholesterol was added to each of the diets. Significant positive associations between CETP activity and VLDL and LDL cholesterol concentrations were found whereas significant negative associations were seen between CETP activity and HDL cholesterol in each of the diet groups. The strength of these associations was highest in the Sat group. Cholesteryl ester (CE) fatty acid composition of lipoproteins varied widely among diet groups, with the more polyunsaturated CE of the Poly group being associated with a higher rate of CE transfer to endogenous acceptor apolipoprotein B-containing lipoproteins. Finally, only the Sat diet group showed significant positive correlations of CETP activity with LDL particle diameter (r = 0.76), cholesteryl ester percentage (r = 0.67), and a strong negative correlation (r = -0.86) with LDL receptor function, estimated as the difference between native and methylated LDL turnover rates. We speculate that strong associations between CETP and LDL metabolism may explain, at least in part, the increased atherogenicity of dietary saturated fat.     

Cholesterol absorption and transport in thoracic duct lymph lipoproteins of nonhuman primates. Effect of dietary cholesterol level

The effect of dietary cholesterol level on cholesterol absorption and its subsequent transport in thoracic duct lymph lipoproteins was studied in two species of nonhuman primates, namely the African green monkey (Cercopithecus aethiops) and the cynomolgus macaque (Macaca fascicularis). Each animal served as its own control because each received sequential, intraduodenal infusions of two fat-rich liquid diets that differed only in the amounts of cholesterol. The percentage of dietary cholesterol absorbed was calculated by dividing the rate of appearance of exogenous cholesterol radioactivity in lymph by the rate of infusion of dietary cholesterol radioactivity at a time when lymph cholesterol specific activity was constant relative to that of diet. The percentage of dietary cholesterol absorbed was similar during both diet infusions in African green monkeys but was significantly decreased during the high cholesterol diet infusion in cynomolgus macaques. Rates of appearance of lymph total cholesterol mass were significantly increased during infusion of high cholesterol diets due to a statistically significant increase in cholesteryl ester transport rates. This increase was due in large part to the preferential esterification of exogenous cholesterol that was incorporated into lymph chylomicrons and VLDL. The rate of appearance in lymph of exogenous cholesterol significantly increased during the high cholesterol diet infusion while that of endogenous cholesterol decreased. This decrease or compensation in endogenous cholesterol transport occurred during absorption of increased levels of dietary cholesterol and apparently was due to an approximate 50% decrease in the absorption of lumenal cholesterol. Our data demonstrated that there was not a simple one-to-one relationship between the amount of isotopic dietary cholesterol absorbed from the intestinal lumen and the mass of cholesterol moved into lymph via the intestine. Rather, the amount of sterol transported into the body depends on the degree of the decrease in endogenous cholesterol transport and probably on the efficiency of cholesterol esterification during absorption.     

Effects of chronic glucagon administration on rat lipoprotein composition

Male adult rats of the Wistar strain received daily at 9 a.m. and 5 p.m. 10 micrograms of Zn-protamine glucagon (Novo) for 21 days by subcutaneous injections. Plasma levels of cholesterol, triacylglycerol and phospholipids were decreased by 47, 40 and 21%, respectively. Lipoproteins were separated by sequential ultracentrifugation. Concentrations of cholesterol, phospholipids and proteins were decreased in chylomicrons, VLDL, LDL2 (1.040-1.063 g/ml) and HDL, LDL2 being the most affected by glucagon treatment (-70%). Triacylglycerol levels were decreased only in chylomicrons and VLDL. The relative proportions of cholesterol, triacylglycerol, phospholipids and proteins in lipoproteins were virtually unchanged by glucagon, suggesting a reduced number of some lipoprotein particles in plasma. However, lipoproteins of glucagon-treated rats were depleted in cholesteryl esters, while the proportion of triacylglycerol increased in LDL and HDL. Apo E contents were decreased in plasma, LDL1 (1.006-1.040 g/ml), LDL2 and HDL, whereas apo B100 proportions increased in VLDL and LDL1 in glucagon-treated rats. Glucagon appeared to be a potent hypolipidemic agent affecting mainly the apo-E-rich lipoproteins.     

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

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

Interstitial fluid lipoproteins

While a wide variety of techniques has been used to collect samples of interstitial fluid, most of our detailed knowledge about the composition of interstitial fluid lipoproteins has come from lymph collection studies. The considerable variability of lymph data probably reflects the effect of variable metabolic modification and different capillary permeabilities on the lipoprotein composition of interstitial fluid. All density classes of plasma lipoproteins are present in lymph. In peripheral lymph, the lymph/plasma concentration ratios of lipoproteins vary from 0.03 for VLDL-sized particles to 0.2 for HDL. Lymph from more permeable vascular beds, such as lung and myocardium, contains proportionately more lipoproteins. Their lymph/plasma concentration ratios vary from 0.1 to 0.6. In general, lymph lipoproteins are more heterogeneous in size than their plasma counterparts. Lymph HDL and LDL contain larger and smaller particles than their plasma equivalents. Lymph lipoproteins have unusual shapes (square packing and discoidal), chemical compositions, and molecular charge, which suggest de novo formation and/or extensive peripheral modification. Lymph HDL and LDL are enriched in free cholesterol. Lymph HDL also has increased cholesterol/protein and phospholipid/protein (especially sphingomyelin) ratios (Sloop, C.H., L. Dory, and P.S. Roheim, unpublished observations). Lymph HDL apoprotein composition differs from that of plasma, with an increase in apoE and apoA-IV content relative to apoA-I. These discoidal HDL particles may be products of an initial stage of reverse cholesterol transport. We believe further study of their metabolic fate would give important information concerning the later stages of reverse cholesterol transport.     

Lipoprotein abnormalities associated with lipopolysaccharide-induced lecithin: cholesterol acyltransferase and lipase deficiency

Density gradient ultracentrifugation was used to isolate and characterize the plasma lipoproteins from African green monkeys before and 24 and 48 h after subcutaneous injection of 300 micrograms/kg lipopolysaccharide (LPS) to induce an acute phase response. Compared with 0 h values, reductions occurred in plasma cholesterol (39%), high density lipoprotein (HDL) cholesterol (54%), lecithin:cholesterol acyltransferase (LCAT) activity (55%), and post-heparin plasma lipase activity (68%) 48 h after LPS injection while plasma triglyceride concentrations increased 700%. Cholesterol distribution among lipoproteins shifted from 7 to 41% in very low density lipoproteins (VLDL), 65 to 38% in low density lipoproteins (LDL), and 28 to 21% in HDL after LPS injection. At 48 h after LPS injection, all lipoprotein classes were relatively enriched in phospholipid and triglyceride and depleted of cholesteryl ester. The plasma concentration of all chemical constituents in VLDL was increased 3-9-fold within 48 h after LPS injection. By negative stain electron microscopy, HDL were discoidal in shape while VLDL and LDL appeared to have excess surface material present. Even though total HDL protein concentration in plasma was unaffected, the plasma mass of the smallest HDL subfractions (HDL3b,c) doubled while the mass of intermediate-sized subfractions (HDL3a) was dramatically decreased within 24 h after treatment. HDL became enriched in apoE, acquired apoSAA, and became depleted of apoA-I, A-II, and Cs by 48 h after LPS injection while apoB-100 remained the major apoprotein of VLDL and LDL. We conclude that administration of LPS to monkeys prevents normal intravascular metabolism of lipoproteins and results in the accumulation of relatively nascent forms of lipoproteins in plasma. These immature lipoproteins resemble those isolated from the recirculating perfusion of African green monkey livers, which are relatively deficient of LCAT activity and those isolated from the plasma of patients with familial LCAT deficiency.     

Characterization of plasma lipoproteins of grain- and cholesterol-fed White Carneau and Show Racer pigeons

Plasma cholesterol concentrations from White Carneau (WC) and Show Racer (SR) pigeons consuming a cholesterol-free grain diet averaged about 300 mg/dl, approximately 200 mg/dl as high density lipoproteins (HDL) and the remainder as low density lipoproteins (LDL). Consumption of a cholesterol-containing diet increased plasma cholesterol concentrations in both breeds to greater than 2000 mg/dl. Approximately one-half of this increase was as LDL with the remainder as beta-migrating very low density lipoproteins (beta-VLDL). There was little change in HDL concentration. LDL from cholesterol-fed animals had a greater net negative charge than control LDL, and was larger (Mr = 10 X 10(6) vs 3.2 X 10(60)) due to an increase in the number of cholesteryl ester molecules per particle. The principal apoprotein of LDL was apoB-100 with smaller amounts of apoA-I and several minor unidentified apoproteins. beta-VLDL was cholesteryl ester-rich, could be separated into two size populations by gel chromatography, and contained apoB-100 as its principal apoprotein. Apoprotein E was not detected in any of the plasma lipoproteins. HDL from control and cholesterol-fed animals was composed of a single class of particles with virtually identical composition resembling HDL2. The major apoprotein of HDL was apoA-I. There were no consistent quantitative or qualitative differences in the lipoproteins of the two breeds of pigeons that could help to explain the susceptibility to atherosclerosis of the WC or the resistance of the SR.     

The lipid composition of serum lipoproteins in the harbour seal, Phoca vitulina

The density profile of serum lipoproteins and their lipid composition was studied in 12 adult, female harbour seals. The animals were sampled after an approximate 20 hr fast. The density profile of lipoproteins showed that the harbour seals displayed a distinct VLDL (density less than 1.006 g/ml) and HDL band (density about 1.125 g/ml), but no clear LDL band. There was a rather diffuse population of lipoproteins in the density range of 1.019-1.100 g/ml. Mean serum total cholesterol concentration was 5.7 mmol/l; about 60% of this cholesterol was located in the HDL fraction (density greater than 1.063 g/ml). The fasted seals were found to carry 4% of serum total lipids in chylomicrons. These lipoproteins consisted of 51% of triaclyglycerols (on the basis of total chylomicron lipids). The LDL (defined as heparin-manganese precipitable lipoproteins in VLDL and chylomicron-deficient serum) contained 49% of cholesterol and 43% of phospholipids (on the basis of total LDL lipids). The HDL (defined as heparin-manganese soluble lipoproteins in VLDL and chylomicron-deficient serum) contained 36% of cholesterol and 58% of phospholipids (on the basis of total HDL lipids).     

Postprandial chylomicrons: potent vehicles for transporting cholesterol from endogenous LDL+HDL and cell membranes to the liver via LCAT and CETP

We examined whether postprandial (PP) chylomicrons (CMs) can serve as vehicles for transporting cholesterol from endogenous cholesterol-rich lipoprotein (LDL+HDL) fractions and cell membranes to the liver via lecithin:cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) activities. During incubation of fresh fasting and PP plasma containing [(3)H]cholesteryl ester (CE)-labeled LDL+HDL, both CMs and VLDL served as acceptors of [(3)H]CE or cholesterol from LDL+HDL. The presence of CMs in PP plasma suppressed the ability of VLDL to accept [(3)H]CE from LDL+HDL. In reconstituted plasma containing an equivalent amount of triglycerides from isolated VLDL or CMs, a CM particle was about 40 times more potent than a VLDL particle in accepting [(3)H]CE or cholesterol from LDL+HDLs. When incubated with red blood cells (RBCs) as a source for cell membrane cholesterol, the cholesterol content of CMs, VLDL, LDL, and HDL in PP plasma increased by 485%, 74%, 13%, and 30%, respectively, via LCAT and CETP activities. The presence of CMs in plasma suppressed the ability of endogenous lipoproteins to accept cholesterol from RBCs. Our data suggest that PP CMs may play an important role in promoting reverse cholesterol transport in vivo by serving as the preferred ultimate vehicle for transporting cholesterol released from cell membranes to the liver via LCAT and CETP.     

Distribution of cell-derived cholesterol among plasma lipoproteins: a comparison of three techniques

Three fractionation procedures (immunoaffinity chromatography, two-dimensional nondenaturing electrophoresis, and heparin-agarose affinity chromatography) have been compared in determining the kinetics of free and ester cholesterol transfer in normolipemic native plasma. Similar results were obtained in each case. Cell-derived free cholesterol is initially enriched in high density lipoproteins (HDL) (mainly HDL without apoE); at longer time periods (greater than 10 min) greater proportions are observed in very low density lipoproteins (VLDL) and low density lipoproteins (LDL). The major part of cholesteryl ester (about 90%) was retained in HDL, while VLDL and LDL, which contained about 75% of total cholesteryl ester mass, received only about 10% of cell-derived cholesteryl ester. Within HDL, almost all cholesteryl ester was in the apoE-free fraction. These data provide evidence that lipoprotein free and esterified cholesterol are not at chemical equilibrium in normal plasma, and that cell-derived cholesterol is preferentially directed to HDL. The techniques used had a comparable effectiveness for the rapid fractionation of labile lipoprotein lipid radioactivity.     

Effects of chylomicron remnants and beta-VLDL on the class and composition of newly secreted lipoproteins by HepG2 cells

The regulation of lipoprotein secretion in the cell line HepG2 was studied. HepG2 cells were preincubated with chylomicron remnants (triglyceride- and cholesterol-rich) or with beta very low density lipoproteins (beta-VLDL) (cholesterol-rich). The medium was removed and the cells were incubated for and additional 24 hr in a lipoprotein-free medium that contained either [2-3H]glycerol or DL-[2-3H]mevalonate. Cells and media were harvested, and lipoproteins were separated and fractionated. The mass and radioactivity of the lipids in cells and in the lipoproteins were measured. The activities of cellular acyl-CoA:cholesterol acyltransferase (ACAT) and 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase were also determined. Preincubation with chylomicron remnants induced an increase in cellular triglyceride and stimulated both HMG-CoA reductase and ACAT. Preincubation with beta-VLDL induced an increase in cellular free and esterified cholesterol, inhibited HMG-CoA reductase and stimulated ACAT. Although the absolute amount of VLDL is small, chylomicron remnants induced large relative increases in the amount of triglyceride and phospholipid secreted in VLDL and decreases in the amount of triglyceride secreted in low density (LDL) and high density (HDL) lipoproteins as well as a decrease in the amount of phospholipid secreted in HDL. In contrast, preincubation with beta-VLDL did not affect triglyceride secretion, but markedly stimulated the amount of phospholipid secreted in HDL. Comparison of the mass of glycerolipid actually secreted with that calculated from the cellular specific activity suggested that glycerolipids are secreted from single, rapidly equilibrating pools. Cholesterol and cholesteryl ester secretion were affected differently. Preincubation with chylomicron remnants increased the amount of free cholesterol secreted in both VLDL and LDL, but did not alter cholesteryl ester secretion. Preincubation with beta-VLDL increased free cholesterol secretion in all lipoprotein fractions and increased cholesteryl ester secretion in VLDL and LDL, but not HDL. Comparison of isotope and mass data suggested that the cholesteryl ester secreted came primarily from a preformed, rather than an newly synthesized, pool. In summary, these data provide insight to the mechanism whereby a liver cell regulates the deposition of exogenous lipid.     

Alterations of the plasma lipoproteins and apoproteins following cholesterol feeding in the rat.

The feeding of cholesterol to rats resulted in marked alterations in the type and distribution of the plasma lipoproteins and their apoproteins. The hyperlipoproteinemia was characterized by an increase in the d < 1.006 lipoproteins (B-VLDL and VLDL), an increase in the intermediate and low density lipoproteins (LDL), and the appearance of HDL(c). Associated with these lipoproteins was a prominence of the arginine-rich apoprotein. The high density lipoproteins (HDL) were decreased. A two-dimensional immunoelectrophoretic procedure was adapted to quantitate the changes in distribution of the arginine-rich apoprotein in the plasma and various ultracentrifugal fractions obtained from control and cholesterol-fed rats. In rats fed the cholesterol diet, the total plasma arginine-rich apoprotein increased from a control value of approximately 29 mg/dl to 47 mg/dl. The method of ultracentrifugation, however, was found to markedly alter the quantitative results. When the 60 Ti rotor was used at maximum speed to isolate the ultracentrifugal fractions, less than 50% of the total plasma arginine-rich apoprotein was associated with the lipoproteins in the d < 1.006 or the d 1.006-1.02, 1.02-1.063, or 1.063-1.21 ultracentrifugal fractions. By contrast, after limited ultracentrifugation with the 40 rotor, much less arginine-rich apoprotein was lost, with approximately 20% of the arginine-rich apoprotein in control rats and 10% in cholesterol-fed rats found in the d > 1.21 fraction. Significant alterations in the arginine-rich apoprotein quantitation notwithstanding, the observations of increased arginine-rich apoprotein in the B-VLDL, intermediate fraction, and HDL(c) following cholesterol feeding remained valid. However, precise quantitation awaits refinements in lipoprotein isolation techniques.     

Effects of the degree of saturation of dietary fat on the hepatic production of lipoproteins in the African green monkey

The cholesteryl ester content of plasma low density lipoproteins (LDL) in monkeys has previously been shown to be related to the rate of hepatic cholesterol secretion and cholesteryl ester content of newly secreted lipoproteins in the isolated perfused liver. In the present studies, African green monkeys were fed diets containing cholesterol and 40% of calories as either butter or safflower oil in order to determine the effects of saturated versus polyunsaturated dietary fat on hepatic lipoprotein secretion. The rate of cholesterol accumulation in liver perfusates was correlated with the size of the donor's plasma LDL, but for any rate, a smaller plasma LDL was found in donor animals of the safflower oil group than in those of the butter group. Hepatic very low density lipoproteins (VLDL) were smaller in the safflower oil group but contained more cholesteryl ester and fewer triglyceride molecules per particle than those from the butter group. Livers from the safflower oil group contained more cholesteryl ester and less triglyceride than those from the butter group. The cholesteryl ester percentage composition of hepatic VLDL resembled that of the liver in each group. The data show that dietary polyunsaturated fat decreased plasma LDL size even though it increased the cholesteryl ester content of lipoproteins secreted by the liver. Therefore, intravascular formation of plasma LDL from hepatic precursor lipoproteins appears to include the removal of relatively greater amounts of cholesteryl esters from the precursor lipoproteins in polyunsaturated fat-fed animals.     

Triacylglycerol increase in plasma very low density lipoproteins in cyclophosphamide-treated rabbit: relationship with cholesteryl ester transfer activity

We have studied the cholesteryl ester transfer between HDL and VLDL in cyclophosphamide-treated rabbits, in order to explain the abnormal cholesteryl ester partition between these two lipoprotein classes. The hypertriglyceridemia caused by treatment with the drug was associated with cholesteryl ester- and triacylglycerol-rich VLDL and with HDL poor in esterified cholesterol but relatively enriched in triacylglycerol. These two lipoprotein classes were characterized by their chemical composition and by gel filtration chromatography. VLDL particles were slightly larger in size, compared with controls. Different transfer combinations were envisaged between these abnormal lipoproteins and control ones. The transfer study involved the plasma fraction of d greater than 1.21 g/ml containing the cholesteryl ester transfer protein (CETP). It appeared that the chemical composition of lipoproteins was responsible for the level of cholesteryl ester transfer between lipoproteins. Actually, when the cholesteryl ester acceptor lipoproteins (VLDL) were enriched in triacylglycerol, the transfer was enhanced. Therefore, the effect of lipolysis on the transfer has also been explored. Lipoprotein lipase seemed to enhance the transfer of cholesteryl ester from HDL to VLDL when these lipoproteins were normal, but an important decline was obtained when triacylglycerol-rich VLDL were lipolyzed. This study defines the relationship between lipoprotein chemical composition and transfer activity of cholesteryl ester from HDL to VLDL.     

Secretion of cholesteryl-ester-rich lipoproteins by the perfused livers of rabbits fed a wheat-starch-casein diet

Rabbits fed a cholesterol-free semi-synthetic wheat-starch-casein diet had a high plasma cholesterol concentration; most of the cholesterol was associated with low-density lipoproteins (LDL). Chemical analyses of plasma lipoproteins revealed that very-low-density lipoproteins (VLDL), intermediate lipoproteins and LDL from casein-fed rabbits contained more cholesteryl ester than that of lipoproteins isolated from chow-fed animals. The fatty acid composition of cholesteryl esters of plasma lipoproteins showed that there were higher contents of oleic acid than linoleic acids in lipoproteins from casein-fed rabbits. Lipoproteins isolated from liver perfusates of casein-fed rabbits had higher cholesteryl oleate content than lipoproteins from chow-fed rabbit liver perfusates. There was a marked increase in secretion of apolipoproteins from perfused livers of casein-fed rabbits. We conclude that the high levels of plasma cholesterol in casein-fed rabbits are of hepatic origin and that one of the hypercholesterolemic actions of dietary casein in rabbits is the induction of hepatic synthesis and secretion of cholesteryl-ester-rich lipoproteins.     

The apo E/apo CIII molar ratio affects removal of cholesterol ester from modified human lipoproteins injected into cebus monkeys

The removal of postprandial (PP) and postabsorptive (PA) human LDL and HDL cholesterol was examined in cebus monkeys (Cebus albifrons) following in vitro labelling of these lipoproteins by 3H-cholesterol in the presence or absence of DTNB. The removal of LDL cholesteryl ester was 3.5 and 2 times greater than that of HDL in male and female monkeys, respectively. Incubation with DTNB reduced cholesteryl ester removal by 45 and 52% for LDL and HDL, respectively. Cholesteryl ester from PA lipoproteins was removed 80% faster than that PP particles only when plasma was incubated without DTNB. Cholesterol removal from these lipoproteins was positively (r = 0.941) and significantly (P less than 0.001) correlated with the molar apo E/apo CIII ratio. The data suggest that density of lipoproteins was less important than their apoprotein composition in dictating their removal from circulation.     

Impaired plasma cholesteryl ester transfer with accumulation of larger high density lipoproteins in some families of baboons (Papio sp.)

Baboons from some families have a higher concentration of plasma high density lipoproteins (HDL) on a chow diet and accumulate large HDL (HDL1) when challenged with a high cholesterol and high saturated fat (HCHF) diet. HDL1 from high HDL1 animals contained more (1.5-fold) cholesteryl ester than HDL (HDL2 + HDL3) from high or low HDL1 animals. HDL from high HDL1 baboons had lower triglyceride content than that from low HDL1 baboons. HDL3 or HDL labeled with [3H]cholesteryl linoleate was incubated with entire lipoprotein fraction (d less than 1.21 g/ml) or very low density lipoprotein + low density lipoprotein (VLDL + LDL) (d less than 1.045 g/ml) and with lipoprotein-deficient serum (LPDS), and the radioactive cholesteryl ester and mass floating at d 1.045 g/ml (VLDL + LDL) after the incubation was measured. The transfer of cholesteryl esters from either HDL or HDL3, prepared from plasma of high HDL1 animals fed chow or the HCHF diet, was slower than the transfer from either HDL or HDL3 of low HDL1 animals, regardless of the source of transfer activity or the ratio of LDL:HDL-protein used in the assay. Addition of HDL from high HDL1 baboons into an assay mixture of plasma components from low HDL1 baboons decreased the transfer of cholesteryl ester radioactivity and mass from HDL to VLDL and LDL. In addition to HDL, a fraction of intermediate density lipoprotein (IDL) and denser HDL were also effective in inhibiting the transfer. These observations suggest that accumulation of HDL1 in high HDL1 baboons fed an HCHF diet is associated with a slower transfer of cholesteryl esters from HDL to LDL.(ABSTRACT TRUNCATED AT 250 WORDS)