In vivo influx, tissue esterification and hydrolysis of free and esterified plasma cholesterol in the cholesterol-fed rabbit

Measurement of reverse cholesterol transport pathways in humans: in vivo rates of free cholesterol efflux, esterification, and excretion

BACKGROUND

Reverse cholesterol transport from peripheral tissues is considered the principal atheroprotective mechanism of high-density lipoprotein, but quantifying reverse cholesterol transport in humans in vivo remains a challenge. We describe here a method for measuring flux of cholesterol though 3 primary components of the reverse cholesterol transport pathway in vivo in humans: tissue free cholesterol (FC) efflux, esterification of FC in plasma, and fecal sterol excretion of plasma-derived FC.

METHODS AND RESULTS

A constant infusion of [2,3-(13)C(2)]-cholesterol was administered to healthy volunteers. Three-compartment SAAM II (Simulation, Analysis, and Modeling software; SAAM Institute, University of Washington, WA) fits were applied to plasma FC, red blood cell FC, and plasma cholesterol ester (13)C-enrichment profiles. Fecal sterol excretion of plasma-derived FC was quantified from fractional recovery of intravenous [2,3-(13)C(2)]-cholesterol in feces over 7 days. We examined the key assumptions of the method and evaluated the optimal clinical protocol and approach to data analysis and modeling. A total of 17 subjects from 2 study sites (n=12 from first site, age 21 to 75 years, 2 women; n=5 from second site, age 18 to 70 years, 2 women) were studied. Tissue FC efflux was 3.79±0.88 mg/kg per hour (mean ± standard deviation), or ≍8 g/d. Red blood cell-derived flux into plasma FC was 3.38±1.10 mg/kg per hour. Esterification of plasma FC was ≍28% of tissue FC efflux (1.10±0.38 mg/kg per hour). Recoveries were 7% and 12% of administered [2,3-(13)C(2)]-cholesterol in fecal bile acids and neutral sterols, respectively.

CONCLUSIONS

Three components of systemic reverse cholesterol transport can be quantified, allowing dissection of this important function of high-density lipoprotein in vivo. Effects of lipoproteins, genetic mutations, lifestyle changes, and drugs on these components can be assessed in humans. (J Am Heart Assoc. 2012;1:e001826 doi: 10.1161/JAHA.112.001826.).

Effect of reserpine treatment on low-density lipoproteins in arterial wall and internal organs of rats

The effects were determined in rats of single injections of reserpine at increasing doses (0.5, 1.58, and 5.0 mg/kg) on low-density lipoprotein (LDL) and cholesterol in aortic wall, heart, liver, kidney, and adrenal gland. Catecholamine levels in plasma, heart, and liver, arterial blood pressure, and heart rate were also monitored. Reserpine was injected intraperitoneally, followed immediately by the administration of [3H]cholesterol by gavage. Twelve hours later, homologous 125I-tyramine cellobiose-labeled LDL (125I-TC-LDL) was injected intravenously. Twenty-four hours later, the rats were killed, and the radioactivities of aortic walls, heart, liver, kidney, and adrenal glands were determined. The results showed that after reserpine treatment the accumulation of both the 125I-TC label derived from LDL and total [3H]cholesterol was significantly reduced in aortic wall and heart, increased in liver, and unchanged in the kidney and adrenal gland. At higher doses (1.58 and 5.0 mg/kg), reserpine significantly accelerated the plasma clearance of radiolabelled LDL. Plasma noradrenaline in reserpine-treated animals decreased maximally (86%) by 12 h and by 61-71% at 36 h compared with the control. Plasma adrenaline increased transiently after injection of reserpine and then returned to the basal levels. Reserpine greatly decreased noradrenaline and adrenaline levels in heart and liver. Arterial blood pressure was decreased significantly (0.001 < p < 0.05) at 12 h by the two lower doses of reserpine and then returned to normal values over the next 24 h. The results indicate that reserpine decreases LDL cholesterol in artery wall and heart and increases it in liver. These findings suggest that reserpine could find a new use as a cholesterol-lowering drug for the prevention of atherosclerosis.

Nifedipine inhibits accumulation of LDL and cholesterol in the aorta of the normocholesterolemic rabbit

The effect of 5 days of oral nifedipine treatment (approximately 1 mg/kg per day in drinking water) on low density lipoprotein (LDL) and cholesterol accumulation in rabbit arteries was determined. Compared with control aortas, nifedipine treatment (n = 5) significantly reduced homologous 125I-tyramine cellobiose-LDL accumulation (control versus nifedipine: 45.93 +/- 4.3 versus 20.14 +/- 3.1 ng LDL per milligram dry weight x10(-3), p = 0.001) while the reduction of human LDL accumulation (n = 5) was not significant (49.1 +/- 6.1 versus 35.5 +/- 4.1 ng LDL per milligram dry weight x10(-3)). Aortic accumulation of orally administered [3H]cholesterol was also inhibited by nifedipine (352 +/- 34 versus 257 +/- 16 ng cholesterol per milligram dry weight x10(-3), n = 10, p = 0.022). These findings suggest a possible mechanism for the antiatherosclerotic effect of nifedipine.

Aortic permeability to LDL as a predictor of aortic cholesterol accumulation in cholesterol-fed rabbits

The aim of this study was to investigate the possibility that the permeability characteristics of the arterial wall are related to the development of atherosclerosis. The in vivo regional variation of aortic permeability to iodinated human low density lipoprotein (LDL) in normal rabbits was compared with the regional variation in aortic cholesterol accumulation in cholesterol-fed rabbits. Aortas were divided into the aortic arch, thoracic aorta, and abdominal aorta, and each of these three parts was further subdivided into four segments of similar size. The permeability to LDL was 40 +/- 7 nl.cm-2.hr-1 (mean +/- SEM, n = 11) in the most proximal segment of the aortic arch and decreased throughout the length of the aorta to 3 +/- 1 nl.cm-2.hr-1 in the most caudal segment of the abdominal aorta. In such normal rabbits the aortic cholesterol content was similar in all 12 arterial segments at 0.08 +/- 0.005 mumol/cm2 (mean +/- SEM, n = 3 x 12). Aortic cholesterol accumulation was determined in other rabbits with an average plasma cholesterol level of 32 +/- 1 mmol/l for 96 days; the cholesterol content in the most proximal segment of the aortic arch was 2.7 +/- 0.5 mumol/cm2 (mean +/- SEM, n = 11) and decreased with increasing distance from the heart to 0.17 +/- 0.03 mumol/cm2 in the most caudal segment of the abdominal aorta. Linear regression analysis showed a close positive association between the permeability to LDL of a given aortic segment and the cholesterol accumulation in that same aortic segment after cholesterol feeding (r2 = 0.96, p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Accelerated cholesterol accumulation in homologous arterial transplants in cholesterol-fed rabbits. A surgical model to study transplantation atherosclerosis

Accelerated coronary artery disease has become a major complication to heart transplantation in humans. Therefore, we have developed a surgical model in the rabbit, with transplantation of the thoracic aorta as a bypass graft onto the abdominal aorta of another rabbit. The model permits the study of cholesterol metabolism in transplanted arteries. The graft did not accumulate cholesterol for as long as 298 days, provided that the rabbits were normocholesterolemic, i.e., with plasma cholesterol levels of 0.3-0.7 mmol/l. However, after a few weeks of cholesterol feeding resulting in plasma cholesterol levels of 2-5 mmol/l, the homologous graft accumulated cholesterol compared with intact aortic tissue in the rabbits and also compared with autologous aortic grafts. The intimal clearance of plasma cholesteryl ester, mainly high density lipoprotein cholesteryl ester, in the luminal layer of the aortic graft was 60-150 nl x cm-2 x hr-1 1-2 hours after transplantation. The intimal clearance in the corresponding intact thoracic aorta of the recipient animal was 5-20 nl x cm-2 x hr-1. The values were 1,500-3,000 nl x cm-2 x hr-1 51-298 days after transplantation, while the intimal clearance of the rabbit's own aorta remained unchanged. A pronounced increase in plasma lipoprotein permeability is thus an early event in transplanted arteries. It results in a higher cholesteryl ester influx that leads to cholesterol accumulation in the artery, but only if the rabbits are fed a cholesterol-enriched diet. This rabbit model may be useful in the search for interventional measures to prevent or diminish the accelerated coronary artery disease in transplanted hearts in humans.

Mechanisms of hypertriglyceridemia in the coconut oil/cholesterol-fed rabbit. Increased secretion and decreased catabolism of very low density lipoprotein

Rabbits fed a 14% coconut oil/0.5% cholesterol (CNO/Chol) diet develop mild to severe hypertriglyceridemia compared with rabbits fed a 14% olive oil/0.5% cholesterol (OO/Chol) diet. Lipids and apolipoprotein (apo) B were significantly higher in the very low density lipoprotein (VLDL) and intermediate density lipoprotein fractions from CNO/Chol than from OO/Chol rabbits. Yet, the particle diameters of these lipoproteins were similar in both diet groups, indicating that CNO/Chol rabbits had a much larger number of VLDL and intermediate density lipoprotein particles in plasma. Although the composition of CNO/Chol VLDL differed from that of OO/Chol VLDL, the rates of triglyceride hydrolysis of CNO/Chol VLDL and OO/Chol VLDL by postheparin lipoprotein lipase in vitro were the same, suggesting that VLDLs from the two diet groups were equally good substrates for lipoprotein lipase. To determine the mechanisms of hypertriglyceridemia in the CNO/Chol rabbit, triglyceride and apo B of CNO/Chol VLDL and OO/Chol VLDL were labeled with tritium-containing triolein and iodine-131 and injected intravenously into CNO/Chol and OO/Chol rabbits. The fractional clearance rate for triglyceride in OO/Chol rabbits was twice that of CNO/Chol rabbits, which parallels the previously observed differences in postheparin lipoprotein lipase activity. Although the average fractional removal of apo B did not differ between diet groups, there was a significant inverse relation between plasma cholesterol and apo B fractional clearance rate. We conclude that the hypertriglyceridemia and the enhanced hypercholesterolemia in the CNO/Chol rabbit results primarily from increased hepatic secretion of VLDL and a modest decrease in VLDL triglyceride clearance capacity.

A surgical model to study in vivo efflux of cholesterol from porcine aorta. Evidence for cholesteryl ester transfer through the aortic wall

We describe a surgical procedure in pigs which makes it possible to follow the influx into, the penetration through and the efflux from the arterial wall of labeled lipoproteins. After 4 h exposure of the luminal side of the arterial wall to labeled lipoproteins, labeled esterified cholesterol was found in all layers of the aortic wall, whereas labeled free cholesterol gained access only to the most luminal layer. The data suggest that at least 40%, if not 80-90%, of the cholesteryl ester that enters the aortic wall from the luminal side, passes through the entire wall and leaves the aortic wall through vasa vasorum and lymphatics. They also suggest that free cholesterol in the lipoproteins exchanges extensively with cellular free cholesterol while the lipoproteins penetrate through the most luminal layer. More than 90% of the labeled cholesteryl ester disappeared from the arterial wall during a cold chase period of 4 days. By the simultaneous use of [3H]cholesterol and [14C]cholesterol it was shown that only 10-20% of this disappearance can be explained by cholesteryl ester hydrolysis in the arterial wall.

The arterial barrier to lipoprotein influx in the hypercholesterolemic rabbit. 2. Long-term studies in deendothelialized and reendothelialized aortas

These studies consider whether a mild aortic injury that does not increase cholesteryl ester influx during the first few days promotes atheromatosis in the hypercholesterolemic rabbit. The cholesteryl ester influx in uninjured, deendothelialized, and reendothelialized aorta was also measured in order to account for the different cholesteryl ester contents in these areas. By 32-33 days after localized aortic injury which was made after 5-7 days of cholesterol feeding, uninjured (control) areas of the thoracic aortas had accumulated 48 micrograms/cm2 each of esterified and nonesterified cholesterol due to continued cholesterol feeding. However, the previously injured deendothelialized and reendothelialized areas of the aortas had accumulated 6 and 10 times as much esterified cholesterol, and 2 and 5 times as much nonesterified cholesterol, respectively, as the adjacent uninjured areas. Esterified cholesterol influx was low during the second day after injury but increased with time so that 30-31 days later the esterified cholesterol influx in deendothelialized and reendothelialized aorta was respectively 44 and 7 times as great as the 0.1 microgram/h/cm2 in the adjacent uninjured aorta. However, expressed per mg aortic cholesteryl ester, cholesteryl ester influx in reendothelialized aorta was no greater than in noninjured aorta. These studies suggest that an injury that does not initially increase cholesteryl ester influx can promote atheromatosis and that the higher rate of cholesteryl ester influx that develops with time in reendothelialized areas is closely related to its increased cholesteryl ester content.

The arterial barrier to lipoprotein influx in the hypercholesterolemic rabbit. 1. Studies during the first two days after mild aortic injury

These studies examine the hypothesis that removal of aortic endothelium eliminates a barrier to lipoprotein cholesterol influx. The aortas of rabbits fed a cholesterol-rich diet from 7 to 16 days before balloon injury were studied 1 or 2 days after deendothelialization of specific areas of the aorta. By this design the aortic sterol content was near normal on the day of injury, but areas of injured and noninjured aorta were exposed to identical levels of elevated plasma cholesterol. Measuring the arterial [3H]- and [14C] cholesterol fractions accumulated during 2 different intervals after dosage in the same animals permitted calculation of total influx and fractional loss of aortic cholesterol. During the first 2 days after deendothelialization, total (unidirectional) cholesteryl ester influx in deendothelialized aorta was similar to that in adjacent uninjured aorta, but total influx of nonesterified cholesterol was increased. The calculated increase in influx of nonesterified cholesterol was not a result of an increase in hydrolysis of entered cholesteryl ester but probably represents increased exchange of labeled cholesterol between artery and plasma. These results suggest that subendothelial layers of the aorta of short-term cholesterol-fed rabbits function as significant barriers to lipoprotein influx and that processes other than increased permeation by lipoproteins initiate injury-induced lesions.

Effects of dietary beef and soy protein on tissue composition and low density lipoprotein uptake in young pigs

To study effects of dietary protein source on plasma cholesterol homeostasis, six littermate pairs of 8-wk-old, castrated male pigs were assigned randomly to high fat diets containing either lean beef or soy protein isolate, with beef tallow as the major fat source in both diets. Diets were fed for 6 wk, with blood samples taken weekly and analyzed for lipid composition. At 6 wk, each pig was injected with [3H]cholesterol- and [14C]sucrose-labeled low density lipoproteins, and serial blood samples were taken. Pigs were then killed, and several tissues were sampled. Results show that dietary beef or soy protein, when fed with beef tallow as a fat source, has differential effects on rates of uptake of intact LDL by heart, M. sartorius muscle and subcutaneous adipose tissue, and on rates of uptake of LDL cholesterol by net free cholesterol exchange in subcutaneous and perirenal adipose tissues. Dietary protein source affected the composition of only two tissues analyzed. Pigs fed the beef-based diet had greater lipid concentrations in M. sartorius muscle, whereas pigs fed the soy protein-based diet had greater lipid concentrations in heart. Source of dietary protein did not influence in vivo kinetics of LDL cholesterol removal from blood plasma.

Reduced atherogenesis in cholesterol-fed diabetic rabbits. Giant lipoproteins do not enter the arterial wall

In cholesterol-fed rabbits, alloxan-diabetes has an anti-atherogenic effect, which is associated with severe elevation of plasma triglyceride concentrations. To study this effect, we measured lipoprotein sizes and aortic permeability coefficients for cholesteryl ester and for albumin in hypertriglyceridemic diabetic cholesterol-fed rabbits and in normotriglyceridemic cholesterol-fed rabbits. With the same high cholesterol concentration in plasma, hypertriglyceridemic diabetic rabbits had 70% of plasma cholesterol in very large lipoproteins (diameter greater than 75 nm), whereas normotriglyceridemic rabbits had only about 10% of plasma cholesterol in these giant lipoproteins. The aortic permeability coefficients for cholesteryl ester in hypertriglyceridemic diabetic cholesterol-fed rabbits was only 10% to 50% of that in normotriglyceridemic cholesterol-fed rabbits. Aortic permeability coefficients for albumin did not differ significantly between the hypertriglyceridemic and normotriglyceridemic rabbits. The results suggest that the large size of a major fraction of plasma lipoproteins in the hypertriglyceridemic diabetic cholesterol-fed rabbits is responsible for the relatively low aortic permeability coefficient for cholesteryl ester from plasma and hence for reduced atherogenesis in these animals.

Enhanced accumulation and turnover of esterified cholesterol in injured rabbit aorta

Arterial injury exacerbates experimental atherogenesis. This report evaluates cholesteryl ester influx and loss during the first 5 days after arterial injury. Selected areas of aortas from rabbits made hypercholesterolemic by 12 to 16 days of cholesterol feeding were injured with a balloon catheter. This allowed measurements and a relatively precise comparison of cholesterol and cholesteryl ester influx into the uninjured and injured arterial wall within the same animal. The animals received oral doses of 3H- or 14C-cholesterol either just before injury or 1 day later. Most animals were given the other isotope of cholesterol 1 day before sacrifice, which took place 2 to 5 days after injury. Measurement of accumulated labelled esterified cholesterol within the same animal during two different time periods allowed the estimation of total influx and fractional loss of entered esterified cholesterol within the artery. Between 2 and 5 days after injury, total influx into areas of injured artery averaged 30 to 60 times the total influx into the adjacent uninjured areas. By the fifth day after injury, the esterified cholesterol content was about 10 times greater in injured areas than in adjacent, uninjured areas. The nonesterified cholesterol content of injured areas was about 70% greater than that of adjacent noninjured areas. Total influx and fractional loss of arterial cholesteryl ester calculated from radioactivity data could account for 117 +/- 6 (mean +/- SEM) percent of the cholesteryl ester mass that accumulated during the interval from injury to sacrifice. The total influx rates and daily increments in cholesteryl ester content of injured areas were positively related.

Relationships between lymphocyte cholesterol homeostasis and LDL-cholesterol

The homeostasis of cholesterol was studied in lymphocytes freshly isolated from the blood and cultured with or without low-density lipoprotein (LDL). The content of cholesterol decreased in the lymphocytes cultured without LDL, whereas LDL substituted for cellular cholesterol losses, in spite of almost suppressed LDL-receptor and lymphocyte cholesterol synthesis. Free cholesterol was taken up from LDL mainly via cholesterol exchange and, in contrast to esterified cellular cholesterol, rapidly excreted into the medium. In vitro stimulation of lymphocyte cholesterol synthesis was correlated to the ratio of esterified to free LDL-cholesterol in the blood from which the lymphocytes had been isolated. This result probably reflects the different rates of influx and efflux of esterified or free cholesterol between plasma lipoproteins and lymphocytes. These effects should be taken into account if LDL-cholesterol is determined in plasma for the evaluation of an individual's atherosclerotic risk.

Uptake and transport of lipid substrates in the heart

Between 1955-1960 it was realized that the fatty acids circulating in the blood, after transport into the cardiac cells, then beta-oxidation in the myocyte mitochondria, were the major source of energy behind the impressive hydraulic performance of the heart. Only albumin-bound fatty acids and, to a lesser extent, cholesterol ester and triglyceride fatty acids have access to the cardiac cells. Circulating phospholipid fatty acids are excluded. From experiments with isolated perfused hearts it was concluded that fatty acid uptake by the myocardium was essentially an energy independent process. An important question still pending in the literature concerns the mechanisms of fatty acid transport through the capillary endothelium, through the cardiac cell plasma membrane and then through the intracellular compartments. The most plausible model now emerging considers that specific fatty acid-binding proteins, sequentially disposed along this cascade of barriers, might facilitate and drive the flux of fatty acids entering the cardiac cells.

Effect of verapamil on accumulation of free and esterified cholesterol in the thoracic aorta of cholesterol-fed rabbits

In order to investigate the effect of the calcium antagonist verapamil on atherogenesis in cholesterol-fed rabbits, 3 groups of 11 animals were fed a 2% cholesterol-enriched diet for 10 weeks. One group received verapamil in a daily dose of 16 mg/kg orally plus 2 mg/kg subcutaneously. This dosage resulted in plasma concentrations of verapamil in the same range as the usual therapeutic levels in humans. Another group received verapamil in a daily dose of 8 mg/kg orally and 0.5 mg/kg subcutaneously. The third group received placebo capsules orally and isotonic saline subcutaneously. Total cholesterol concentrations in plasma over the 10 weeks were 37 +/- 4, 42 +/- 4 and 45 +/- 3 mM (mean +/- SE) in the high verapamil-, in the low verapamil- and in the placebo group, respectively. These values were not significantly different. The distribution of cholesterol between HDL, LDL and VLDL in plasma was similar in the 3 groups. The high verapamil group had a significantly (P less than 0.05) lower concentration of cholesterol in the thoracic aorta than the placebo group (29 +/- 5 vs 43 +/- 9 mumol/g wet weight). The low verapamil group and the placebo group had the same aortic cholesterol concentrations. Neither dosage of verapamil affected the permeability of the aortic endothelium to plasma lipoproteins and albumin, as measured by use of radioactive tracers at the end of the experiment.

Cholesterol transport and uptake in miniature swine fed vegetable and animal fats and proteins. 2. LDL uptake and cholesterol distribution in tissues

In a 2 X 2 factorial arrangement, miniature pigs were fed four diets containing vegetable protein/fat (soybean) and animal protein (egg white)/fat (beef tallow) to demonstrate the effects of protein and fat source on tissue cholesterol concentrations, uptake of intact low density lipoproteins (LDL) and free cholesterol exchange from LDL to tissues. Soybean oil feeding, compared with beef tallow feeding, resulted in greater concentrations of cholesterol in aorta, heart, and large and small intestines. Similar trends were seen in liver, adipose tissue and skeletal muscle. Dietary protein source had little or no effect on tissue cholesterol concentrations. Uptake of intact LDL, as measured by using [14C]sucrose-LDL, tended to be greater in pigs fed soybean oil or soy protein. Net exchange of free cholesterol from LDL, as measured with [3H]cholesterol, tended to be greater when vegetable products were fed. Relative contributions of whole tissues to total uptake by either mechanism were not influenced by diet. Mechanisms in addition to uptake of cholesterol from LDL seem to be involved in the greater accumulation of tissue cholesterol resulting from polyunsaturated fat feeding.

In vivo influx of free and esterified plasma cholesterol into human aortic tissue without atherosclerotic lesions

In order to determine the in vivo influx of plasma cholesterol into human aortic intimamedia tissue, specimens of the ascending aortic wall without visible atherosclerosis were obtained from patients undergoing aortic valve replacement. Before the operation the patients were intravenously injected with autologous plasma in which the lipoproteins were labeled with radioactive cholesterol. The influence of the duration of the exposure time (0.3-114 h) and of the distribution of radioactivity between free and esterified cholesterol in plasma on the amount of radioactivity found in the arterial wall was studied by the simultaneous use of 3H- and 14C-cholesterol. It was shown that the influx of free and esterified cholesterol into the intima-media layer of the tissue could be calculated from a set of linear equations that relate the labeled sterols in the tissue to the average specific activities in plasma. In nine patients between 50 and 70 yr of age with 4.2-5.9 mM total cholesterol in plasma, the influx of free cholesterol and of esterified cholesterol was 1.2-8.8 and 1.0-12.5 nmol X cm-2 X d-1, respectively. Both hydrolysis and esterification of the sterol fractions in the aortic tissue and exchange of free cholesterol between the plasma lipoproteins and the tissue were demonstrated. The cholesterol content of the intima-media layer was 0.6-2.3 mumol X cm-2. This corresponds to the influx of esterified cholesterol during a period of only 0.1-3.5 yr, which is short compared with the lifespan of the patient. Our data thus suggest that removal of esterified cholesterol from aortic tissue without visible atherosclerosis represents a major importance for the cholesterol concentration in the tissue.

No effect of nifedipine on atherogenesis in cholesterol-fed rabbits

It was recently reported that the calcium antagonist nifedipine suppresses aortic cholesterol accumulation in cholesterol-fed rabbits without reducing hypercholesterolemia. We extended this study on plasma lipoprotein levels and aortic influx of cholesteryl ester. We gave 40 mg per day of nifedipine orally to 17 rabbits fed a 2% cholesterol diet for 8 weeks. For the same period of time 15 control rabbits received placebo capsules and the same diet. During the study, total cholesterol, HDL, LDL, and VLDL cholesterol concentrations in plasma were not significantly different in the experimental and control animals. At the end of the study we found no difference in the two groups in accumulation of cholesterol in the intima media of the proximal thoracic aorta, the distal thoracic aorta, and the corresponding media layers. Furthermore, aortic influx of free cholesterol, cholesteryl ester, and albumin from plasma measured by radioactive tracers was not significantly affected by nifedipine.

Retention of apolipoprotein B and cholesterol by perfused heart during lipolysis of very-low-density lipoprotein

The fate and mechanism of removal of apolipoproteins and lipids of human very-low-density lipoproteins were determined in the perfused rat heart. Approx. 50% of the VLDL triacylglycerol was hydrolyzed during a 2 h perfusion. Phospholipid phosphorus, apolipoproteins C-II, C-III and E were quantitatively recovered in the medium. However, there was a loss of unesterified (17 +/- 6%) and esterified (19 +/- 8%) cholesterol from the perfusion medium. Apolipoprotein B was retained by the heart, as determined by the loss of immunoassayable apolipoprotein B (30 +/- 5%) or the uptake of 125I-labelled apolipoprotein of VLDL (9 +/- 2%) from the perfusion medium. The discrepancy in the two methods for estimating apolipoprotein removal was shown to be due to the modification of apolipoprotein B-containing lipoproteins, which was such that they were no longer precipitated with antibodies to apolipoprotein B. The labelled apolipoprotein B, retained by the heart, could be partially released by perfusion of the heart with buffer containing heparin (14 +/- 2%) or trypsin (50 +/- 2%). Labelled apolipoprotein uptake by the heart was reduced by 90% when lipoprotein lipase was first released by heparin or when VLDL was treated with 1,2-cyclohexanedione to modify arginine residues of apolipoproteins. Very little extensive degradation of the apoprotein to low molecular weight material occurred during the 2 h perfusion, since 95% of the tissue label was precipitated by trichloroacetic acid. It is concluded that there is retention of apolipoprotein B, cholesteryl ester and cholesterol by the perfused heart during catabolism of VLDL. The data are consistent with the concept that the retention of apolipoprotein B requires membrane-bound lipoprotein lipase or an interaction with the cell surfaces that is modified by heparin. The overall process also involves arginine residues of apolipoproteins. At least 50% of the labelled apolipoprotein retained in the tissue is associated with lipoprotein lipase and other cell surface sites, while the remainder may be taken up by the cells.

Comparison of cholesteryl ester transfer from chylomicrons and other plasma lipoproteins to aortic intima media of cholesterol-fed rabbits

Lymph chylomicrons labeled with 14C-cholesterol and plasma very low and intermediate density lipoproteins (d less than 1.019) labeled with 3H-cholesterol were injected simultaneously into recipient cholesterol-fed rabbits. The cholesteryl ester of chylomicron origin (14C) found in the tissues 1 to 4 hours after the injection is the sum of both direct and indirect uptake of chylomicron cholesteryl ester. The direct uptake of chylomicron cholesteryl ester is defined as occurring in situ during the interaction between chylomicrons and lipoprotein lipase in endothelial cells. The indirect uptake of chylomicron cholesteryl ester results from the uptake of chylomicron remnants and of other plasma lipoproteins which become labeled during the degradation of the injected chylomicrons by net lipid transfer or by exchange. The indirect contribution of chylomicron cholesteryl ester to tissues was calculated from tissue 3H-cholesteryl ester derived from injected plasma d less than 1.019 lipoproteins. In rabbits fed 0.5 g cholesterol daily for 40-60 days, with plasma cholesterol concentrations between 20 and 30 mM (800-1200 mg/dl), less than 1% of the cholesteryl ester influx into aortic intima media and less than 10% in liver was derived from the direct uptake of chylomicron cholesteryl ester.

Uptake of [3H]cholesterol from low density lipoprotein by cultured human fibroblasts

The uptake of [3H]cholesterol from low density lipoprotein (LDL) was studied in LDL receptor-positive and receptor-negative human fibroblasts. In both cell lines the uptake depended upon temperature, time of incubation and the concentration of LDL in the medium. Although the incorporation of 125I-labeled LDL was minimal after 2 h of incubation in the receptor-negative (homozygous familial hypercholesterolemia, FH) cells, the uptake of [3H]cholesterol was only slightly less than that of the receptor-positive (WI-38) cells. With longer periods of incubation, a larger difference in labeled cholesterol incorporation was observed; this appeared to be due to a continued accumulation of the steroid in the WI-38 cells. After 8 and 24 h of incubation, some of the [3H]cholesterol was present as the ester in the WI-38 cells, but not the FH cells. Modified (reduced and methylated) LDL did not enter WI-38 cells by the receptor-mediated pathway during 2 h of incubation, as indicated by 125I uptake. [3H]Cholesterol uptake, however, was not significantly different from modified and unmodified LDL. While experiments indicated that significant amounts of cholesterol moved rapidly from LDL to cultured cells with a dependence on time and LDL concentration, no increase in total cell cholesterol was detected in either cell line. FH cells contained less total cholesterol and had a higher 3H specific activity than the WI-38 cells. These data suggest that there may be important mechanisms in addition to the LDL pathway for the movement of lipids into cells.