Cholestyramine-induced changes in low density lipoprotein composition and metabolism. I. Studies in the guinea pig

Effect of colesevelam hydrochloride on glycemia and insulin sensitivity in men with the metabolic syndrome

Colesevelam hydrochloride (colesevelam) lowers low-density lipoprotein (LDL) cholesterol and glycated hemoglobin in patients with type 2 diabetes mellitus. The present study examined the effects of colesevelam treatment in nondiabetic men with metabolic syndrome. Twenty men completed the study, which consisted of two 8-week phases of treatment with colesevelam (3.75 g/day) or placebo and a 6-week washout between study phases. Of the 20 men, 17 took statins throughout. The fasting plasma LDL cholesterol, triglyceride, glucose, and glycated hemoglobin levels were measured in the last 2 weeks of each study phase. Nonesterified fatty acids and 3-hydroxybutyrate, insulin, and glucose were measured hourly for 5 hours during fasting and during an extended glucose tolerance test. The colesevelam treatment reduced LDL cholesterol from 96 ± 28 mg/dl to 78 ± 32 mg/dl (p <0.006) and non-high-density lipoprotein cholesterol by 8.2% (p = 0.07). Triglycerides increased by 17% (p <0.02). The fasting plasma glucose was reduced by 5 mg/dl (p <0.03), and glycated hemoglobin remained unchanged by colesevelam. No significant treatment changes were noted for the 2-hour glucose test or insulin sensitivity. The fasting nonesterified fatty acid level was significantly reduced with treatment but the 3-hydroxybutyrate level was unchanged. Insulin-mediated suppression of nonesterified fatty acids during extended glucose tolerance test was significantly less effective during treatment than during placebo. In conclusion, colesevelam significantly reduced the LDL cholesterol levels, even though the baseline LDL cholesterol level was low owing to statin treatment. The fasting and postprandial blood glucose level but not the glycated hemoglobin level was lowered by colesevelam therapy. The effect on fasting glucose was unrelated to the changes in insulin resistance or fatty acid oxidation. Finally, an increase in triglycerides with colesevelam therapy might have been related to a lesser suppression of nonesterified fatty acids levels in the postprandial state.

A novel function of lipoprotein [a] as a preferential carrier of oxidized phospholipids in human plasma

Oxidized phospholipids (OxPLs) on apolipoprotein B-100 (apoB-100) particles are strongly associated with lipoprotein [a] (Lp[a]). In this study, we evaluated whether Lp[a] is preferentially the carrier of OxPL in human plasma. The content of OxPL on apoB-100 particles was measured with monoclonal antibody E06, which recognizes the phosphocholine (PC) headgroup of oxidized but not native phospholipids. To assess whether OxPLs were preferentially bound by Lp[a] as opposed to other lipoproteins, immunoprecipitation and ultracentrifugation experiments, in vitro transfer studies, and chemiluminescent ELISAs were performed. Immunoprecipitation of Lp[a] from human plasma with an apolipoprotein [a] (apo[a])-specific antibody demonstrated that more than 85% of E06 reactivity (i.e., OxPL) coimmunoprecipitated with Lp[a]. Ultracentrifugation experiments showed that nearly all OxPLs were found in fractions containing apo[a], as opposed to other apolipoproteins. In vitro transfer studies showed that oxidized LDL preferentially donates OxPLs to Lp[a], as opposed to LDL, in a time- and temperature-dependent manner, even in aqueous buffer. Approximately 50% of E06 immunoreactivity could be extracted from isolated Lp[a] following exposure of plasma to various lipid solvents. These data demonstrate that Lp[a] is the preferential carrier of PC-containing OxPL in human plasma. This unique property of Lp[a] suggests novel insights into its physiological function and mechanisms of atherogenicity.

Guinea pigs: a suitable animal model to study lipoprotein metabolism, atherosclerosis and inflammation

Numerous animal models have been used to study diet effects on cholesterol and lipoprotein metabolism. However, most of those models differ from humans in the plasma distribution of cholesterol and in the processing of lipoproteins in the plasma compartment. Although transgenic or knock-out mice have been used to study a specific pathway involved in cholesterol metabolism, these data are of limited use because other metabolic pathways and responses to interventions may differ from the human condition. Carbohydrate restricted diets have been shown to reduce plasma triglycerides, increase HDL cholesterol and promote the formation of larger, less atherogenic LDL. However, the mechanisms behind these responses and the relation to atherosclerotic events in the aorta have not been explored in detail due to the lack of an appropriate animal model. Guinea pigs carry the majority of the cholesterol in LDL and possess cholesterol ester transfer protein and lipoprotein lipase activities, which results in reverse cholesterol transport and delipidation cascades equivalent to the human situation. Further, carbohydrate restriction has been shown to alter the distribution of LDL subfractions, to decrease cholesterol accumulation in aortas and to decrease aortic cytokine expression. It is the purpose of this review to discuss the use of guinea pigs as useful models to evaluate diet effects on lipoprotein metabolism, atherosclerosis and inflammation with an emphasis on carbohydrate restricted diets.

Free phytosterols facilitate excretion of endogenous cholesterol in gerbils

To determine whether phytosterols (PST) facilitate excretion of whole body cholesterol and whether dietary fat or enhancing gallbladder contraction with curcumin might influence this process, four experiments were conducted in gerbils. In Experiment 1, naive gerbils received cholesterol-free purified diets with 30% energy from fat and 0% or 0.75% free PST from tall oil for 4 weeks. In Experiment 2, body cholesterol pools were expanded by feeding a diet containing 0.3% cholesterol for 3 weeks. Subsequently, PST was provided in either fat-free or normal-fat diets without cholesterol for only 2 h each morning, followed by a low-fat diet for the rest of the day and food restriction overnight. In Experiment 3, gerbils were preloaded with cholesterol, followed by either PST alone or PST+curcumin to enhance gallbladder contraction. In Experiment 4, curcumin or curcumin+PST were fed with 30% as fat and 0.15% cholesterol throughout the study. Because of the small whole body cholesterol pool in Experiment 1, the impact of PST was limited. When whole body cholesterol was expanded in Experiments 2 and 3, subsequent reductions of liver esterified cholesterol by PST were significant. In the presence of dietary fat, PST caused a greater reduction (23%) than in a fat-free diet (8%) compared to respective controls. Curcumin (Experiments 3 and 4) proved ineffective in reducing liver or plasma cholesterol pools, and the 3:1 ratio between PST/diet cholesterol was less effective at blocking cholesterol absorption than a 5:1 ratio previously employed. Thus, free PST removed whole body cholesterol, which was enhanced by concomitant fat intake, but was unaffected by a gallbladder contracting agent.

Guinea pigs as models to study the hypocholesterolemic effects of drugs

Guinea pigs are useful models to investigate the mechanisms of the hypocholesterolemic effects of drugs. Like humans, guinea pigs are one of the few species that carry the majority of cholesterol in LDL. This animal model has also been shown to develop atherosclerosis when challenged with hypercholesterolemic diets. In addition, plasma lipid profiles in males, females and ovariectomized guinea pigs, a model for menopause, follow similar patterns to those observed in humans. In this report, drugs aimed at lowering plasma cholesterol and triglycerides in hyperlipidemic individuals are reviewed. Studies analyzing the hypolipidemic effect of HMG-CoA reductase inhibitors, acyl CoA cholesterol acyltransferase inhibitors, fibrates, bile acid resins, apical sodium bile acid transporter inhibitors, and others show that guinea pigs and humans have comparable responses to drug therapy. In addition, results from the limited clinical reports addressing specific effects of drugs on LDL catabolism or VLDL synthesis are in agreement with observations in guinea pigs. From the review of these studies, it is apparent that the guinea pig is a useful animal model to further explore the mechanisms of action of lipid lowering drugs including effects on specific receptors and regulatory enzymes involved in cholesterol metabolism and on early atherosclerosis development.

ABBREVIATIONS

ACAT, acyl-CoA:cholesterol acyltransferase; ASBT, apical sodium co-dependent bile acid transporter; ApoB, apolipoprotein B; CHD, coronary heart disease; CYP7, cholesterol 7alpha-hydroxylase; HDL, high density lipoprotein; HMG-CoA, 3-hydroxy-3-methylglutaryl coenzyme A; FCR, free catabolic rate; LDL, low density lipoprotein; PPAR, peroxisome proliferators-activated receptor; TC, total cholesterol; TG, triglycerides; VLDL, very low density lipoprotein.

Nonesterified phytosterols dissolved and recrystallized in oil reduce plasma cholesterol in gerbils and humans

When free phytosterols are adequately heated and then cooled in fat, they recrystallize and are rendered bioavailable for blocking cholesterol absorption. To extend the application of phytosterols to fried foods, the activity of these modified crystals was assessed in 2 experiments with 26 male gerbils fed purified diets containing 0.15 g/100 g cholesterol with or without 0.75 g/100 g free phytosterols. The heat-modified soybean sterols were added directly to the diet (Expt. 1) or as phytosterol-enriched potato chips (Expt. 2). In the gerbil experiments, only the diet containing phytosterols significantly reduced plasma cholesterol (35-48%) and the total cholesterol/HDL cholesterol (HDL-C) ratio (40%), as well as hepatic cholesterol esters (80%). In a subsequent human study, subjects (n = 7) consumed two 28-g servings of tortilla chips fried in oil with or without phytosterols that provided 0 or 1.5 g/d for 4-wk periods in a crossover design (Expt. 3). During consumption of the phytosterol-enriched chips, significant reductions in plasma cholesterol (10%) and LDL cholesterol (15%) were achieved without affecting HDL-C. This novel means of delivering free phytosterols proved to be both functionally efficient and effective.

Growth Hormone Induces Low-Density Lipoprotein Clearance but not Bile Acid Synthesis in Humans

OBJECTIVE

Growth hormone (GH) induces hepatic low-density lipoprotein (LDL) receptors and lowers plasma cholesterol. We characterized the influence of GH treatment on plasma LDL clearance in normal humans and investigated the relative role of LDL receptor (LDLR) activity and stimulation of bile acid synthesis in subjects with different LDLR expression.

METHODS AND RESULTS

Plasma clearance of autologous 125I-LDL was measured before and during 3 weeks of treatment with GH (0.1 IU/kg per day) in 9 healthy young males. Plasma LDL cholesterol was reduced by 13% and the fractional catabolic rate of LDL increased by 27%. More marked changes were seen in a patient with hypopituitarism substituted with GH (0.07 IU/kg per day) for 3 months. In a second study, GH dose-dependently reduced LDL cholesterol and increased Lp(a) levels in 3 groups of males: younger and elderly healthy subjects and heterozygous familial hypercholesterolemia (FH). No effect on bile acid synthesis measured by the plasma marker 7alpha-hydroxy-4-cholesten-3-one was observed. In an LDLR-deficient FH homozygote, LDL cholesterol was not affected by GH.

CONCLUSIONS

GH treatment reduces plasma LDL cholesterol by inducing LDL clearance. In humans, LDLR expression is a prerequisite for this effect, whereas it is not related to stimulation of bile acid synthesis.

Betaine in sub-acute and sub-chronic rat studies

To evaluate the sub-acute and sub-chronic effect of large doses of betaine, this trimethyl glycine compound was fed to rats. Initial studies at BIBRA in the UK evaluated intakes of 0, 1, 2, and 5% betaine added to a maintenance chow designed for use in toxicology studies. Male and female Sprague-Dawley rats were followed for up to 90 days. No toxicity occurred, but at higher betaine intakes several serum chemistries were altered slightly, the MCV, MCH, and MCHC of red cells were reduced, and hepatocytes developed fatty droplets in direct proportion to betaine intake. Females were more affected than males. In a second study to assess reversibility in females, betaine effects were induced for 28 days, followed by a 28 day betaine-free period. All perturbations, except the reduced MCV and MCH, were reversed. As a follow up to BIBRA investigations, both 28 and 90 day feeding trials were conducted at Brandeis University using a rat chow with higher levels of energy, protein, and fat, with betaine added at 0, 0.5, 0.75, 1.0 and 5.0% of the diet. A similar broad range of clinical chemistries and physiological parameters were monitored, and hepatic lipid droplets were investigated in more detail. Liver lipid was actually reduced by betaine, and no significant adverse effects of clinical importance resulted from any dose. However, the MCV was again reduced at 5% betaine in the 28 day study. By 90 days all parameters were normal and comparable to controls. Based on these collective data, it was concluded that even at these high doses, betaine is nontoxic. Differences observed between the BIBRA and Brandeis studies were attributed to differences in the dietary formulations. Significant betainexdietxgrowth interactions were thought to reflect primary disparities in protein and energy concentrations, more than the addition of betaine per se.

Inhibition of both the apical sodium-dependent bile acid transporter and HMG-CoA reductase markedly enhances the clearance of LDL apoB

Discovery of the ileal apical sodium-dependent bile acid transporter (ASBT) permitted development of specific inhibitors of bile acid reabsorption, potentially a new class of cholesterol-lowering agents. In the present study, we tested the hypothesis that combining the novel ASBT inhibitor, SC-435, with the HMG-CoA reductase inhibitor, atorvastatin, would potentiate reductions in LDL cholesterol (LDL-C) and LDL apolipoprotein B (apoB). ApoB kinetic studies were performed in miniature pigs fed a typical human diet and treated with the combination of SC-435 (5 mg/kg/day) plus atorvastatin (3 mg/kg/day) (SC-435+A) or a placebo. SC-435+A decreased plasma total cholesterol by 23% and LDL-C by 40%. Multicompartmental analysis (SAAM II) demonstrated that LDL apoB significantly decreased by 35% due primarily to a 45% increase in the LDL apoB fractional catabolic rate (FCR). SC-435+A significantly decreased hepatic concentrations of free cholesterol and cholesteryl ester, and increased hepatic LDL receptor mRNA consequent to increased cholesterol 7alpha-hydroxylase expression and activity. In comparison, SC-435 (10 mg/kg/day) monotherapy decreased LDL apoB by 10% due entirely to an 18% increase in LDL apoB FCR, whereas atorvastatin monotherapy (3 mg/kg/day) decreased LDL apoB by 30% due primarily to a 22% reduction in LDL apoB production. We conclude that SC-435+A potentiates the reduction of LDL-C and LDL apoB due to complementary mechanisms of action.

1-[4-[4[(4R,5R)-3,3-Dibutyl-7-(dimethylamino)-2,3,4,5-tetrahydro-4-hydroxy-1,1-dioxido-1-benzothiepin-5-yl]phenoxy]butyl]-4-aza-1-azoniabicyclo[2.2.2]octane methanesulfonate (SC-435), an ileal apical sodium-codependent bile acid transporter inhibitor alters hepatic cholesterol metabolism and lowers plasma low-density lipoprotein-cholesterol concentrations in guinea pigs

Male Hartley guinea pigs (10/group) were assigned either to a control diet (no drug treatment) or to diets containing 0.4, 2.2, or 7.3 mg/day of an ileal apical sodium-codependent bile acid transporter (ASBT) inhibitor, 1-[4-[4[(4R,5R)-3,3-dibutyl-7-(dimethylamino)-2,3,4,5-tetrahydro-4-hydroxy-1,1-dioxido-1-benzothiepin-5-yl]phenoxy]butyl]-4-aza-1-azoniabicyclo[2.2.2] octane methanesulfonate (SC-435). Based on food consumption, guinea pigs received 0, 0.8, 3.7, or 13.4 mg/kg/day of the ASBT inhibitor. The amount of cholesterol in the four diets was maintained at 0.17%, equivalent to 1200 mg/day in the human situation. Guinea pigs treated with 13.4 mg/kg/day SC-435 had 41% lower total cholesterol and 44% lower low-density lipoprotein (LDL)-cholesterol concentrations compared with control (P < 0.01), whereas no significant differences were observed with either of the lower doses of SC-435. Hepatic cholesterol esters were significantly reduced by 43, 56, and 70% in guinea pigs fed 0.8, 3.7, and 13.4 mg/kg/day of the ASBT inhibitor, respectively (P < 0.01). In addition, the highest dose of the inhibitor resulted in a 42% increase in the number of very low-density lipoprotein (VLDL) triacylglycerol molecules and a larger VLDL diameter compared with controls (P < 0.05). Acyl-CoA cholesterol/acyltransferase activity was 30% lower with the highest dose treatment, whereas cholesterol 7alpha-hydroxylase, the regulatory enzyme of bile acid synthesis, was 30% higher with the highest ASBT inhibitor dose (P < 0.05). Furthermore, bile acid excretion increased 2-fold with the highest dose of SC-435 compared with the control group (P < 0.05). These results suggest that the reduction in total and LDL-cholesterol concentrations by the ASBT inhibitor is a result of alterations in hepatic cholesterol metabolism due to modifications in the enterohepatic circulation of bile acids.

Free phytosterols effectively reduce plasma and liver cholesterol in gerbils fed cholesterol

The potential of free phytosterols (including 20% stanols) to lower plasma and liver lipids was assessed in three experiments with gerbils fed diets containing cholesterol. The first explored the ability of phytosterols (0.5%) to block absorption of 0.05, 0.10, and 0.5% cholesterol. The second assessed the importance of consuming phytosterols (0.75%) simultaneously with cholesterol (0.15%). The third compared free phytosterols (0.75%) with similar levels of esterified sterols or stanols using diets containing 0.15% cholesterol. A 5:1 ratio of phytosterols:cholesterol effectively blocked cholesterol absorption when the dietary cholesterol load was moderate. Consuming a 5:1 ratio with every meal was more effective than receiving equal phytosterols in a 10:1 ratio every other day. Finally, free phytosterols dissolved in fat were as effective as esterified sterols and stanols in lowering plasma and liver cholesterol, and all were equally effective at blocking cholesterol absorption as shown by increased fecal cholesterol output. Plant sterol accumulation in the liver was minimal for all test groups.

Effects of F2833 on cholesterol metabolism in the genetically hyperlipidemic rat

The effects of the new hypolipidemic agent, F2833 or (chloro 2' (1-1') biphenyl-4)-2 propanol-2, on cholesterol metabolism were studied in genetically hyperlipidemic rats (RICO). Cholesterolemia decreased after 2 days of treatment to 60% of its initial value (1.20+/-0.10 g/l vs. 1.99+/-0.08, P < 0.001) and then stabilised within 10 days. This hypocholesterolemic action was effective for as long as 3 months. Concerning the different classes of lipoproteins, a significant drop was observed in HDL (high density lipoproteins) (25%, 0.49 +/- 0.02 g/l vs. 0.66 +/- 0.007, P < 0.01) and particularly in LDL (low density lipoproteins) (70%, 0.30 +/- 0.04 g/l vs. 0.92 +/- 0.05, P < 0.001). Whole body cholesterol showed a higher fractional catabolic rate (0.25 +/- 0.02 vs. 0.17 +/- 0.005 day(-1), P < 0.01) together with an increased cholesterol synthesis (60 +/- 5 vs. 36 +/- 4 mg/day, P < 0.01). LDL kinetics showed that the decrease in these lipoproteins is essentially caused by an increase in the fractional catabolic rate (10.6 +/-0.1%/h vs. 5.2 +/- 0.1%/h, P < 0.001) and by a lesser decrease in the LDL production rate. This cholesterol metabolic profile created by treatment suggests an effect through stimulation of cholesterol output (biliary cholesterol elimination or cholesterol transformation into bile acids).

Guinea pigs as models for cholesterol and lipoprotein metabolism

Guinea pigs carry the majority of their plasma cholesterol in LDL, making them a unique animal model with which to study hepatic cholesterol and lipoprotein metabolism. In this review, the benefits and advantages of using this particular model are discussed. How dietary factors such as soluble fiber, cholesterol and fatty acids that vary in saturation and chain length affect hepatic cholesterol homeostasis and influence the synthesis, intravascular processing and catabolism of lipoproteins is reviewed. In addition, alterations in hepatic cholesterol metabolism and plasma lipoproteins as affected by treatment with cholestyramine or 3-hydroxyl-3-methylglutaryl coenzyme A reductase inhibitors, exercise, marginal intake of vitamin C, ovariectomy (a model for menopause) and similarities to the human situation are addressed. A review of guinea pigs as models for early atherosclerosis development is also presented.

Low-density lipoprotein and oxidised low-density lipoprotein: their role in the development of atherosclerosis

Oxidation of low-density lipoprotein (LDL) may be implicated in the development of atherosclerotic disease. Oxidised LDL is taken up more readily by monocyte-derived macrophages than LDL. Antibodies to oxidised LDL are found in atherosclerotic lesions, Increased risk of ischaemic heart disease is associated with a preponderance of small dense LDL particles, which are more susceptible to oxidation. Proatherogenic alterations in cell biochemistry and signalling pathways occur in the presence of LDL and more markedly oxidised LDL. In vitro antioxidants inhibit changes in cell biochemistry, while in vivo, they have been shown to attenuate or reverse development of atherosclerosis.

Specific reduction of plasma large, light low-density lipoprotein by a bile acid sequestering resin, cholebine (MCI-196) in type II hyperlipoproteinemia

The effect of a bile acid sequestrant, cholebine (3 g/day), on plasma lipoprotein subfractions was investigated in 16 patients with type II hyperlipoproteinemia. Activities of low density lipoprotein (LDL)-receptor and activities of lecithin:cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) were assayed to address the mechanism of cholebine-induced changes in plasma lipoprotein subfractions. Twelve weeks of treatment with cholebine reduced plasma levels of total cholesterol (TC) and LDL-cholesterol (C) by 8.3 +/- 8.1% (mean +/- S.D.) and 14.4 +/- 11.9%, respectively (P < 0.001), but did not affect plasma levels of high density lipoprotein (HDL)-C. Cholebine significantly reduced plasma levels of LDL1-C (1.019 < d < 1.045) by 22.9 +/- 18.9% (P < 0.001) but did not affect plasma levels of very low density lipoprotein (VLDL)-C, intermediate density lipoprotein (IDL)-C, LDL2-C (1.045 < d < 1.063), HDL2-C, and HDL3-C (d > 1.125). Gradient polyacrylamide gel electrophoresis (PAGE) revealed that cholebine reduced large LDL in plasma but had almost no effects on small LDL and HDL subfractions. Cholebine did not alter the activities of LCAT and CETP. LDL-receptor activities of cultured lymphocytes negatively correlated with the reduction in plasma levels of LDL-C (r = -0.500, P < 0.05), IDL-C (r = -0.581, P < 0.02), and LDL1-C (r = -0.610, P < 0.01), respectively. Thus, cholebine seems to reduce further the plasma levels of IDL and large, light LDL in patients with lower LDL-receptor activities. We conclude that cholebine only reduces plasma levels of large, light LDL. This may be due to the stimulation of hepatic LDL-receptor activity.

Effects of colestipol alone and in combination with simvastatin on apolipoprotein B metabolism

The effects of colestipol therapy alone (20 g/d) or combined with simvastatin (20 mg/d) were examined in a group of eight male patients with primary moderate hypercholesterolemia (total cholesterol > or = 6.5 mmol/L [> or = 250 mg/dL]) who had undergone coronary artery bypass grafting more than 3 months previously. Colestipol therapy decreased total cholesterol by 14% (P < .001) and LDL cholesterol (LDL-C) by 23% (P < .001), while dual therapy decreased total cholesterol by 38% and LDL-C by 52% (both P < .001 versus baseline). No significant changes were observed in plasma triglyceride, VLDL cholesterol, or HDL cholesterol levels. VLDL subfraction turnovers were conducted at baseline and again on each regimen. ApoB kinetic parameters derived from a multicompartmental model suggested that colestipol therapy resulted in an expansion of the total VLDL apoB pool (36%, P < .05) that was largely due to a fall in the clearance rate of VLDL1 apoB (49%), while the LDL apoB pool decreased 23% as a result of diminished direct LDL input. The model used also revealed that addition of simvastatin to the resin therapy caused increases in the fractional transfer rates of VLDL2 to IDL and IDL to LDL together with a 37% increment in the LDL apoB fractional catabolic rate. Compared with baseline, combined therapy generated falls in both IDL (35%, P = .01) and LDL (37%, P < .04) apoB pools due to enhanced clearance of IDL (214%, P < .03) and reduced total input of LDL (39%, P < .003).

Effect of feeding psyllium and cholestyramine in combination on low density lipoprotein metabolism and fecal bile acid excretion in hamsters with dietary-induced hypercholesterolemia

We wished to determine the effectiveness of submaximal doses of cholestyramine and psyllium given in combination in reversing dietary-induced hypercholesterolemia in Golden Syrian hamsters, and to investigate the mechanism or mechanisms of action through which these agents together decrease plasma low density lipoprotein cholesterol (LDL-C) levels in this model. For 30 days, male hamsters were fed a cholesterol-rich cereal-based diet containing either a submaximal dose of cholestyramine (1% wt/wt) alone or in combination with psyllium (either 2 or 4%), or a high dose of cholestyramine (3%) alone. Although the greatest cholesterol-reducing action was achieved with 3% resin alone, in the animals fed one third as much cholestyramine combined with psyllium (4%) LDL-C production decreased from 288 +/- 15 to 187 +/- 17 micrograms/h per 100 g body weight, the suppression of LDL-receptor activity was almost fully reversed, plasma LDL-C levels were reduced from 90 +/- 8 to 41 +/- 5 mg/dl, and hepatic cholesterol content decreased from 17.1 +/- 1.9 to 2.4 +/- 0.1 mg/g. In the group that received 1% resin alone, the plasma LDL-C and hepatic cholesterol levels were 60 +/- 3 mg/dl and 7.2 +/- 0.6 mg/g, respectively. As compared with animals that received 1% resin alone, those fed both agents manifested higher rates of fecal bile acid excretion and lower levels of intestinal cholesterol absorption. A significant cholesterol-lowering benefit can be derived from using these nonsystemic agents in combination at lower, more tolerable doses.

Decreased affinity of low density lipoprotein (LDL) particles for LDL receptors in patients with cholesteryl ester transfer protein deficiency

We have reported that the disorder of lipoprotein metabolism in hyperalphalipoproteinaemic patients with a deficiency of cholesteryl ester transfer protein (CETP) is characterized by the polydisperse low density lipoprotein (LDL) particles and the accumulation of cholesteryl ester (CE) in high density lipoprotein (HDL) particles, forming cholesterol-induced HDL (HDLc)-like particles. In the present study we have investigated the interaction of these abnormal LDL with LDL receptors of normal human fibroblasts. Since the ultracentrifugally separated LDL fraction (1.019 < d < 1.063 g mL-1) from the CETP-deficient patients contained HDLc-like particles, these particles were removed by anti-apolipoprotein (apo) A-I immunoaffinity column chromatography. The lipoproteins eluted in the unbound fraction of this column did not contain apo A-I, so this fraction was considered to be authentic LDL. The authentic LDL of the patients were deficient in CE and rich in triglycerides and apo B. The authentic LDL itself showed polydispersity, ranging in size from 23 nm to 30 nm. The affinity of these abnormal LDL particles for LDL receptors was analysed by a competitive assay in which cold LDL from the patients or control compete with 125I-labelled LDL for fibroblast LDL receptors. The concentration of LDL particles at which 50% of 125I-labelled normal LDL was replaced was two to three times higher for the patients than for the normal control. Therefore, the affinity of patient LDL was thought to be reduced compared to that of control LDL. These results demonstrate that CETP may play an important role in making LDL particles homogeneous and rich in CE.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of LDL-cholesterol lowering action of psyllium hydrophillic mucilloid in the hamster

Psyllium hydrophillic mucilloid (psyllium) is a soluble fiber that significantly lowers plasma low-density lipoprotein (LDL)-cholesterol levels in humans and experimental animals. These studies were designed to determine whether this action is the result of a reduction in LDL-cholesterol production, an increase in receptor-mediated LDL clearance by the tissues, or a combination of these mechanisms. Adult male Golden Syrian hamsters were fed ad libitum for 30 days a cereal-based diet containing added cholesterol (0.1%) and hydrogenated coconut oil (10%), as well as either microcrystalline cellulose (Avicel) (7.5%) or psyllium (7.5%). In contrast to their Avicel-fed controls, the hamsters given psyllium had markedly lower plasma total (122.1 +/- 4.1 vs. 399.4 +/- 39.4 mg/dl) and LDL-cholesterol (46.0 +/- 2.2 vs. 143.5 +/- 12.0 mg/dl) levels. Psyllium feeding also prevented both the dramatic increase in hepatic total cholesterol levels (2.6 +/- 0.1 vs. 16.6 +/- 1.1 mg/g), and the suppression of hepatic cholesterol synthesis (165.1 +/- 27.1 vs. 26.1 +/- 1.2 nmol/h per g) that occurred in the animals given Avicel. Compared to their controls, the psyllium-fed animals also manifested a 44% lower rate of LDL-cholesterol production (167.6 +/- 8.1 vs. 300.2 +/- 16.0 micrograms/h per 100 g bw), and a 2.2-fold higher rate of hepatic LDL clearance (50.1 +/- 2.3 vs. 22.6 +/- 2.1 microliters/h per g). When expressed as a percentage of corresponding values obtained for hamsters fed the basal diet without any additions, the relative rate of LDL-cholesterol production was 175 +/- 10% and 99 +/- 4% for the Avicel- and psyllium-fed groups, respectively. It was similarly determined that the level of whole animal relative LDL receptor activity was marginally higher in the hamsters given psyllium (55.9 +/- 1.4%) than in those fed Avicel (47.5 +/- 3.3%). Thus, it was concluded that while the LDL-cholesterol lowering action of psyllium in the hamster is mediated through two mechanisms, the major effect is exerted at the level of LDL-cholesterol production.

Triggering of plaque disruption and arterial thrombosis in an atherosclerotic rabbit model

BACKGROUND

It is now recognized that plaque disruption and thrombosis, a process often triggered by activities of the patient, is generally the cause of the onset of acute coronary syndromes. Understanding of disease onset could be greatly enhanced by the availability of a suitable animal model of plaque disruption and thrombosis. The aim of this study was to replicate and further characterize an atherosclerotic rabbit model of triggering of arterial thrombosis that was introduced by Constantinides and Chakravarti more than 30 years ago but not subsequently used. Aortic plaques were induced by a high-cholesterol diet, by mechanical balloon injury of the artery, or by a combination of the two. Triggering was attempted by injection of Russell's viper venom (RVV), which is a proteolytic procoagulant, and histamine.

METHODS AND RESULTS

A total of 53 New Zealand White rabbits were exposed to one of four preparatory regimens: rabbits in group I (n = 9) were fed a regular diet for 8 months; rabbits in group II (n = 13) were fed a diet of 1% cholesterol for 2 months alternated with 2 months of a regular diet for a total of 8 months; rabbits in group III (n = 5) underwent balloon-induced arterial wall injury, then were given a regular diet for 8 months; and rabbits in group IV (n = 14) underwent balloon-induced arterial wall injury, then were given a diet of 1% cholesterol for 2 months followed by a regular diet for 2 months for a total of 4 months. After completion of the preparatory regimen, triggering of plaque disruption and thrombosis was attempted by injection of RVV (0.15 mg/kg IP) and histamine (0.02 mg/kg IV). In group I, normal control rabbits without atherosclerosis, only one small thrombus was noted in 1 of 9 rabbits. In group II, cholesterol-fed rabbits, thrombosis occurred in 3 of 13 rabbits. Thrombus occurred in all rabbits in group III (5 of 5) and in 10 of 14 rabbits in group IV. Although the frequency of thrombosis was not significantly different between groups I and II, possibly due to a small sample size, it was significantly different among all four groups (P < .001). Also, the frequency and amount of thrombus formation were significantly different among all four groups (P < .001; P < .0001) but not between groups I and II. Rabbits with atherosclerosis (those in groups II and IV) demonstrated plaque disruption and overlying platelet-rich thrombus formation similar to that observed in patients with acute coronary syndromes. The surface area covered by thrombus was 2 mm2 in group I, 15.3 +/- 19.2 mm2 in group II, 223 +/- 119 mm2 in group III, and 263 +/- 222 mm2 in group IV. Rabbits in groups III and IV had the greatest amount of thrombus, and this amount was significantly greater than in rabbits in groups I and II (P < .001 and P < .03, respectively).

CONCLUSIONS

A suitable animal model is available for the study of plaque disruption and arterial thrombosis. Hypercholesterolemia and mechanical arterial wall injury seemed to produce plaques vulnerable to triggering of disruption and thrombosis, whereas normal arteries were relatively resistant to triggering. This model provides a method to evaluate agents that might decrease the occurrence of vulnerable plaques or the amount of thrombus formed after triggering. Most important, the model can be used to identify the features of vulnerable plaques and the pharmacological stressors that trigger plaque disruption and thrombus formation.

Low-density lipoprotein particle size in type 2 diabetic patients and age matched controls

Non-enzymatic glycation of low-density lipoprotein (LDL), and the predominance of small dense LDL particles may together contribute to the increased risk of atherosclerosis in diabetes. We aimed to establish whether the size of LDL particles is related to plasma triglyceride concentration, and to the extent of LDL glycation in type 2 diabetic patients. Sixteen men with type 2 diabetes and 16 age matched non-diabetic controls were studied. LDL size was measured by rapid density gradient ultracentrifugation, and LDL glycation by affinity chromatography. Modal LDL density correlated with plasma triglyceride concentrations in both diabetic and control groups (r = 0.86, P < 0.0001, and r = 0.76, P < 0.0008, respectively). There was no significant difference in these variables between the groups. LDL modal density showed no correlation with HbA1, serum fructosamine or plasma glucose in either group. In the diabetic group the degree of LDL glycation correlated with serum fructosamine (r = 0.74, P < 0.001), HbA1 (r = 0.65, P < 0.008), and with plasma glucose (r = 0.64, P < 0.008). Our results suggest that, in well controlled type 2 diabetic patients LDL size is independent of short-term glycaemic control but can be predicted by plasma triglyceride concentrations.

Lipoprotein lipases, lipoprotein density gradient profile and LDL receptor activity in miniature pigs fed fish oil and corn oil

The effects of fish oil and corn oil on plasma lipoprotein concentrations, the lipolytic enzymes, lipoprotein lipase and hepatic triacylglycerol lipase, the density distribution of the plasma lipoproteins and LDL receptor activity were studied. These experiments were designed, in part, to define the mechanism(s) responsible for the increased conversion of plasma VLDL apolipoprotein B to LDL and a decreased LDL apolipoprotein B fractional catabolic rate described in previous apolipoprotein B kinetic studies. Miniature pigs were fed diets for 3 to 6 weeks containing supplements of corn oil or fish oil as Maxepa. Triacylglycerol and cholesterol in plasma and VLDL were significantly reduced by the fish oil diet. LDL and HDL cholesterol were not significantly changed. The fish oil diet significantly reduced post-heparin plasma lipoprotein lipase and hepatic triacylglycerol lipase activities, which may be an adaptive response to the low concentration of substrates (triacylglycerol-rich lipoproteins) for these enzymes. No differences were observed in the density of VLDL, LDL or HDL as determined by density gradient ultracentrifugation with the fish oil diet. No major changes in percent lipid composition of VLDL, LDL and HDL were observed. No differences were found with respect to LDL uptake by J774 macrophages. Receptor mediated clearance of LDL in vivo, as assessed by measuring the difference in fractional catabolic rate of native vs. methylated LDL decreased significantly by 17% (P < 0.032). We conclude that the increased conversion of VLDL apolipoprotein B to LDL in miniature pigs fed fish oil is not related to an increase in lipolytic enzymes or density distribution of VLDL, but may be due in part to a decrease in LDL receptor activity.

Catabolism of low-density lipoprotein is altered in experimental chronic renal failure

Catabolism of low-density lipoprotein is altered in experimental chronic renal failure. In patients with chronic renal failure, cardiovascular disease accounts for a significant proportion of all deaths. Several factors contribute to the "accelerated" atherosclerosis in this population, including hyperlipidemia, the pathogenesis of which is multifactorial. We investigated low-density lipoprotein (LDL) metabolism in a remnant model of chronic renal failure in the guinea pig. After one and two-thirds nephrectomy, creatinine clearance decreased to one-sixth normal. Plasma cholesterol and triglyceride (TG) levels increased with induction of renal failure. Analysis of lipoprotein composition disclosed significant TG enrichment of both uremic very-low-density lipoprotein (VLDL) and uremic LDL compared with control lipoproteins. Plasma clearance of homologous LDL was evaluated in turnover studies in control and uremic guinea pigs. To discriminate between differences in catabolism related to the uremic lipoprotein particle and to the uremic host milieu, a crossover protocol was used comparing the fractional catabolic rate (FCR) after simultaneous injection into control and uremic animals of 125I-control LDL and 131I-uremic LDL. The FCR of native LDL was slower in uremic animals than in controls. In addition, FCR of uremic LDL was significantly less than that of control LDL in both groups. Degradation studies in cultured fibroblasts indicated substantially reduced degradation of uremic LDL compared with control LDL. These results suggest dual abnormalities of LDL catabolism in renal failure that are not only related to alterations in clearance mechanisms in the uremic environment, but also suggest important functional differences in the LDL particle itself isolated from uremic animals.

Effects of simvastatin on apoB metabolism and LDL subfraction distribution

Seven moderately hypercholesterolemic subjects were studied before and after 10 weeks of simvastatin therapy (20 mg/day). Therapy reduced low density lipoprotein (LDL) cholesterol by 39% (p < 0.001), whereas high density lipoprotein and very low density lipoprotein (VLDL) cholesterol were unchanged. Apolipoprotein (apo) B-containing lipoproteins were divided into VLDL1 (Sf 60-400), VLDL2 (Sf 20-60), intermediate density lipoprotein (IDL) (Sf 12-20), and LDL (Sf 0-12), and metabolic changes were sought in dual-tracer VLDL1 and VLDL2 turnover studies. VLDL1 apoB pool size was unaltered by therapy, as were its rates of synthesis, catabolism, and delipidation to VLDL2. Similarly, the VLDL2 apoB pool size was unchanged, but its metabolic fate was altered. The IDL pool size fell significantly (27%, p < 0.01) due entirely to an increased fractional catabolism of the lipoprotein. In our subjects, the circulating mass of LDL apoB decreased (49%, p < 0.01) primarily due to a reduction in its synthesis. Before therapy, 30% of the apoB entering the delipidation cascade in these hyperlipidemic subjects was converted to LDL. On therapy the input remained the same, but direct catabolism from VLDL2 and IDL was increased (p < 0.05), and as a result only 16% eventually appeared in LDL. These kinetic changes were associated with a fall in particle cholesteryl ester content throughout the delipidation cascade. We also observed a link between LDL kinetics and its subfraction distribution. Simvastatin influences the metabolism of LDL, IDL, and VLDL2 but not VLDL1.

Comparison of methods for incorporating a radioiodinated residualizing cholesteryl ester analog into low density lipoprotein

Two different methods were evaluated for incorporating [125I]cholesteryl iopanoate ([125I]CI), a non-hydrolyzable cholesteryl ester analog, into LDL. The first procedure was an organic solvent delipidation-reconstitution procedure (R[125I-CI]LDL) while the second involved incubation of detergent (Tween-20)-solubilized [125I]CI with whole plasma (D[125I-CI]LDL). R[125I-CI]LDL behaved similar to native LDL in vitro, but was markedly different in vivo, apparently due to a heterogeneity in particle size. D[125I-CI]LDL, however, was metabolized normally both in vitro and in vivo. These results, combined with the residualizing nature of [125I]CI, demonstrate that D[125I-CI]LDL is appropriate for tracing LDL uptake in vivo.

Metabolism of plasma low density lipoproteins in familial combined hyperlipidaemia: effect of acipimox therapy

Ten male patients with familial combined hyperlipidaemia (FCHL) were studied with regard to LDL metabolism and composition. The FCHL patients had higher LDL levels than healthy controls (5.4 +/- 1.4 vs. 3.7 +/- 0.7 mmol l-1; P < 0.005) and a higher rate of production of the lipoprotein (15.8 +/- 3.1 mg kg-1 d-1 in FCHL vs. 13.1 +/- 1.8 mg kg-1 d-1 in the normals; P < 0.005). The fractional catabolic rate of LDL was low-normal in the FCHL patients, with a high level of interindividual variation. The actual individual LDL cholesterol level within the FCHL patient group appeared to be more closely associated with the LDL apoB FCR value than the rate of production of the particle. Analysis of the LDL particles from FCHL patients revealed a relative enrichment in triglycerides, while the cholesterol content of the lipoprotein was normal. Institution of acipimox therapy in 8 patients reversed the high rate of synthesis of LDL (15.2 +/- 3.5 mg kg-1 d-1) to a more normal level (13.9 +/- 4.0 mg kg-1 d-1; P = 0.08), while the FCR did not change significantly. In conclusion, patients with FCHL show an apparent overproduction of LDL apoB, while the actual degree of LDL elevation appears to be dependent on the clearance capacity of the lipoprotein, measured as LDL apoB FCR. The overproduction defect of LDL apoB can, at least in part, be managed by treatment with the nicotinic acid analogue acipimox.

Effectiveness of low-dose colestipol therapy in patients with moderate hypercholesterolemia

Recommended doses of bile-acid binding resins have an established hypocholesterolemic effect, but data on responses to low doses, especially in women and subjects with moderate hypercholesterolemia, are sparse. A double-blind, placebo-controlled, randomized trial of 3 low doses of colestipol hydrochloride was conducted in women and men with moderate hypercholesterolemia. Men and women with plasma low-density lipoprotein (LDL) cholesterol concentrations greater than 4 mmol/liter (155 mg/dl) and triglyceride concentrations less than 2.82 mmol/liter (250 mg/dl) were recruited for the study. Eligible patients (54 women and 98 men) were placed on the American Heart Association step I diet 6 weeks before randomization. Participants were subsequently assigned to 1 of 4 drug treatment groups (placebo, and 5, 10 and 15 g/day of colestipol in 2 divided doses) for an additional 12 weeks. Of the 152 patients randomized, 141 completed all aspects of the study. For the treatment groups--placebo, and 5, 10 and 15 g of colestipol--LDL cholesterol reductions (mmol/liter) were observed respectively (n = 141): 0.10 +/- 0.49 (2.7%), 0.65 +/- 0.41 (16.3%), 0.98 +/- 0.36 (22.8%) and 1.17 +/- 0.47 (27.2%) (p less than 0.001). Similar changes were observed in total cholesterol and apolipoprotein B concentrations. The apolipoprotein B/LDL cholesterol ratio increased significantly with increasing colestipol dosage. Modest but insignificant changes in plasma triglyceride levels occurred, and high-density lipoprotein cholesterol levels remained unchanged. A dose of 5 g/day of colestipol achieved 51% of the LDL cholesterol reduction noted with 15 g/day. Low-dose colestipol therapy is effective in the treatment of patients with moderate hypercholesterolemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cholestyramine and acipimox on subfractions of plasma low density lipoprotein. Studies in normolipidaemic and hypercholesterolaemic subjects

Two independent studies were designed to investigate the separate and combined effects of acipimox and cholestyramine on plasma low density lipoprotein subfractions. In the first study, normolipidaemic subjects were given cholestyramine (16 g day-1, 4 weeks), followed, after an 8-week wash-out period, by acipimox (750mg day-1, 4 weeks). In the second study, moderately hypercholesterolaemic subjects were prescribed acipimox (1250mg day-1, 10 weeks), followed by acipimox in combination with low dose cholestyramine (12g day-1) for a further 10 weeks. In the normal subjects, cholestyramine decreased total LDL mass (density (d) = 1.019-1.063g ml-1) by selectively reducing the largest, least dense LDL-I (d 1.025-1.034 g ml-1, P less than 0.05) and LDL-II (d 1.034-1.044 g ml-1, P less than 0.005) subfractions. The small, dense LDL-III (d 1.044-1060 g ml-1) showed a variable response to the resin. In the same subjects acipimox produced no overall change in total LDL mass but showed a tendency to redistribute LDL towards LDL-I (+10%) and LDL-II (+10%) in a manner related to the changes in plasma triglyceride (TG) (TG vs. LDL-III r = 0.75, P less than 0.05). In the hypercholesterolaemic subjects acipimox induced a substantial redistribution of LDL subfractions (LDL-I +84% P less than 0.05; LDL-III -50%) without affecting total LDL mass. The addition of cholestyramine produced a significant decrease in total LDL mass which was again confined to the LDL-I (-28%) and LDL-II (-23%) subfractions.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypolipidemic drugs reduce lipoprotein susceptibility to undergo lipid peroxidation: in vitro and ex vivo studies

Oxidized LDL, which has been discovered in vivo in areas of proximity to the atherosclerotic lesion, has been shown to enhance macrophage cholesterol accumulation. We studied the anti-oxidant potential of pravastatin, bezafibrate and cholestyramine in 18 patients with hypercholesterolemia. In addition, we examined the electrophoretic mobility and the uptake of LDL by macrophages before and after drug therapy. Pravastatin lowered plasma levels of LDL cholesterol by 57%, cholestyramine by 27% and bezafibrate by 25%. Pravastatin and bezafibrate also altered the composition of LDL as evidenced by the reduction of its cholesterol/apo B100 ratio. Pravastatin and bezafibrate reduced plasma triglyceride levels by 45% and 25%, respectively, whereas cholestyramine raised plasma triglyceride concentrations by 28%. LDL propensity for in vitro oxidation was analyzed following lipoprotein incubation with 10 microM copper ions and determination of LDL malondialdehyde (MDA), peroxides (PD) and conjugated dienes (CD) content. All drugs inhibited the susceptibility to in vitro oxidation of LDL isolated after drug therapy in comparison to LDL isolated before commencing drug therapy. Pravastatin reduced MDA content by 22%, PD by 18% and CD by 20%. Cholestyramine reduced LDL content of MDA by 41%, PD by 25% and CD by 63%. Bezafibrate reduced MDA by 41%, PD by 38% and CD by 45%. LDL vitamin E content was reduced after treatment with bezafibrate, pravastatin and cholestyramine by 49%, 36% and 8%, respectively. The electrophoretic mobility of LDL after all drug therapies was reduced in comparison to LDL obtained before therapy. Macrophage uptake of LDL assessed by either the cellular cholesterol esterification rate or by lipoprotein degradation was not affected by drug therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cholestyramine on lipoprotein levels and metabolism in Syrian hamsters

Oral administration of cholestyramine to adult male hamsters not only induced a marked decrease in plasma concentrations of cholesterol and LDL but had a similar lowering effect on plasma triacyglycerol and VLDL concentrations. The hypotriglyceridaemic effects of resin administration were not due to an increase in lipoprotein lipase, as post-heparin plasma lipoprotein lipase activities were unchanged, but rather to a 35% decrease in VLDL synthesis. Measurement of the disappearance rate of apolipoprotein B from VLDL after i.v. injection of 125I-labelled hamster or human VLDL into control and cholestyramine-fed recipient animals showed a 2-times lower T1/2 in the drug-treated animals. The fraction of VLDL apolipoprotein B, recovered at any time after injection in the LDL, was equal or higher in cholestyramine-fed animals as compared to controls. These data indicate that the lowering in plasma LDL by cholestyramine in male hamsters is due not only to LDL receptor up-regulation but also to a lower rate of VLDL synthesis. No indications were found for a decreased efficiency of VLDL to LDL conversion in cholestyramine-fed animals.

Influence of lovastatin therapy on metabolism of low density lipoproteins in mixed hyperlipidaemia

To determine the mechanisms whereby HMG-CoA reductase inhibitors lower the levels of low density lipoproteins (LDL) in patients with mixed hyperlipidaemia, LDL turnover studies were conducted in 12 such patients during placebo and then during treatment with lovastatin. Drug therapy reduced total cholesterol and triglyceride concentrations by 33% and 32%, respectively. During lovastatin therapy, LDL-cholesterol levels fell by 37%, and LDL-apo B concentrations decreased by an average of 29%. The decrease in LDL-apo B concentrations on lovastatin therapy was largely due to an increase in fractional catabolic rates (FCRs) for LDL apo B. The average increase in FCRs was 34%, whereas transport rates (production rates) for LDL apo B remained unchanged. These results strongly suggest that an increase in LDL-receptor activity is the major mechanism whereby LDL levels are lowered during lovastatin therapy. The data do not indicate that this drug inhibited the input of apo B-containing lipoproteins, which would have been expected to result in a decrease in the rate of production of LDL.

Metabolism of lipoprotein remnants in humans. Studies during intestinal infusion of fat and cholesterol in subjects with varying expression of the low density lipoprotein receptor

To study the possible importance of the low density lipoprotein (LDL) receptor in regulating the degree of postprandial lipemia, a cholesterol-rich fat emulsion was infused into the duodenum of subjects who were divided into four groups based on the expected variation in the expression of the LDL receptor: young men (n = 11), elderly men (n = 7), male patients on estrogen therapy (n = 5), and patients with familial hypercholesterolemia (n = 9). In familial hypercholesterolemia, fasting plasma levels of lipoproteins of d less than 1.006 g/ml, intermediate density lipoproteins, and LDLs were increased. During the fat infusion, the cholesterol and triglyceride contents in the d less than 1.006 g/ml fraction increased to a similar extent in all groups, whereas a concomitant reduction of LDL cholesterol levels was observed. The degree of the decrease in LDL cholesterol was positively correlated with the observed increase in triglycerides in the d less than 1.006 g/ml fraction. There were no signs of accumulation of intermediate density lipoproteins during infusion in any of the groups studied. The results indicate that the capacity for clearance of chylomicrons and chylomicron remnants is not affected by variation in LDL receptor expression.

Dietary fat saturation in rhesus monkeys affects LDL concentrations by modulating the independent production of LDL apolipoprotein B

In a recent study from this laboratory, rhesus monkeys fed a 90% palm oil/10% soybean oil-containing diet (PS), rich in 16:0 and 18:1 fatty acids, had decreased total and LDL cholesterol concentrations compared to monkeys fed a 90% coconut oil/10% soybean oil-containing diet (CS), rich in 12:0 and 14:0 fatty acids. To investigate the metabolic basis of these changes, homologous 125I-VLDL and 131I-LDL were injected simultaneously into eight monkeys (four per dietary group). Analysis of apo B specific activity curves revealed that PS monkeys had an increased pool size of VLDL apo B (P less than 0.02), a 3-fold increase in the total VLDL apo B transport rate (P less than 0.001), a decreased pool size of LDL apo B (P less than 0.01) and a 2-fold decrease in the total transport rate of LDL apo B (P less than 0.001), while the irreversible FCR for VLDL apo B and LDL apo B was similar between dietary groups. PS monkeys derived a greater percentage of LDL apo B from VLDL catabolism resulting in a greater transport rate of LDL apo B from VLDL catabolism (P less than 0.055), in comparison to CS monkeys. For CS monkeys the proportion as well as the amount of LDL apo B derived from VLDL-independent catabolism (i.e., LDL apo B derived from sources other than VLDL catabolism) was higher (P less than 0.001) than the values obtained in PS monkeys. In both dietary groups the proportion of VLDL apo B converted to LDL apo B was similar, although the absolute amount was higher for the PS monkeys (P less than 0.06). The proportion of VLDL apo B directly removed from the circulation was similar for both dietary groups, with the absolute amount being higher for the PS monkeys (P less than 0.001). Consistent with the lower pool size of LDL apo B and the higher pool size of VLDL apo B observed in PS monkeys, plasma and LDL cholesterol concentrations tended to be lower, whereas plasma triacylglycerol and VLDL cholesterol concentrations tended to be higher, but these changes were not statistically significant. Although total apo B and VLDL apo B transport rates were increased 2-3-fold in PS monkeys, LDL apo B concentration was reduced by 40% (P less than 0.02) attributed to a significant reduction in the mass and proportion of LDL apo B derived independent of VLDL catabolism.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of fluvastatin (XU 62-320), an HMG-CoA reductase inhibitor, on the distribution and composition of low density lipoprotein subspecies in humans

We studied the effect of fluvastatin (XU 62-320), a new HMG-CoA reductase inhibitor, on the distribution of low density lipoprotein (LDL) subspecies and composition in humans. As expected, fluvastatin significantly lowered serum LDL levels (25% after 6 weeks of therapy). In addition, treatment with fluvastatin changed the LDL subspecies. In the group treated with fluvastatin, 38.5% of the individuals showed changes in the shape of LDL absorbance profile obtained from density gradient ultracentrifugation and 54% of the group showed changes in the electrophoretic mobility of the LDL bands. Of those showing changes in electrophoretic mobility, the majority (78%) shifted to slightly larger, less dense LDL after drug therapy. However, the LDL-cholesterol/apo B ratio changes were relatively small in all fluvastatin-treated individuals including the group with changes in electrophoretic mobility, confirming that HMG-CoA reductase inhibitor causes relatively small and subtle changes in the distribution of LDL subspecies.

Reduction of LDL production rate in ileal bypassed rabbits treated with lovastatin

The effect of lovastatin on the low density lipoprotein metabolism in bypassed rabbits was investigated. Partial ileal bypass effectively reduced total and low density lipoprotein (LDL) cholesterol by 44 and 48% respectively. The LDL drop was due to an increased fractional catabolic rate (FCR) of apolipoprotein B (apo B) from 0.74 to 1.27 pools per day with no effect on the apo B absolute catabolic rate and an increased expression of liver LDL receptors (+71%). Association of lovastatin with PIB resulted in a further decrease of total and LDL cholesterol (56 and 75% respectively) as compared to bypassed animals, without effects on the LDL FCR (1.27 +/- 0.11 versus 1.42 +/- 0.13 pools/day) or the expression of LDL receptors by the liver. The reduction of LDL was due to a decrease of the apolipoprotein B absolute synthetic rate (8.5 +/- 1.7 versus 13.6 +/- 1.7 mg/day). From these data we conclude that in bypassed rabbits lovastatin lowers total and LDL cholesterol mainly by reducing apolipoprotein B production rate.

Detection of two species of low density lipoprotein particles in cholesteryl ester transfer protein deficiency

By equilibrium density gradient ultracentrifugation, we analyzed the chemical composition and particle size of low density lipoproteins (LDLs) in 16 subfractions separated from the LDL fractions (1.019 less than d less than 1.063 g/ml) of two hyperalphalipoproteinemic patients who had a deficiency of cholesteryl ester transfer protein (CETP). The LDLs of these patients comprised a group of heterogeneous lipoprotein particles distributed almost equally in a wide density range from d = 1.025 g/ml to d = 1.053 g/ml, whereas LDLs from normal controls were a homogeneous group of lipoprotein particles distributed in a narrow density range from d = 1.030 g/ml to d = 1.046 g/ml. The LDL in each subfraction derived from the patients' plasma samples was poor in cholesteryl ester and rich in triglycerides and apolipoproteins. Each subfraction of normal control LDL contained only one species of homogeneous LDL particles, which progressively decreased in size with an increase in the density of the fraction. In contrast, each subfraction of patient LDL contained two species of LDL particles: smaller LDLs existed, in addition to those that were found to be identical to the normal control LDL particles observed in the corresponding subfractions. The intermediate density lipoproteins of the two patients were also composed of two species of lipoproteins. From these results, we speculate that two metabolic pathways may exist in the LDL formation process. In this process, the transfer of cholesteryl ester from high density lipoproteins by CETP may convert the smaller lipoprotein particles to the larger ones, forming the homogeneous LDL species.

Effects of long-term treatment with simvastatin on plasma lipids and lipoproteins in patients with primary hypercholesterolemia

We investigated long-term hypolipidemic effects and clinical safety of simvastatin, a new competitive inhibitor of 3-hydroxy-methylglutaryl coenzyme A reductase in 24 patients with familial and non-familial hypercholesterolemia. Patients received up to 40 mg simvastatin for a period of 30 months. Significant decreases were noted in plasma cholesterol (30%), plasma triglycerides (25%), very low density lipoprotein-cholesterol (26%), and low density lipoprotein-cholesterol (40%), whereas an increase in plasma high density lipoprotein-cholesterol (11%) was observed. Furthermore, the percentage decrease in plasma low density lipoprotein cholesterol was independent of individual baseline concentrations. Simvastatin did not alter the composition of low density lipoproteins or high density lipoproteins. The percentage decrease in total plasma and low density lipoprotein-cholesterol was independent of apoprotein E isoforms and low density lipoprotein-receptor activity as assayed in cultured fibroblasts. The drug therapy was well tolerated and clinical examinations revealed no adverse effects. Clinical chemistry indices and hematological, as well as endocrinological parameters remained within normal limits and ranges.

Bile acid sequestrants

The bile acid sequestrants, cholestyramine and colestipol, are the drugs of choice for the treatment of patients with hypercholesterolemia caused by increases in LDL-cholesterol levels without concurrent hypertriglyceridemia (type IIA and type IIB hyperlipoproteinemia). Longitudinal clinical studies with these drugs have shown their ability to slow the progression of atherosclerosis and to limit the consequences of the disease. Bile acid sequestrants can be used with other lipid-lowering drugs such as nicotinic acid or HMG CoA reductase inhibitors, to maximize the cholesterol-lowering effects. The side effect profile of the bile acid sequestrants is tolerable, with most complaints related to effects on the gastrointestinal tract and the bulkiness of the resins.

Pravastatin therapy in primary moderate hypercholesterolaemia: changes in metabolism of apolipoprotein B-containing lipoproteins

This study examined the actions of pravastatin on the metabolism of apolipoprotein B (apo B) in very low-, intermediate-, and low-density lipoproteins (VLDL, IDL, and LDL) in 10 patients with primary moderate hypercholesterolaemia. 131I-VLDL apo B was used as a tracer, and appearance of label was followed into IDL apo B and LDL apo B. Compared to placebo, pravastatin therapy reduced levels of cholesterol in total plasma. LDL, VLDL, and IDL cholesterol by 25%, 29%, 31%, and 47%, respectively. Pravastatin treatment also significantly decreased concentrations of apo B in LDL, IDL, and VLDL. The drug significantly reduced the mean production rate for VLDL apo B by 40%, and decreased production rates for LDL apo B in eight of 10 patients. In contrast, fractional catabolic rates (FCRs) were not altered significantly in any of the three lipoprotein fractions on pravastatin therapy. Further, pravastatin produced no consistent changes in LDL particle size, composition, or LDL subclass pattern. Thus pravastatin seemingly reduced input rates for all apo B-containing lipoproteins. Consistent with previous studies, this response was most likely the result of enhanced removal of nascent lipoproteins by increased activity of LDL receptors, although decreased synthesis of apo B in the liver is a possible second action.

Dietary fat-mediated changes in hepatic apoprotein B/E receptor in the guinea pig: effect of polyunsaturated, monounsaturated, and saturated fat

These studies investigated the effects of dietary fat quality on guinea pig plasma total and low-density lipoprotein (LDL) cholesterol and triglyceride levels; LDL composition, density, and binding affinity to hepatic apolipoprotein (apo) B/E receptors; and hepatic apo B/E receptor levels. Animals were fed semi-synthetic diets containing 7.5% (weight per weight) fat--either corn oil (CO), olive oil (OL), or lard. Plasma cholesterol levels were significantly lower on the CO diet compared with the OL and lard diets. The isolated LDL had mean peak densities that ranged from 1.071 for LDL from lard-fed animals to 1.075 for LDL from CO- and OL-fed animals. The cholesterol ester to protein ratio of the LDL particle decreased in CO-fed guinea pigs. Binding studies showed that these compositional changes of the LDL had no effect on the binding affinity of the particles to a standardized hepatic membrane preparation. Membrane phospholipid fatty acid compositions were significantly different among the three dietary fat groups. When hepatic membranes were incubated with 125I-labeled LDL, the receptor-mediated binding of LDL to membranes from CO-fed animals increased 1.5-fold compared with binding to membranes from OL- and lard-fed guinea pigs. Scatchard plots indicated an increase of 50% in receptor number in membranes of animals fed the CO diet, whereas the affinity of the hepatic apo B/E receptor for LDL (Kd) was virtually identical for all membranes.(ABSTRACT TRUNCATED AT 250 WORDS)