Low Density Lipoprotein Catabolism Research Articles

Lifibrol: first member of a new class of lipid-lowering drugs?

Management of dyslipoproteinaemia is one of the key strategies in the prevention of cardiovascular disease. The major target of hypolipidaemic drugs is the reduction of low density lipoprotein (LDL) cholesterol. Lifibrol, a novel lipid-lowering agent, is highly potent in reducing total cholesterol, LDL cholesterol, and apolipoprotein B. Its efficacy in lowering serum triglycerides, lipoprotein(a) and fibrinogen implies additional benefit in the prophylaxis and treatment of coronary heart disease. Thus, lifibrol appears to be a multivalent anti-atherosclerotic agent. The hypolipidaemic properties of lifibrol have been examined in several clinical trials and in various animal models. The mode of action of lifibrol involves at least three mechanisms: lifibrol enhances LDL catabolism by sterol-independent stimulation of LDL receptor activity, reduces cholesterol absorption from the intestine, and slightly decreases hepatic cholesterol biosynthesis. Lifibrol’s lipid-lowering profile and putative mode of action clearly distinguish it from other classes of hypolipidaemic drugs, such as HMG-CoA reductase inhibitors or fibric acid derivatives. Thus, lifibrol may represent a new class of agents affecting lipid metabolism.

Reversal of cyclosporine-inhibited low-density lipoprotein receptor activity in HepG2 cells by 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors.

Previously we have shown that cyclosporine inhibits low-density lipoprotein (LDL) catabolism in HepG2 cells. This inhibition mainly occurs through reduced LDL-receptor activity. 3-Hydroxy-3-methylglutarylcoenzyme A (HMG-CoA) reductase inhibitors up-regulate LDL receptor activity with a subsequent increase in LDL uptake and degradation. In this study, in HepG2 cells, we investigated the effects of HMG-CoA reductase inhibitors on cellular LDL catabolism in the presence of cyclosporine. Different concentrations of cyclosporine and HMG-CoA reductase inhibitors, which were within the range of therapeutic concentrations used in humans, were added to the culture medium and the cellular LDL receptor activity was then measured. The results show that HMG-CoA reductase inhibitors reverse the down-regulatory effect of cyclosporine on LDL receptor activity, thus further supporting our previous findings and also providing a rationale for the already established treatment in cyclosporine-induced hypercholesterolemia with HMG-CoA reductase inhibitors.

Long-term Treatment With Pravastatin Alone and in Combination With Gemfibrozil in Familial Type IIB Hyperlipoproteinemia or Combined Hyperlipidemia.

BACKGROUND: Pravastatin inhibits 3-hydroxy-3-methylglutaryl-coenzyme A reductase. It prevents mevalonate synthesis, reducing endogenous cholesterol production, and reduces cholesterol content in the liver, thus resulting in a down-regulation of low-density lipoprotein receptor production. Gemfibrozil reduces very low-density lipoprotein production and low-density lipoprotein-cholesterol level and increases very low-density lipoprotein catabolism. Therefore, it was suggested that combination therapy with both drugs could effect greater reduction of cholesterol levels as compared to pravastatin alone. The present study was carried out to evaluate the efficacy and safety of pravastatin as a monotherapy or in combination with gemfibrozil in the treatment of patients with familial type IIb hyperlipoproteinemia or familial combined hyperlipidemia. METHODS AND RESULTS: Forty-one patients were included in the study. All patients initially followed 6 weeks of hypolipidemic diet; subsequently they were randomized and received either 20 mg once daily of pravastatin alone (n = 13) or 20 mg of pravastatin together with 600 mg of gemfibrozil twice daily (n = 14). As a control, 14 patients were treated with diet only. The treatment lasted 24 months and clinical evaluation and laboratory tests were done at given time points. Both groups of treated patients showed an early reduction (3 months) of total (about 30% P <.01 vs controls), low-density lipoprotein (about 35%, P <.01 vs controls) and very low-density lipoprotein cholesterol levels (about 18%, P = NS). In contrast, high-density lipoprotein cholesterol levels increased significantly in patients treated with pravastatin and gemfibrozil (about 20%, P <.05 vs controls). Pravastatin treatment alone reduced the level of serum triglycerides as efficiently as in combination with gemfibrozil. Data showed a sustained normalization of lipid profile until 24 months. However, this effect was achieved in patients that had rather low levels of triglycerides. During the treatment we did not observe any difference in the incidence of possible drug-related side effects. Severe myopathy or rhabdomyolysis was not observed at the doses of the drugs used in our study. CONCLUSIONS: Therapy with pravastatin and in combination with gemfibrozil resulted in significant and sustained normalization of lipid profile in high-risk patients with familial type IIb hyperlipoproteinemia or familial combined hyperlipidemia.

Leukocyte low density lipoprotein receptor (LDL-R) does not contribute to LDL clearance in vivo: bone marrow transplantation studies in the mouse

The targeted disruption of the low density lipoprotein (LDL) receptor gene in mice results in accumulation of plasma LDL cholesterol and in predisposition to diet-induced aortic atherosclerosis. Although the liver is the central organ for receptor mediated clearance of LDL, the in vivo role of other organs and tissues in LDL catabolism has not been directly studied. Since bone marrow-derived cells such as blood leukocytes and tissue macrophages express LDL receptors and contribute a large mass to the body, we designed bone marrow transplantation (BMT) experiments to reconstitute LDL receptor null mice [LDL-R(-/-)] with marrow obtained from LDL-R wild-type mice [LDL-R(+/+)] and evaluate the effects on parameters of plasma lipid metabolism. Although reconstitution of the transplanted mice with donor bone marrow cells was complete, no differences in plasma lipid levels and lipoprotein distribution were found between groups, irrespective of the diet used, and turnover studies using 125I-labeled LDL showed that LDL receptor expression by leukocytes and macrophages does not significantly contribute to plasma LDL clearance. The complementary experiment of transplanting LDL-R(-/-) marrow into C57BL/6 recipients [LDL-R(-/-)-->LDL(+/+)], performed to evaluate the role of leukocyte LDL-R in normocholesterolemic condition, also produced no effects on plasma lipid parameters. LDL binding studies using macrophages isolated from transplanted mice showed a lack of LDL-R expression. Thus, despite their large number and wide distribution, bone marrow-derived cells do not significantly influence receptor-mediated clearance of plasma LDL.

Low density lipoprotein catabolism is enhanced by the cleaved form of alpha-1-antitrypsin

The frequent occurrence of hypocholesterolaemia following inflammatory processes is well known but unexplained. Elevated plasma levels of serine proteinase inhibitors (serpins) and their complexes with target enzymes have been demonstrated in inflammatory, malignant and infectious diseases which are also often accompanied by low plasma cholesterol levels. Under inflammatory conditions, uncomplexed, but cleaved inactive serpins arising from slow deacylation of the serpin-proteinase complex may be present in the circulation. To determine the influence of native and cleaved forms of serpins, such as alpha-1-antitrypsin (AAT), on lipoprotein metabolism, we investigated the effect of these forms on low density lipoprotein (LDL) catabolism in human HepG2 cell line. We have found that the cleaved form of AAT in concentrations from 125 to 2000 nmol 1−1 stimulates LDL binding to the HepG2 cells, by up to 49% with a subsequent increase in LDL uptake and degradation of up to 79 and 65% respectively. Native AAT was also found to increase LDL binding and internalization by 20–25%, independently of the amount of AAT added, an effect most probably due to the cleaved form of AAT produced by local proteolysis of native AAT incubated in the cell culture. Moreover, we have shown that the cleaved form of AAT interacts with LDL in vitro, and that such an interaction abolishes AAT ability to stimulate LDL binding and internalization. This study for the first time describes the ability of the cleaved form of AAT to stimulate LDL binding and internalization in HepG2 cell culture, and provides evidence that hypocholesterolaemia occurring during inflammatory processes may be mediated by cleaved forms of serpins.

Approaches for the design of novel anti-atherogenic compounds.

Atherosclerosis, a chronic disease related to the vascular system, is the most common cause of morbidity and mortality in Western society. Epidemiological studies have established that the serum cholesterol/triglyceride level is one of the major causative factors for its occurrence. At present, patients with persistent high levels of serum cholesterol and/or triglyceride are therapeutically treated with various types of drugs. Frequently applied drugs are inhibitors of a rate-limiting enzyme in the cholesterol synthesis pathway, i.e., 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, or bile acid sequestrants (fibrates). These drugs lead to an enhanced expression level of low-density lipoprotein (LDL) receptors, thereby stimulating the catabolism of LDL, the main carrier of cholesterol in human blood. Despite the general therapeutic efficacy of these drugs, a significant number of patients, including those suffering from familial hypercholesterolemia and dysbetalipoproteinemia, do not respond sufficiently to treatment with these drugs. In addition, HMG-CoA reductase inhibitors may induce unwanted side effects, since they only interfere with cholesterol synthesis but also lower the synthesis of other physiologically active isoprenoids. These considerations have prompted efforts to explore new frontiers in anti-atherosclerotic theraphy in order to expand the therapeutic scope for treating hyperlipidemia and hypercholesterolemia.

Terminalia arjuna: an Ayurvedic Cardiotonic, Regulates Lipid Metabolism in Hyperlipaemic Rats

The lipid lowering action of the bark powder of Terminalia arjuna (T. arjuna) has been studied in triton and cholesterol fed rats. Serum lipids were found to be lowered by T. arjuna (100 mg/kg, b.w.) in triton induced hyperlipaemia. Chronic feeding of this powder (100 mg/kg, b.w., p.o.) in animals simultaneously fed with cholesterol (25 mg/kg, b.w.) for 30 days, caused lowering in lipids and protein levels of β-lipoproteins followed by an increase in high density lipoprotein-cholesterol compared with the cholesterol fed groups. T. arjuna alters lipolytic activities in plasma, liver, heart and adipose tissues of hyperlipaemic rats. The lipid lowering action of this natural product is mediated through inhibition of hepatic cholesterol biosynthesis, increased faecal bile acid excretion and enhanced plasma lecithin:cholesterol acyltransferase activity and stimulation of receptor mediated catabolism of low density lipoprotein.

Lipid Lowering Activity of Guggulsterone fromCommiphora mukul in Hyperlipaemic Rats

The lipid lowering action of guggulsterone, the active constituent of guggulipid, has been studied in triton and cholesterol fed hyperlipaemic rats. Serum lipids were found to be lowered by gug%sterone (50 mgkg, b.w.) in triton WR-1339 induced hyperlipaemia. Chronic feeding of this drug (5 mg/kg, b.w.) in animals simultaneously fed with cholesterol (25 m%kg, b.w.) for 30 days, caused lowering in the lipid and apoprotein levels of very low density and low density lipoproteins in experimental animals. Guggulsterone activates lipolytic enzymes in plasma and liver as well as stimulated receptor mediated catabolism of low density lipoprotein. The hypolipidaemic activity of this drug is mediated through inhibition of hepatic cholesterol biosynthesis, increased faecal bile acid excretion and enhanced plasma 1ecithin:cholesterol acyltransferase activity.

Cyclosporine inhibits catabolism of low-density lipoproteins in HepG2 cells by about 25%

The aim of this study was to elucidate the possible causes of elevated low-density lipoprotein (LDL)-cholesterol levels in transplanted patients treated with the immunosuppressant drug, cyclosporine. HepG2 cells, from a well-differentiated cell-line of hepatoma cells, were cultured and used as a model for in vitro hepatocytic LDL uptake. Different concentrations of cyclosporine, which were within the range of concentrations found in humans treated with cyclosporine, were added to tissue culture medium together with 125I-LDL. The results showed that cyclosporine reduced LDL uptake and degradation in HepG2 cells by about 25%. The cells were also pretreated with cyclosporine for 1 to 24 hours and then incubated with new medium containing labeled LDL for 2 hours at 4 degrees C in an LDL-binding assay. The data showed that cyclosporine reduced the subsequent LDL binding. Cyclosporine has no toxic effects on HepG2 cells, as shown by unchanged growth capacity of the cells. By means of a 50-fold excess of unlabeled LDL, a monoclonal anti-LDL receptor antibody, and dextran sulfate, we also evaluated if this inhibition of LDL binding occurred through the LDL receptor- mediated pathway, through non-LDL receptor-mediated pathways, or through both. The results show that cyclosporine reduces LDL binding and uptake by mainly inhibiting the LDL receptor-mediated pathway. We also studied the effect of the LDL-cyclosporine complex on the binding of labelled LDL. The presence of cyclosporine in the LDL particle does not influence the binding behaviour of LDL to its receptor. We also found that cyclosporine reduces the expression of the LDL receptor messenger RNA (mRNA) by about 40%. Thus, the interpretation of this study is that cyclosporine can cause an increase in LDL-cholesterol in the plasma of transplantation patients by reducing the catabolism of LDL in the liver by inhibiting mainly the LDL receptor-mediated catabolism through an effect on LDL receptor synthesis. (Hepatology 1996 Sep;24(3):613-9)

Cyclosporine Inhibits Catabolism of Low–Density Lipoproteins in Hepg2 Cells by About 25%

The aim of this study was to elucidate the possible causes of elevated low-density lipoprotein (LDL)-cholesterol levels in transplanted patients treated with the immunosuppressant drug, cyclosporine. HepG2 cells, from a well-differentiated cell-line of hepatoma cells, were cultured and used as a model for in vitro hepatocytic LDL uptake. Different concentrations of cyclosporine, which were within the range of concentrations found in humans treated with cyclosporine, were added to tissue culture medium together with 125I-LDL. The results showed that cyclosporine reduced LDL uptake and degradation in HepG2 cells by about 25%. The cells were also pretreated with cyclosporine for 1 to 24 hours and then incubated with new medium containing labeled LDL for 2 hours at 4 degrees C in an LDL-binding assay. The data showed that cyclosporine reduced the subsequent LDL binding. Cyclosporine has no toxic effects on HepG2 cells, as shown by unchanged growth capacity of the cells. By means of a 50-fold excess of unlabeled LDL, a monoclonal anti-LDL receptor antibody, and dextran sulfate, we also evaluated if this inhibition of LDL binding occurred through the LDL receptor-mediated pathway, through non-LDL receptor-mediated pathways, or through both. The results show that cyclosporine reduces LDL binding and uptake by mainly inhibiting the LDL receptor-mediated pathway. We also studied the effect of the LDL-cyclosporine complex on the binding of labelled LDL. The presence of cyclosporine in the LDL particle does not influence the binding behaviour of LDL to its receptor. We also found that cyclosporine reduces the expression of the LDL receptor messenger RNA (mRNA) by about 40%. Thus, the interpretation of this study is that cyclosporine can cause an increase in LDL-cholesterol in the plasma of transplantation patients by reducing the catabolism of LDL in the liver by inhibiting mainly the LDL receptor-mediated catabolism through an effect on LDL receptor synthesis.

Lipoprotein Lipase Binds to Low Density Lipoprotein Receptors and Induces Receptor-mediated Catabolism of Very Low Density Lipoproteins in Vitro

Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of plasma triglycerides, promotes binding and catabolism of triglyceride-rich lipoproteins by various cultured cells. Recent studies demonstrate that LPL binds to three members of the low density lipoprotein (LDL) receptor family, including the LDL receptor-related protein (LRP), GP330/LRP-2, and very low density lipoprotein (VLDL) receptors and induces receptor-mediated lipoprotein catabolism. We show here that LDL receptors also bind LPL and mediate LPL-dependent catabolism of large VLDL with Sf 100-400. Up-regulation of LDL receptors by lovastatin treatment of normal human foreskin fibroblasts (FSF cells) resulted in an increase in LPL-induced VLDL binding and catabolism to a level that was 10-15-fold greater than in LDL receptor-negative fibroblasts, despite similar LRP activity in both cell lines. This indicates that the contribution of LRP to LPL-dependent degradation of VLDL is small when LDL receptors are maximally up-regulated. Furthermore studies in LRP-deficient murine embryonic fibroblasts showed that the level of LPL-dependent degradation of VLDL was similar to that in normal murine embryonic fibroblasts. LPL also promoted the internalization of protein-free triglyceride emulsions; lovastatin-treatment resulted in 2-fold higher uptake in FSF cells, indicating that LPL itself could bind to LDL receptors. However, the lower induction of emulsion catabolism as compared with native VLDL suggests that LPL-induced catabolism via LDL receptors is only partially dependent on receptor binding by LPL and instead is primarily due to activation of apolipoproteins such as apoE. A fusion protein between glutathione S-transferase and the catalytically inactive carboxyl-terminal domain of LPL (GST-LPLC) also induced binding and catabolism of VLDL. However GST-LPLC was not as active as native LPL, indicating that lipolysis is required for a maximal LPL effect. Mutations of critical tryptophan residues in GST-LPLC that abolished binding to VLDL converted the protein to an inhibitor of lipoprotein binding to LDL receptors. In solid-phase assays using immobilized receptors, LDL receptors bound to LPL in a dose-dependent manner. Both LPL and GST-LPLC promoted binding of VLDL to LDL receptor-coated wells. These results indicate that LPL binds to LDL receptors and suggest that the carboxyl-terminal domain of LPL contributes to this interaction.

Estrogen increases low-density lipoprotein receptor—independent catabolism of apolipoprotein B in hyperlipidemic rabbits

Estrogen has been reported to increase the catabolism of low-density lipoprotein (LDL) apolipoprotein (apo) B by increasing LDL receptor activity. To determine the effect of estrogen on LDL receptor—independent pathways, paired turnover studies of native LDL and chemically modified LDL (methyl-LDL) were performed before and during estrogen administration in female New Zealand rabbits consuming a diet containing 0.5% (wt/wt) cholesterol. Rabbits were matched by plasma cholesterol concentration and assigned randomly to receive estrogen (estradiol cypionate 0.5 mg/kg/wk) or placebo. The residence time of both the native LDL apo B tracer and the methyl-LDL apo B tracer in plasma was decreased by estrogen but not by placebo. Multicompartmental modeling of the paired, double-labeled turnover studies indicated that an increase in fractional catabolic rate (FCR) of the fast-turnover pool, a kinetically distinct LDL subpopulation in plasma, accounted for the observed decrease in residence time in plasma for both tracers. These data support the hypothesis that, in addition to any effect on the LDL receptor, estrogen promotes the activity of LDL receptor—independent pathways.

High plasma insulin is associated with lower LDL cholesterol in elderly individuals

To investigate possible relationships between plasma low density lipoprotein (LDL) cholesterol and fasting plasma insulin in the elderly, cross-sectional random samples of age cohorts (65, 75, 80 and 85 years, n = 1188, M F 38 62 % ) were studied in the neighbouring cities of Helsinki and Vantaa, Finland. Plasma total and high density lipoprotein (HDL) cholesterol, plasma triglycerides, blood glucose and plasma insulin were measured after an overnight fast. LDL cholesterol was calculated using the Friedewald equation. Statistical analyses were performed separately in subjects with non-insulin-dependent diabetes mellitus (NIDDM, n = 219) and non-diabetic subjects ( n = 969). Comparison of lipid levels by insulin quartile (I < 7.4 IU/l, II 7.4–10.0, III 10.1–15.0, IV > 15.0) showed that total and LDL cholesterol decreased in the highest insulin quartile ( P = 0.003). This trend prevailed after adjustments for age, gender, body mass index, blood glucose and serum triglycerides, and it was significant also in normotriglyceridemic (serum triglycerides <2.3 mmol/l) subjects. Furthermore, the association between high insulin and lower cholesterol was seen in normoglycemic (fasting blood glucose <6.7 mmol/l) and diabetic subjects. Lower LDL cholesterol in elderly subjects with higher fasting insulin may reflect poor health or a ‘harvesting’ effect, but the results may also be due to effects of insulin on LDL catabolism and/or cholesterol absorption

Endotoxin inhibits catabolism of low density lipoproteins in vivo: an experimental study in the rat

Hyperlipidaemia frequently accompanies infectious diseases and may be due to an increase in lipoprotein production or a decrease of lipoprotein clearance. The administration of endotoxin has been used to mimic infection and previous studies have demonstrated that endotoxin induces an increase in low density lipoprotein (LDL) levels. In the present study in rats, the dose of endotoxin (055:B5, 50 μg 100 g body weight−1) induced hyperlipidaemia without shock and death. The dearance of 125I-LDL was measured after simultaneous injection of 100 μg LDL protein with endotoxin or without endotoxin (control). Endotoxin sidcantly inhibited the clearance of 125I-LDL from blood at all time intervals between 1 and 24 h. The total 125I and trichloroacetic acid-(TCA)precipitable 125I in the liver was higher in endotoxin-treated animals than in controls after 0.5, 4 and 24 h. An increased tissue radioactivity was also seen in several other tissues at various time intervals in the endotoxin treated group. The increase in plasma triglyceride induced by endotoxin reached a maximum after 8 h. Endotoxin thus causes a rapid inhibition of both the LDL disappearance from blood and of the LDL degradation after the uptake by the liver. This effect precedes the increase in triglyceride levels. The inhibition of the LDL catabolism may contribute to the rise in LDL levels in endotoxin induced hyperlipidaemia.

Identification of scavenger receptor SR-BI as a high density lipoprotein receptor.

High density lipoprotein (HDL) and low density lipoprotein (LDL) are cholesterol transport particles whose plasma concentrations are directly (LDL) and inversely (HDL) correlated with risk for atherosclerosis. LDL catabolism involves cellular uptake and degradation of the entire particle by a well-characterized receptor. HDL, in contrast, selectively delivers its cholesterol, but not protein, to cells by unknown receptors. Here it is shown that the class B scavenger receptor SR-BI is an HDL receptor. SR-BI binds HDL with high affinity, is expressed primarily in liver and nonplacental steroidogenic tissues, and mediates selective cholesterol uptake by a mechanism distinct from the classic LDL receptor pathway.

Endotoxin inhibits catabolism of low density lipoproteins in vivo: an experimental study in the rat.

Hyperlipidaemia frequently accompanies infectious diseases and may be due to an increase in lipoprotein production or a decrease of lipoprotein clearance. The administration of endotoxin has been used to mimic infection and previous studies have demonstrated that endotoxin induces an increase in low density lipoprotein (LDL) levels. In the present study in rats, the dose of endotoxin (055:B5, 50 micrograms 100 g body weight-1) induced hyperlipidaemia without shock and death. The clearance of 125I-LDL was measured after simultaneous injection of 100 micrograms LDL protein with endotoxin or without endotoxin (control). Endotoxin significantly inhibited the clearance of 125I-LDL from blood at all time intervals between 1 and 24 h. The total 125I and trichloroacetic acid-(TCA) precipitable 125I in the liver was higher in endotoxin-treated animals than in controls after 0.5, 4 and 24 h. An increased tissue radioactivity was also seen in several other tissues at various time intervals in the endotoxin treated group. The increase in plasma triglyceride induced by endotoxin reached a maximum after 8 h. Endotoxin thus causes a rapid inhibition of both the LDL disappearance from blood and of the LDL degradation after the uptake by the liver. This effect precedes the increase in triglyceride levels. The inhibition of the LDL catabolism may contribute to the rise in LDL levels in endotoxin induced hyperlipidaemia.

Regulation of Human Plasma Vitamin E

Publisher Summary There is renewed interest in the role of vitamin E in human nutrition because of the demonstration that vitamin E deficiency occurs in humans consuming apparently adequate diets. It has been seen that a neurologic disorder is caused by genetic defects in a liver protein necessary for the incorporation of vitamin E into plasma lipoproteins. The studies leading to this conclusion are highlighted. Information about the plasma transport of vitamin E was gleaned using stable isotopes and gas chromatography/mass spectrometry to analyze lipoprotein fractions. Techniques of molecular biology were instrumental in the isolation of the hepatic tocopherol transport protein and in the identification of genetic defects in this protein, which ultimately causes vitamin E deficiency in humans. These studies now open approaches in investigating the function of vitamin E, which may include more than just antioxidant function. The chapter details vitamin E and low density lipoprotein absorption, chylomicron secretion and catabolism, turnover rates, discrimination between tocopherols, and the tocopherol transfer protein including the genetic defect in the α-tocopherol transfer protein. A kinetic model of plasma vitamin E transport was developed from the results of studies in humans using deuterium-labeled stereoisomers of α-tocopherol. In normal subjects given equimolar amounts of these two labeled stereoisomers, plasma was preferentially enriched in RRR-α-tocopherol within 24 hr. It is explained how although labeled RRR-α-tocopherol apparently leaves the plasma compartment slowly, but the opposite is true. The term vitamin E includes eight naturally occurring molecules: four tocopherols and four tocotrienols. The four forms of tocopherols and tocotrienols differ in the number of methyl groups on the chromanol nucleus. The γ-tocopherol, which has a phytyl tail in the natural conformation, and SRR-α-tocopherol, which has the opposite conformation at the 2-position, has similar plasma disappearance kinetics and disappears from the plasma more rapidly than RRR-α-tocopherol.

Sialic acids and the metabolism of low density lipoprotein.

Sialic acid-poor low density lipoprotein (LDL) is suggested to be atherogenic because it causes in vitro accumulation of cholesterol into epithelial cells and macrophages. We studied whether the whole-body catabolism of LDL varies according to its sialic acid content by analyzing the sialic acids in total (d 1.019-1.063 g/ml), light (d 1.019-1.036 g/ml), dense (d 1.037-1.055 g/ml), and very dense (d 1.056-1.063 g/ml) LDL, and the kinetics of total and dense LDL apolipoprotein (apo) B in 20 non-insulin-dependent diabetic and 10 non-diabetic men of similar age, both groups without and with coronary artery disease (CAD). The sialic acid/apoB ratio was significantly higher in the diabetics compared to the controls in every LDL fraction. Fractional catabolic rate (FCR) for LDL apoB was significantly faster in the diabetics than in the non-diabetics, and was positively associated with the sialic acid ratio in the total and dense LDL. In multiple linear regression analyses with FCR for total and dense LDL apoB as the dependent variable and the presence of diabetes, body mass index, LDL cholesterol, triglycerides, and sialic acid/apoB ratio as the independent variables, LDL cholesterol and serum triglycerides were the only variables entering significantly to these models. The sialic acid/apoB ratio of total and dense LDL was similar in subjects without and with CAD. These results suggest that in this population the sialic acid content of LDL was not associated with clinical signs of atherosclerosis, but the catabolic rate of dense LDL apoB was positively related to the sialic acid content of the respective lipoproteins.

Metabolism of plasma lipoproteins in the genetically hypercholesterolemic rat (RICO)

Experiments were performed to determine the turnover processes of plasma cholesterol in genetically hypercholesterolemic rats (RICO). Specific activity of plasma cholesterol was monitored during 4 months following an intravenous injection of tritiated cholesterol. The results were subjected to two-pool model analysis. Cholesterol production in the RICO rat was significantly higher (28.9 ± 1.7 mg/d) than in the SW control (18.5 ± 0.7, P < .01). The study also revealed a 30% decrease in the rate constant for cholesterol movement from the plasma toward the majority of organs in the RICO rat versus the SW control. Very—low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) turnover were investigated following injection of labeled lipoproteins (on cholesteryl ester or apolipoproteins). Results from these experiments showed that the higher HDL cholesterol concentration in the RICO rat as compared with the control is due to the greater production rate of esterified cholesterol in these lipoproteins (1.3 ± 0.05 mg/h v 0.8 ± 0.03 in the control, P < .001). The fractional catabolic rate (FCR) or production rate for VLDL were not significantly different between the two groups (3.4 ± 0.01 and 3.6 ± 0.01 h −1 and 2.6 ± 0.4 and 3.3 ± 0.1 mg/h, respectively). However, radioactivity of VLDL recovered in LDL at death was considerably higher in RICO rats (14% ± 1% v 6% ± 1%, P < .01). The greater concentration of LDL cholesterol in RICO rats is due to a higher LDL production (0.40 ± 0.05 v 0.19 ± 0.03 mg/h, P < .01) together with a lower catabolism (FCR, 5.5 ± 0.6 v 7.9 ± 0.8%/h, P < .05). Cross-injection experiments showed that this lower catabolism of LDL is partly due to the nature of the lipoprotein particle. Taken together, these data are consistent with the hypothesis of a reduced uptake of apolipoprotein (apo)E-containing lipoproteins (VLDL and LDL), which results in a higher LDL cholesterol concentration in RICO rats.

A pilot study of ex vivo gene therapy for homozygous familial hypercholesterolaemia

The outcome of the first pilot study of liver-directed gene therapy is reported here. Five patients with homozygous familial hypercholesterolaemia (FH) ranging in age from 7 to 41 years were enrolled; each patient tolerated the procedure well without significant complications. Transgene expression was detected in a limited number of hepatocytes of liver tissue harvested four months after gene transfer from all five patients. Significant and prolonged reductions in low density lipoprotein (LDL) cholesterol were demonstrated in three of five patients; in vivo LDL catabolism was increased 53% following gene therapy in a receptor negative patient, who realized a reduction in serum LDL equal to approximately 150 mg dl-1. This study demonstrates the feasibility of engrafting limited numbers of retrovirus-transduced hepatocytes without morbidity and achieving persistent gene expression lasting at least four months after gene therapy. The variable metabolic responses observed following low-level genetic reconstitution in the five patients studied precludes a broader application of liver-directed gene therapy without modifications that consistently effect substantially greater gene transfer.