Hepatic LDL Receptor Activity Research Articles

The emerging concept of "individualized cholesterol-lowering therapy": A change in paradigm.

High LDL-cholesterol concentrations constitute a risk for atherosclerotic cardiovascular disease. By consensus, cholesterol-lowering therapy is initiated with a statin that reduces endogenous cholesterol synthesis, upregulates hepatic LDL receptor activity, increases LDL clearance and lowers LDL-cholesterol concentrations in the bloodstream. The efficacy of statin treatment is dose dependent and achieves a risk reduction of up to 50%. However, a substantial body of evidence suggests that a quarter of statin-treated patients do not respond adequately as a result of low endogenous cholesterol synthesis. In humans fractional cholesterol absorption varies from 20% to 80%. High cholesterol absorbers, which are characterized by a low-to-normal cholesterol synthesis, exhibit poor responsiveness to statin treatment. On the other hand, the cholesterol absorption inhibitor ezetimibe effectively reduces serum cholesterol levels in these patients. On this background, we suggest to "get personal" and individualize cholesterol-lowering therapies, according to the individual's status of cholesterol synthesis and absorption.

Citrus flavonoid 5‐demethylnobiletin suppresses scavenger receptor expression in THP‐1 cells and alters lipid homeostasis in HepG2 liver cells

Nobiletin, a polymethoxyflavone from the peel of citrus fruits, has been reported to inhibit modified LDL uptake in macrophages and enhance hepatic LDL receptor expression and activity. We report the anti-atherogenic effect and mechanism of 5-demethylnobiletin, an auto-hydrolysis product of nobiletin. 5-Demethylnobiletin significantly attenuated phorbol 12-myristate 13-acetate-induced gene expression and activity of scavenger receptors, CD36, scavenger receptor-A and lectin-like oxidized LDL receptor-1. The inhibitory effect is partly associated with the inhibition of protein-kinase C activity and c-Jun NH(2) -terminal kinase 1/2 phosphorylation, thereby inhibiting the activation of activator protein-1 and nuclear factor-κB. 5-Demethylnobiletin treatment also led to reduction of oxidized LDL-induced CD36 mRNA expression and blockade of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanide perchlorate-modified LDL uptake in THP-1-derived macrophages. In the human hepatoma cell line HepG2, 5-demethylnobiletin significantly induced LDL receptor activity and transcription, at least in part, through steroid-response element-binding protein-2 activation. 5-Demethylnobiletin also decreased the mRNA expression of acyl CoA:diacylglycerol acyltransferase 2, the key enzyme involved in the hepatic triacylglycerol biosyntheses. Current results suggest that 5-demethylnobiletin has diverse anti-atherogenic bioactivities. It is more potent in inhibiting monocyte-to-macrophage differentiation and foam cell formation than its permethoxylated counterpart, nobiletin. It exhibits similar hypolipidemic activity as nobiletin and both can enhance LDL receptor gene expression and activity and decreased acyl CoA:diacylglycerol acyltransferase 2 expression.

Reduction in intestinal cholesterol absorption by various food components: Mechanisms and implications

A number of different food components are known to reduce plasma and LDL-cholesterol levels by affecting intestinal cholesterol absorption. They include: soluble fibers, phytosterols, saponins, phospholipids, soy protein and stearic acid. These compounds inhibit cholesterol absorption by affecting cholesterol solubilization in the intestinal lumen, interfering with diffusion of luminal cholesterol to the gut epithelium and/or inhibiting molecular mechanisms responsible for cholesterol uptake by the enterocyte. Cholesterol content of intestinal chylomicrons is subsequently reduced, less cholesterol is transported to the liver within chylomicron remnants, hepatic LDL-receptor activity is increased and plasma levels of LDL-cholesterol are decreased. Reduced hepatic VLDL production and less conversion of VLDL to LDL also contribute to lower LDL levels. Certain food components may also affect intestinal bile acid metabolism. Further investigation of the way in which these functional ingredients affect intestinal lipid metabolism will facilitate their use and application as cardiovascular nutraceuticals.

A green tea extract lowers plasma cholesterol by inhibiting cholesterol synthesis and upregulating the LDL receptor in the cholesterol-fed rabbit

Green tea extracts enriched in catechins decrease plasma cholesterol in hamsters, mice and rats. The aims of this study were to determine whether a catechin-enriched extract of green tea could lower plasma cholesterol in the cholesterol-fed rabbit and to determine the mechanism of action. Four groups of six New Zealand White rabbits were initially made hypercholesterolaemic by feeding a 0.25% (w/w) cholesterol diet for 2 weeks before the diet was supplemented with a catechin extract from green tea at 0, 0.5, 1 or 2% (w/w) for 4 weeks. Administration of the crude catechin extract from green tea significantly ( p < 0.05) lowered cholesterol in plasma (−60%), VLDL + IDL (−70%), LDL (−80%), liver (total by −25% and unesterified by −15%) and aorta (−25%) compared to control. There was a significant reduction in the cholesterol synthesis index (−60%) and a significant increase in hepatic LDL receptor activity (+80%) and protein (+70%) but there was no change in the intrinsic capacity to absorb cholesterol from the intestines. These results suggest that green tea catechins lowered plasma, liver and aortic cholesterol in the cholesterol-fed rabbit by lowering cholesterol synthesis and upregulating the hepatic LDL receptor.

Hypercholesterolemia and apolipoprotein B expression: regulation by selenium status.

BackgroundApolipoprotein B (apoB) contains ligand-binding domain for the binding of LDL to LDL-R site, which enables the removal of LDL from circulation. Our recent data showed that selenium (Se) is involved in the lipid metabolism. The present study was aimed to understand the effect of Se deficiency (0.02 ppm) and selenium supplementation (1 ppm) on apoB expression in liver during hypercholesterolemia in male Sprague Dawley rats. Animals were fed with control and high cholesterol diet (2%) for 1 and 2 months. ApoB levels by ELISA and protein expression by western blot was done. Hepatic LDL receptor (LDL-R) activity (in vivo) and mRNA expression by RT-PCR was monitored.ResultsIn selenium deficiency and on high cholesterol diet (HCD) feeding apoB levels increased and LDL-R expression decreased significantly after 2 months. On 1 ppm selenium supplementation apoB expression significantly decreased and LDL-R expression increased after 2 months. But after one month of treatment there was no significant change observed in apoB and LDL-R expression.ConclusionSo the present study demonstrates that Se deficiency leads to up regulation of apoB expression during experimental hypercholesterolemia. Selenium supplementation upto 1 ppm leads to downregulation of apoB expression. Further, this study will highlight the nutritional value of Se supplementation in lipid metabolism.

Cholesterol metabolism and therapeutic targets: rationale for targeting multiple metabolic pathways.

The liver is the major regulator of the plasma low density lipoprotein cholesterol (LDL-C) concentration because it is not only the site of formation of very low density lipoproteins (VLDL), the precursors of most LDL in the circulation, but it is also the organ where the bulk of receptor-mediated clearance of LDL takes place. The liver also initially clears all the cholesterol that is absorbed from the small intestine. The absorption of excess cholesterol can potentially increase the amount of cholesterol stored in the liver. This, in turn, can result in increased VLDL secretion, and hence LDL formation, and also downregulation of hepatic LDL receptor activity. Such events will potentially increase plasma LDL-C levels. The converse situation occurs when cholesterol absorption is inhibited. Cholesterol enters the lumen of the small intestine principally from bile and diet. The major steps involved in the absorption process have been characterized. On average, about half of all cholesterol entering the intestine is absorbed, but the fractional absorption rate varies greatly among individuals. While the basis for this variability is not understood, it may partly explain why some patients respond poorly or not at all to statins and other classes of lipid-lowering drugs. There are few data relating to racial differences in cholesterol absorption. One study reported a significantly higher rate in African Americans compared with non-African Americans. Multiple lipid-lowering drugs that target pathways involving the absorption, synthesis, transport, storage, catabolism, and excretion of cholesterol are available. Ezetimibe selectively blocks cholesterol absorption and lowers plasma LDL-C levels by an average of 18%. When ezetimibe is coadministered with lower doses of statins, there is an additive reduction in LDL-C level, which equals the reduction achieved with maximal doses of statins alone. Dual inhibition of cholesterol synthesis and absorption is an effective new strategy for treating hypercholesterolemia.

Pharmacological and pharmacokinetic features and clinical effects of pitavastatin (Livalo Tablet)

Today 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors (statins) are the most often prescribed drugs among the therapeutics for hypercholesterolemia. Pitavastatin is a novel statin that has been developed entirely in Japan from the biological screening to clinical studies persuing more efficatious statin than hitherto known. Preclinical studies on drug metabolism revealed that pitavastatin is distributed selectively to the liver, excreted into bile without metabolic modification, and efficiently re-circulates to the liver to show a prolonged plasma half-life. In guinea pigs, pitavastatin enhanced hepatic LDL receptor activity and reduced VLDL secretion in a liver perfusion study, and it lowered plasma total cholesterol (TC) levels at 0.3 mg/kg and triglyceride (TG) levels at 1 mg/kg, respectively, and more. From these results, pitavastatin is assumed to lower LDL cholesterol (LDL-C) by promoting LDL receptor expression and further potentiate the cholesterol-lowering effect and exert TG-lowering effect by reducing VLDL secretion. (14)C-Pitavastatin is metabolized with CYP2C9 to 8-hydroxy derivative, but its Vmax /Km was about 2 micro l/min/mg, about 1/8 to 1/100 in comparison to the reported values of other statins, indicating that pitavastatin is hardly metabolized. Also, other human P450 species were not inhibited by pitavastatin. Therefore, pitavastatin is considered to have little interaction with drugs through P450. In the summarized clinical results with 862 patients, pitavastatin lowered TC and LDL-C by 28% and 40%, respectively. There was no difference in the frequency of side effects and no serious adverse effect was observed for pitavastatin. Pitavastatin possesses superior plasma lipid-improving effects, induces little drug interaction, and is expected to make a good contribution to the medication of hypercholesterolemia.

Trans fatty acids and cholesterol metabolism: mechanistic studies in rats and rabbits fed semipurified diets

Studies were conducted in rabbits and rats to investigate the effects of diets rich in oleic (CIS diet), palmitic (SAT diet) and trans fatty acids (TRANS diet) on plasma lipids and lipoprotein metabolism. An important difference between these species is that rabbits possess plasma cholesteryl ester transfer protein (CETP) activity while rats are devoid of transfer activity. In the presence of dietary cholesterol (0.2% w/w) the change in plasma low density lipoprotein-cholesterol (LDL-C) concentration from baseline was significantly higher in rabbits fed the TRANS diet compared with those fed the CIS diet (P < 0.01). Despite this difference, the hepatic LDL-receptor activity was similar in all groups. Also, the fatty acid composition of hepatic phospholipids was affected by diet with lower proportion of palmitic (11%) and higher (19%) linoleic acid despite a similar content in the diet. These effects may represent the maintenance of membrane fluidity within narrow limits to ensure optimal function. The studies in rats showed that the plasma total cholesterol concentration was 20% lower (P < 0.01) in TRANS-fed rats compared with those fed the CIS diet. The results of an in vivo assay of reverse cholesterol transport (RCT) suggested that the three diets gave rise to high density lipoprotein (HDL) particles with similar capacity to accept cellular cholesterol. The differential effects of dietary trans fatty acids in these animal models provide another line of evidence that reinforces the significant role of CETP activity in determining the distribution of plasma cholesterol in response to dietary trans fatty acids.

Detection of LDL receptor by ligand blotting with chylomicron remnants labelled with colloidal gold.

The LDL receptor plays a pivotal role in the clearance of pro-atherogenic lipoproteins, and LDL receptor deficiency may be the underlying cause of several primary and secondary dyslipidaemic conditions. Intervention strategies are often targeted to increase hepatic LDL receptor expression. It is difficult to quantitate hepatic LDL receptor activity and to monitor changes post-therapy. In order to avoid liver biopsy, human skin fibroblasts or circulating mononuclear cells have often been used as surrogate markers for the hepatic receptor. Fibroblasts, and particularly mononuclear cells, are relatively easy to isolate and can be stored for extensive lengths of time without significant loss of LDL receptor expression. Leucocytes or fibroblasts are normally probed with isotopically or gold-labelled LDL. However, the specific activity of the LDL conjugate is usually too low to enable accurate quantitation of differences, or changes, in LDL receptor expression. In this study, we describe an enhanced colloidal gold-labelling procedure for the detection of LDL receptor binding activity. The binding of colloidal gold-labelled chylomicron remnants to human hepatocytes (HepG2 cells) was compared with that of gold-conjugated LDL. Labelled remnants bound specifically to a cell surface protein with a molecular weight of approximately 130 kDa. Binding was blocked in the presence of unlabelled remnants, LDL, or antiserum specific to the LDL receptor. The binding of gold-labelled remnants was substantially greater than that of gold-labelled LDL. Compared with gold-labelled LDL, we found a much clearer demarcation of remnant binding with hepatocytes incubated in the presence or absence of sterols. Our observations suggest that, because of the greater affinity of the LDL receptor for lipoproteins containing apolipoprotein E, changes in LDL receptor expression might be more readily identified using gold-labelled remnants. We conclude that gold-conjugated chylomicron remnants might provide a useful means of detecting subtle changes in LDL receptor expression.

Dietary fat saturation effects on low-density-lipoprotein concentrations and metabolism in various animal models

Saturated vegetable oils (coconut, palm, and palm kernel oil) and fats (butter and lard) are hypercholesterolemic relative to monounsaturated and polyunsaturated vegetable oils. The increase in plasma low-density-lipoprotein-cholesterol (LDL-C) concentrations associated with consumption of saturated vegetable oils and fats is largely explained by a decrease in hepatic LDL receptor activity and an increase in the LDL-C production rate. Hepatic LDL receptor activity may be regulated by the messenger RNA concentration of the LDL receptor. The decrease in hepatic LDL receptor activity with saturated fat feeding is associated with decreased hepatic sterol O-acyltransferase activity and, therefore, a reduced inert pool of cholesteryl ester. A putative regulatory pool of cholesterol is increased with saturated fat feeding and suppresses LDL receptor activity, possibly through hepatic messenger RNA regulation. For most studies, an independent effect of a vegetable oil or fat could not be ascertained because there was no neutral control and at least two of the test oils or fats were varied. Animal data for the effects of individual fatty acids on plasma LDL-C concentrations and metabolism are sparse. The evidence suggests that caproic acid (6:0), caprylic acid (8:0), and capric acid (10:0) are neutral with respect to their LDL-C-raising properties and their ability to modulate LDL metabolism. Lauric acid (12:0), myristic acid (14:0), and palmitic acid (16:0) are approximately equivalent in their LDL-C-raising potential by reducing hepatic LDL receptor activity and increasing the LDL-C production rate, apparently via modulation of sterol O-acyltransferase activity. Stearic acid (18:0) appears to be neutral in its LDL-C-raising potential and how it affects LDL metabolism.

Theoretical considerations of what regulates low-density-lipoprotein and high-density-lipoprotein cholesterol

The concentration of cholesterol carried in low-density-lipoprotein cholesterol (LDL-C) is predominantly dictated by metabolic events occurring in liver. LDL-C is derived from the intravascular metabolism of very-low-density lipoproteins, and, in every species, this lipoprotein particle is predominantly cleared by liver through receptor-dependent mechanisms. In addition to cholesterol absorbed from the diet, sterol is also synthesized within the body and this synthesis occurs predominantly in extrahepatic organs. When the amount of cholesterol input into the body is increased, there is expansion of the pools of sterol within liver cells and down-regulation of the receptors responsible for clearing LDL-C from the bloodstream. As a consequence, the concentration of LDL-C in plasma increases. When dietary cholesterol intake is kept constant, some long-chain saturated fatty acids further suppress hepatic LDL receptor activity whereas several unsaturated fatty acids have the opposite effect. These regulatory events are apparently articulated through the ability of these fatty acids to shift intracellular cholesterol between a regulatory and a storage pool. High-density lipoproteins, in contrast, function primarily to move excess cholesterol from the extrahepatic organs to liver for excretion. Although the concentration of high-density-lipoprotein cholesterol in the plasma may be influenced by the rate of apolipoprotein A-I production or the activity of cholesterol ester transfer protein, it is less clear whether dietary long-chain fatty acids have any effect on these processes. The regulatory effects of the saturated and unsaturated long-chain fatty acids on LDL-C concentrations can be shown in a variety of experimental animals and also in humans.

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 LDL 1-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, LDL 2-C (1.045< d<1.063), HDL 2-C, and HDL 3-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 LDL 1-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.

Treatment of hyperlipidemia in renal transplant recipients.

Treatment of hyperlipidemia in renal transplant recipients.

Inhibitory action of natural compounds of microbial origin on cholesterol metabolism

1) Repeated administration of pravastatin significantly increased serum and liver cholesterol in rats. Hepatic LDL receptor activity was not changed and VLDL cholesterol secretion from the liver was increased. Net cholesterol synthesis in rat liver was increased after 7 days of repeated pravastatin administration. These results suggest that for rats, unlike other animals for which serum cholesterol is decreased, induced HMG-CoA reductase activity due to pravastatin treatment might overcome the inhibitory capability of pravastatin. 2) In the course of screening for squalene synthase inhibitors, novel zaragozic acids-F10863A, B, C and D-containing zaragozic acid D3 were isolated. F10863A was most potent and selectively inhibited cholesterol synthesis in freshly isolated rat hepatocytes among several cultured and isolated cells. It also showed in vivo serum cholesterol-lowering effects in hamsters and marmosets. However, the inhibition for squalene synthase proved to cause acidosis due to the accumulation of farnesol-derived dicarboxylic acids in urines. 3) A novel acyl-CoA: cholesterol acyltransferase (ACAT) inhibitor, designated epi-cochlioquinone A, a stereoisomer of cochlioquinone A, which has been previously reported as a nematocidal agent, was isolated from the fermentation broth of Stachybotrys bisbyi. It inhibited in vivo cholesterol absorption in rats by 50% at 75 mg/kg.

Cholesterol-lowering effect of YM-16638 in cynomolgus monkeys

YM-16638 was found in preclinical studies to be an orally active leukotriene antagonist. Because LY-171883, another leukotriene receptor antagonist with a similar structure to YM-16638, showed a triglyceride-lowering effect with a peroxisomal proliferative effect in monkeys fed a normal diet, we investigated whether YM-16638 also showed a serum triglyceride-lowering effect by examining serum and hepatic lipid levels in cynomolgus monkeys fed a normal diet supplemented with YM-16638 for 4 weeks at a daily dose of 3.75 mg (8.5 μmole), 30 mg (67.7 μmole) or 60 mg (135.4 μmole)/kg body weight. Monkeys given YM-16638 showed a dose-dependent decrease in serum total cholesterol. At 2 weeks of treatment, serum LDL- and HDL-cholesterol in the YM-16638 group showed marked decreases of 35% and 32%, respectively. However, serum triglyceride levels did not change. By contrast, hepatic cholesterol and cholesterol ester levels in this group were only slightly increased, without any effect on hepatic triglyceride level. In vitro investigation of the effect of YM-16638 on LDL-receptor activity and mRNA expression in the human hepatoma cell line HepG2 cells showed that YM-16638 increased LDL-receptor activity in a dose-dependent manner at 44 h of treatment. mRNA level in these cells was also increased 1.7-fold at 8 h of treatment. These results suggest that the decrease in serum cholesterol level in monkeys treated with YM-16638 may be due to an increase in hepatic LDL-receptor activity. Furthermore, they suggest that YM-16638 may represent a potent hypocholesterolemic drug without serious side effects. © 1996 Wiley-Liss, Inc.

Cholesterol lowering action of HOE 402 in the normolipidemic and hypercholesterolemic Golden Syrian hamster

The potent hypolipidemic activity of HOE 402 (4-amino-2-(4,4-dimethyl-2-oxo- l-imidazolidinyl)pyrimidine-5- N-(trifluoromethylphenyl)carboxamide monohydrochloride), which was previously demonstrated in rat and rabbit, was investigated in noncholesterol and cholesterol fed male hamsters. In normolipidemic hamsters fed a low cholesterol chow diet containing 0.10% or 0.15% HOE 402 for 3 weeks, the plasma total cholesterol level fell by 13% and 20% respectively, but no effect on hepatic total cholesterol content was detected. Hepatic sterol synthesis was increased 3-fold in hamsters fed 0.15% HOE 402. In hamsters fed a chow diet containing 0.25% cholesterol for 3 weeks, the plasma cholesterol level increased to 226 mg/dl (compared to 123 mg/dl in their chow fed controls) and the liver cholesterol content was 26.2 mg/g compared to 2.3 mg/g in the control group. However, 0.15% HOE 402 led to a 48% reduction and 0.20% HOE 402 to a 80% reduction, in total hepatic cholesterol concentration. There was a 43% fall in plasma cholesterol level being observed with the higher HOE 402 dose. Using the dual isotope plasma ratio method, no inhibition of intestinal cholesterol absorption by HOE 402 was found, either in the noncholesterol fed or in the cholesterol fed hamsters. Cholesterol feeding diminished the whole LDL animal clearance to 393 ± 17 μl/h per 100 g animal (control 666 ± 81 μl/h per 100 g). When treated with 0.20% HOE 402, the whole animal LDL clearance rate was enhanced 2.3-fold to 824 ± 66 μl/h per 100 g. In the hamsters fed 0.25% cholesterol alone whole liver LDL receptor activity was suppressed to 63 ± 5%, compared to that in the untreated controls (100%). The addition of 0.20% HOE 402 to the cholesterol enriched diet not only reversed this suppression, but resulted in a marked stimulation of liver receptor activity to 165 ± 15% (whole body LDL receptor activity 141 ± 10%). These results indicate that HOE 402 exerts its lipid lowering effect by a more direct activation on hepatic LDL receptor activity rather than by an indirect intestinal effect on cholesterol absorption.

High dose of fluvastatin sodium (XU62-320), a new inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, lowers plasma cholesterol levels in homozygous Watanabe-heritable hyperlipidemic rabbits

The effects of fluvastatin sodium (XU62-320), a new type of inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, on plasma cholesterol and triacylglycerol levels were investigated using homozygous Watanabe-heritable hyperlipidemic (WHHL) rabbit, an LDL-receptor-deficient animal which expresses a hepatic LDL receptor activity less than 5% that of control rabbits. Plasma levels of total, VLDL- and LDL-cholesterol were decreased profoundly after oral administration of fluvastatin at a dose of 50 mg/kg per day for 4 weeks. Plasma triacylglycerol levels were not affected by fluvastatin. Hepatic HMG-CoA reductase activity increased by 3-fold and hepatic LDL receptor activity increased by only 3.7-fold, as calculated by Scatchard plot analysis, with fluvastatin administration for 4 weeks, and the hepatic mRNA level for the rabbit LDL receptor was increased by 3-fold. Combined administration of fluvastatin (50 mg/kg per day) and cholestyramine, a bile acid sequestrant resin, at a level of 2% of the diet for 4 weeks more profoundly decreased plasma total, VLDL- and LDL-cholesterol levels with induction of hepatic cholesterol 7α-hydroxylase and no further induction of the hepatic LDL receptor. Plasma triacylglycerol levels were increased by the combination treatment. These results suggest that high dose of fluvastatin sodium is effective in lowering plasma cholesterol levels in homozygous WHHL rabbits through the shared mechanisms involving decrease in production and secretion of cholesterol from the liver and the induction of hepatic LDL receptor. Additional effect of cholestyramine on decrease in plasma cholesterol levels seems to be due to the further decrease in hepatic cholesterol secretion by up-regulation of hepatic cholesterol 7α-hydroxylase.

Parenchymal and nonparenchymal uptake of technetium-99m, indium-111, and iodine-125 low-density lipoprotein in the normal and estradiol-stimulated rat liver: Tracer validation for quantitative low-density lipoprotein scintigraphy

This study quantifies the parenchymal and nonparen-chymal uptake of technetium-99m (99mTc)- and indium-III (111In)-low-density lipoprotein (LDL) in different states of hepatic LDL-receptor activity to validate quantitative LDL scintigraphy. Iodine-125 (125I)-LDL was used as reference tracer. Four Sprague-Dawley rats with 17-alpha-ethinyl estradiol (EE)-stimulated LDL-receptor activity and five controls received all three tracers simultaneously 90 minutes before collagenase liver perfusion and metrizamide gradient cell separation. Total liver uptake of 99mTc-, 111In-, and 125I-LDL was 1.8 ± 1.0, 1.6 ± 0.8, and 0.2 ± 0.2% injected dose/g organ weight, respectively. The contribution of nonparenchymal cells to total hepatic tracer uptake was 5.4 ± 4.7%, 11.6 ± 10.3%, and 9.6 ± 7.6% in controls. Estradiol treatment increased total liver uptake to 2.4 ± 0.5, 2.0 ± 0.2, and 0.5 ± 0.3% injected dose/g and reduced nonparenchymal cell contribution to 2.3 ± 2.6%, 4.2 ± 4.8%, and 2.6 ± 2.9%, respectively. Dual-isotope scintigraphy in EE-treated and control rats confirmed these data, with a lower total hepatic uptake of 111In-LDL in comparison with 99mTc-LDL but a comparative degree of increase by EE treatment. Both behave quantitatively comparable as residu-alizing tracers, yet 99mTc-LDL shows a higher affinity to the LDL receptor pathway of parenchymal cells. However, the nonspecific uptake of both tracers can be neglected for quantitative LDL scintigraphy, and external imaging of hepatic tracer uptake primarily reflects LDL-receptor activity of parenchymal cells. (Hepatology 1995; 22:1289-1295.).

Parenchymal and nonparenchymal uptake of technetium 99m, indium-111, and iodine-125 low-density lipoprotein in the normal and estradiol-stimulated rat liver: Tracer validation for quantitative low-density lipoprotein scintigraphy

This study quantifies the parenchymal and nonparenchymal uptake of technetium-99m ( 99mTc)- and indium111 ( 111In)-low-density lipoprotein (LDL) in different states of hepatic LDL-receptor activity to validate quantitative LDL scintigraphy. Iodine-125 ( 125I)-LDL was used as reference tracer. Four Sprague-Dawley rats with 17-alpha-ethinyl estradiol (EE)-stimulated LDL-receptor activity and five controls received all three tracers simultaneously 90 minutes before collagenase liver perfusion and metrizamide gradient cell separation. Total liver uptake of 99mTc-, 111In-, and 125I-LDL was 1.8 ± 1.0, 1.6 ± 0.8, and 0.2 ± 0.2% injected dose/g organ weight, respectively. The contribution of nonparenchymal cells to total hepatic tracer uptake was 5.4 ± 4.7%, 11.6 ± 10.3%, and 9.6 ± 7.6% in controls. Estradiol treatment increased total liver uptake to 2.4 ± 0.5,2.0 ± 0.2, and 0.5 ± 0.3% injected dose/g and reduced nonparenchymal cell contribution to 2.3 ± 2.6%, 4.2 ± 4.8%, and 2.6 ± 2.9%, respectively. Dual-isotope scintigraphy in EE-treated and control rats confirmed these data, with a lower total hepatic uptake of 111In-LDL in comparison with 99mTcLDL but a comparative degree of increase by EE treatment. Both behave quantitatively comparable as residualizing tracers, yet 99mTc-LDL shows a higher affinity to the LDL receptor pathway of parenchymal cells. However, the nonspecific uptake of both tracers can be neglected for quantitative LDL scintigraphy, and external imaging of hepatic tracer uptake primarily reflects LDLreceptor activity of parenchymal cells

The mechanism of lack of hypocholesterolemic effects of pravastatin sodium, a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, in rats

In order to clarify the reason why pravastatin, a 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitor, did not show hypocholesterolemic effects in rats, the changes of various parameters affecting the serum cholesterol levels by pravastatin were determined in rats and rabbits, as a comparison. In rabbits, pravastatin administration at 50 mg/kg for 14 days decreased serum and liver cholesterol by 40% and 8%, respectively. The hepatic LDL receptor activity was increased 1.7-fold, and VLDL cholesterol secretion was decreased. Cholesterol 7α-hydroxylase activity was not changed. In contrast, in rats, serum cholesterol was increased by 14% at 50 mg/kg and 27% at 250 mg/kg for 7 days, respectively. At 250 mg/kg, liver cholesterol was significantly increased by 11%. Under these conditions, neither the hepatic LDL receptor activity nor cholesterol 7α-hydroxylase was changed, and VLDL cholesterol secretion was increased. At 250 mg/kg, net cholesterol synthesis in rat liver was increased after 7 days of consecutive administration. These results imply that in rats, stimulated net cholesterol synthesis caused the increase of liver cholesterol followed by the increase of VLDL cholesterol secretion, and resulted in the raise of plasma cholesterol. Although hepatic HMG-CoA reductase was induced almost the same fold in both animals at 50 mg/kg, the induced HMG-CoA reductase activity in rats might overcome the inhibitory capability of pravastatin, resulting in an increase of net cholesterol synthesis, but not in rabbits. This overresponse to pravastatin in rats might cause the lack of hypocholesterolemic effects of this drug.