Hepatic catabolism of low density lipoprotein: mechanisms and metabolic consequences

Lactoferrin and its hydrolysate bind directly to the oleate-activated form of the lipolysis stimulated lipoprotein receptor

The hepatic removal of triglyceride-rich chylomicrons during the postprandial phase represents an important step towards determining the bioavailability of dietary lipids amongst the peripheral tissues. Indeed, elevated postprandial lipemia is often associated with obesity and increased risk of coronary heart disease. The milk protein, lactoferrin, has been shown to inhibit hepatic chylomicron remnant removal by the liver, resulting in increased postprandial lipemia. Despite numerous studies on potential targets for lactoferrin, the molecular mechanisms underlying the effect of lactoferrin remain unclear. We recently demonstrated that the lipolysis stimulated lipoprotein receptor (LSR) contributes to the removal of triglyceride-rich lipoproteins during the postprandial phase. Here, we report that while lactoferrin does not have any significant effect on LSR protein levels in mouse Hepa1-6 cells, this protein colocalizes with LSR in cells but only in the presence of oleate, which is needed to obtain LSR in its active form as lipoprotein receptor. Ligand blotting using purified LSR revealed that lactoferrin binds directly to the receptor in the presence of oleate and prevents the binding of triglyceride-rich lipoproteins. Both C- and N-lobes of lactoferrin as well as a mixture of peptides derived from its hydrolysis retained the ability to bind LSR in its active form. We propose then that the elevated postprandial lipemia observed upon lactoferrin treatment in vivo is mediated in part by its direct interaction with free fatty acid activated LSR, thus preventing clearance of chylomicrons and their remnants through the LSR pathway.

Lipolysis stimulated lipoprotein receptor: a novel molecular link between hyperlipidemia, weight gain, and atherosclerosis in mice

The lipolysis-stimulated lipoprotein receptor, LSR, is a multimeric protein complex in the liver that undergoes conformational changes upon binding of free fatty acids, thereby revealing a binding site (s) that recognizes both apoB and apoE. Complete inactivation of the LSR gene is embryonic lethal in mice. Here we show that removal of a single LSR allele (LSR(-/+)) caused statistically significant increases in both plasma triglyceride and cholesterol levels, a 2-fold increase in plasma triglyceride changes during the post-prandial phase, and delayed clearance of lipid emulsions or a high fat meal. The longer postprandial lipoprotein clearance time observed in LSR(-/+) mice was further increased in LSR(-/+) mice lacking functional low density lipoprotein (LDL) receptors. LSR(-/+) mice placed on a Western-type diet displayed higher plasma triglycerides and cholesterol levels, increased triglyceride-rich lipoproteins and LDL, and increased aorta lipid content, as compared with control mice on the same diet. Furthermore, a direct correlation was observed between the hyperlipidemia and weight gain but only in the LSR(-/+) mice. Knockdown of LSR expression by small interfering RNA in mouse Hepa1-6 cells led to decreased internalization of both DiI-labeled cyclohexanedione-LDL and very low density lipoprotein in the presence of oleate. These data led us to conclude that LSR contributes to the physiological clearance of atherogenic triglyceride-rich lipoproteins and LDL. We propose that LSR cooperates with the LDL receptor in the final hepatic processing of apoB-containing lipoproteins and represents a novel therapeutic target for the treatment of hyperlipidemia associated with obesity and atherosclerosis.

Hepatic secretion of small lipoprotein particles in apobec-1-/- mice is regulated by the LDL receptor

Recent studies have examined the role of the LDL receptor (LDLR) in regulating murine hepatic lipoprotein production and apolipoprotein B (apoB) secretion, with divergent conclusions from in vivo versus in vitro approaches. We have re-examined this question, both in vivo and in vitro, using apobec-1-/- mice to model the pattern of human hepatic apoB-100 secretion. Hepatic triglyceride production in vivo (using Triton WR-1339) was unchanged in wild-type (WT) C57BL/6, apobec-1-/-, ldlr-/-, and [apobec-1-/-, ldlr-/-] mice, while apoB-100 production (using [35S]methionine incorporation) was increased > 2-fold in [apobec-1-/-, ldlr-/-] mice. Although > 90% of newly synthesized apoB floated within the d < 1.006 fraction of serum from all genotypes, fast-performance liquid chromatography separation revealed that nascent triglyceride-rich particles from [apobec-1-/-, ldlr-/-] mice, but not WT, apobec-1-/-, or ldlr-/- mice, distributed into smaller (intermediate and LDL-sized) particles. Studies in isolated hepatocytes from these different genotypes confirmed secretion of smaller particles exclusively from [apobec-1-/-, ldlr-/-] mice, and pulse-chase analysis demonstrated increased secretion of apoB-100 with virtual elimination of posttranslational degradation. These results directly support the suggestion that the LDLR regulates hepatic apoB-100 production and modulates secretion of small, triglyceride-rich particles, both in vivo and in vitro.

Fabry disease: preclinical studies demonstrate the effectiveness of alpha-galactosidase A replacement in enzyme-deficient mice

Preclinical studies of enzyme-replacement therapy for Fabry disease (deficient alpha-galactosidase A [alpha-Gal A] activity) were performed in alpha-Gal A-deficient mice. The pharmacokinetics and biodistributions were determined for four recombinant human alpha-Gal A glycoforms, which differed in sialic acid and mannose-6-phosphate content. The plasma half-lives of the glycoforms were approximately 2-5 min, with the more sialylated glycoforms circulating longer. After intravenous doses of 1 or 10 mg/kg body weight were administered, each glycoform was primarily recovered in the liver, with detectable activity in other tissues but not in the brain. Normal or greater activity levels were reconstituted in various tissues after repeated doses (10 mg/kg every other day for eight doses) of the highly sialylated AGA-1 glycoform; 4 d later, enzyme activity was retained in the liver and spleen at levels that were, respectively, 30% and 10% of that recovered 1 h postinjection. Importantly, the globotriaosylceramide (GL-3) substrate was depleted in various tissues and plasma in a dose-dependent manner. A single or repeated doses (every 48 h for eight doses) of AGA-1 at 0.3-10.0 mg/kg cleared hepatic GL-3, whereas higher doses were required for depletion of GL-3 in other tissues. After a single dose of 3 mg/kg, hepatic GL-3 was cleared for > or =4 wk, whereas cardiac and splenic GL-3 reaccumulated at 3 wk to approximately 30% and approximately 10% of pretreatment levels, respectively. Ultrastructural studies demonstrated reduced GL-3 storage posttreatment. These preclinical animal studies demonstrate the dose-dependent clearance of tissue and plasma GL-3 by administered alpha-Gal A, thereby providing the in vivo rationale-and the critical pharmacokinetic and pharmacodynamic data-for the design of enzyme-replacement trials in patients with Fabry disease.

Molecular cloning of a lipolysis-stimulated remnant receptor expressed in the liver

The lipolysis-stimulated receptor (LSR) is a lipoprotein receptor primarily expressed in the liver and activated by free fatty acids. Antibodies inhibiting LSR functions showed that the receptor is a heterotrimer or tetramer consisting of 68-kDa (alpha) and 56-kDa (beta) subunits associated through disulfide bridges. Screening of expression libraries with these antibodies led to identification of mRNAs derived by alternate splicing from a single gene and coding for proteins with molecular masses matching that of LSR alpha and beta. Antibodies directed against a synthetic peptide of LSR alpha and beta putative ligand binding domains inhibited LSR activity. Western blotting identified two liver proteins with the same apparent molecular mass as that of LSR alpha and beta. Transient transfections of LSR alpha alone in Chinese hamster ovary cells increased oleate-induced binding and uptake of lipoproteins, while cotransfection of both LSR alpha and beta increased oleate-induced proteolytic degradation of the particles. The ligand specificity of LSR expressed in cotransfected Chinese hamster ovary cells closely matched that previously described using fibroblasts from subjects lacking the low density lipoprotein receptor. LSR affinity is highest for the triglyceride-rich lipoproteins, chylomicrons, and very low density lipoprotein. We speculate that LSR is a rate-limiting step for the clearance of dietary triglycerides and plays a role in determining their partitioning between the liver and peripheral tissues.

Biliary lipid composition in heterozygous familial hypercholesterolemia and influence of treatment with probucol

The lipid composition of fasting duodenal bile was determined in 11 healthy subjects with normolipidemia and 15 patients with heterozygous familial hypercholesterolemia (FH) (12 with type IIa, three with type IIb). The age distribution among the groups of subjects was similar. In the patients with heterozygous FH type IIa, the mean value for molar percentage of cholesterol and lithogenic index (LI) of bile were significantly higher than those of controls (8.4 +/- 1.0%, 1.47 +/- 0.18 calculated by Hegard, Dam, and Holzbach vs 4.3 +/- 0.4%, 0.81 +/- 0.07, respectively). The value of LI in the patients with FH type IIb was also found to be significantly higher than that of the controls. In the patients with heterozygous FH type IIa, we observed both a significant decrease in the molar percentages of glycochenodeoxycholic acid, glycoursodeoxycholic acid, and glycolithocholic acid, and a significant increase of taurochenodeoxycholic acid compared to the corresponding values in the controls. Bile analysis of six patients was reexamined during probucol treatment after 16 weeks. Probucol significantly lowered serum cholesterol levels. However, biliary lipid composition and individual bile acid proportions was not altered by the treatment. The results suggest that most of the patients with heterozygous FH have supersaturated bile and are predisposed to cholesterol gallstone formation. In addition, the mechanism by which probucol lowers serum cholesterol appears to be independent of any change in the metabolism of biliary lipid.

Evidence for low-density lipoprotein receptor-mediated uptake of benzoporphyrin derivative

Plasma lipoproteins, such as low-density lipoprotein (LDL), have been proposed to enhance the delivery of hydrophobic photosensitisers to malignant tissue since tumour cells have been shown to have increased numbers of LDL receptors. We have investigated the role of this receptor in the cellular accumulation of the photosensitiser benzoporphyrin derivative (BPD). We observed that: (1) [14C]BPD-LDL accumulation by LDL receptor-negative fibroblast cell lines was insignificant compared with normal cell lines; (2) there was no evidence that BPD dissociated from LDL during incubation with the cells; and (3) chemical acetylation of LDL markedly decreased the uptake of [14C]BPD-LDL. We conclude, therefore, that virtually all of the photosensitiser accumulated by the cells was due to specific binding and internalisation via the LDL receptor. Subsequent in vivo studies in M-1 (methylcholanthrene-induced rhabdomyosarcoma) tumour-bearing DBA/2J mice showed that tumour accumulation of BPD associated with native LDL was significantly (P < 0.01) enhanced over that of acetyl-LDL-associated BPD. These results indicate that the LDL receptor is responsible for the accumulation of LDL-associated BPD both in vitro and in vivo. Thus, utilisation of this delivery system may provide for improvements in photodynamic therapy in clinical practice.

Time course of 125I-labeled LDL accumulation in the healing, balloon-deendothelialized rabbit aorta

We previously showed by qualitative en face autoradiography that after 24 hours of circulation, 125I-labeled low density lipoprotein (LDL) injected in tracer amounts accumulated focally at the edges of regenerating endothelial islands in the balloon catheter-deendothelialized aorta of the normocholesterolemic rabbit. In the present study with the same animal model, we have used quantitative autoradiography to examine 125I-LDL accumulation in the healing aorta as a function of LDL circulation time from 2.5 to 40 hours. The results demonstrated that 125I-LDL accumulation in the healing aorta occurred in two kinetically and biochemically distinct compartments, one of which was in equilibrium with plasma and one of which sequestered LDL. LDL accumulation in the still-deendothelialized aorta (DEA) was diffuse and only moderately intense on autoradiography. It peaked 4 hours after injection; over the following 36 hours the disappearance of 125I-LDL from DEA paralleled the disappearance of 125I-LDL from plasma. In contrast, accumulation of 125I-LDL at the edges of regenerating endothelial islands was focal and intense. LDL accumulation in this compartment also peaked 4 hours after injection but remained elevated even at 40 hours, despite falling plasma levels of LDL. At 24 hours, edge LDL accumulation per unit area was more than five times greater than DEA accumulation. The data indicate that LDL accumulation in specific compartments of the functionally modified arterial wall occurs independently of either acute or chronic hypercholesterolemia. The contrast between labile LDL accumulation in DEA and persistent accumulation at the edges of healing aortic islands indicates that LDL accumulation in the two areas must involve different processes within the arterial wall itself.

Regulation of LDL receptor, apoB, and apoE protein and mRNA in Hep G2 cells

The regulation of low-density lipoprotein (LDL) receptor activity, protein synthesis, and cellular mRNA content was evaluated in the human hepatoma cell line Hep G2. Incubation of the cells with LDL led to a complete downregulation of LDL receptor mRNA and LDL receptor protein synthesis. This LDL regulation of the LDL receptor and its mRNA was both time- and concentration-dependent. In contrast to protein synthesis and cellular mRNA concentrations of the LDL receptor, which were reduced to undetectable levels by prolonged incubation in the presence of LDL, LDL receptor activity was reduced to only 44% of preincubation levels. These findings support the presence of a second metabolic pathway for LDL uptake in human hepatocytic cells. The effect of LDL on cellular LDL receptor expression was specific for LDL because incubation in the presence of HDL did not affect any of these study end points. The potential coordinate regulation of the expression of the LDL receptor with its principal ligands, apolipoproteins (apo) B and E, was also investigated. In contrast to the LDL receptor mRNA downregulation with LDL incubation, cellular apoB and apoE mRNA concentrations were not affected by either LDL or HDL. Secretion of apoB, however, was significantly increased by incubating Hep G2 cells with LDL. These findings indicate that, in contrast to LDL receptor which is regulated at the mRNA level, the ligands for the LDL receptor are regulated either co- or post-translationally.

Evidence for sterol-independent regulation of low-density lipoprotein receptor activity in Hep-G2 cells

The relationship between the serum factor(s)-mediated induction of low-density lipoprotein (LDL) receptor activity and changes in cellular cholesterol metabolism was examined in the human hepatoma cell line Hep-G2. Relative to incubation with serum-free media [Eagle's minimal essential medium (MEM) control], short-term (less than 8 h) incubation with medium containing 15% of either calf serum (MEM + serum) or the d greater than 1.25 fraction of calf serum (MEM + d greater than 1.25) produced a time- and concentration-dependent increase in the uptake of 125I-LDL. Immunoblotting with anti-(LDL receptor) antibodies demonstrated that this was correlated with a 2-fold increase in the amount of the mature 136,000 Da LDL receptor protein in detergent-solubilized Hep-G2 cell membranes. Incubation with MEM + serum, but not MEM + d greater than 1.25, increased the efflux of radiolabelled cholesterol from Hep-G2 cells. However, the induction of 125I-LDL uptake by MEM + d greater than 1.25 (2.3-fold) and MEM + serum (2.2-fold) was virtually identical. Addition of the d less than 1.063 lipoproteins of calf serum to MEM + d greater than 1.25 at their original or three times their serum concentration decreased the induction of 125I-LDL uptake by MEM + d greater than 1.25 by only 20-30%. Together, these results suggest that the stimulation of 125I-LDL uptake was not due to the presence of high-density lipoprotein, the absence of LDL or the stimulation of cholesterol efflux. MEM + serum stimulated 125I-LDL uptake in cells cholesterol-loaded by incubation with rat very-low-density lipoprotein with beta electrophoretic mobility (beta-VLDL). Compared to incubation with the MEM control, either MEM + serum or MEM + d greater than 1.25 produced time-dependent increases in the activity of 3-hydroxy-3-methylglutaryl-CoA reductase which also occurred in cholesterol-loaded cells. However, cholesterol biosynthesis, whether measured from 3H2O, [14C]acetate or [3H]mevalonic acid, was not increased. Incubation with MEM + serum or MEM + d greater than 1.25 did not affect [3H]oleate incorporation into cellular cholesteryl esters, hydrolysis of intracellular [3H]cholesteryl esters or the cellular mass of unesterified or esterified cholesterol. Incubation with MEM + serum or MEM + d greater than 1.25 produced a transient increase in the level of LDL receptor mRNA, reaching a maximum of 5-10-fold by 2 h and decreasing to near baseline levels by 4 h. Actinomycin D blocked the serum-factor-mediated induction of LDL receptor mRNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Modeling of biological and medical systems: a systemic strategy

A strategy in the frame of General System Theory is proposed for the study of biological systems for medical purposes. Its definition and use requires in each experiment the collaboration between physician and system scientists and hence the definition of a common language, by which the real system under study is described. The strategy is based on three intermingled steps: first an ingenuous model is proposed, gathering all the medical knowledge about the studied system, organized within an informal frame derived from the state space approach. Next, a functional model is derived, enlightening the organization of the relations in the medical model. Finally, this organization is formalized by the most suitable algebraic tools, which are thereafter translated into APL programs. This last version is used for simulation, which is exploited not only as a tool to describe and make provisions on the dynamics of the models, but also to deepen and improve the knowledge about the observed system.

Role of a calf thymus preparation in the degradation of native and reductively methylated low density lipoprotein

1. The clearance of low density lipoprotein (LDL) is mediated by a specific LDL receptor pathway and by an alternative metabolic pathway that is responsible for the receptor-independent LDL catabolism. 2. This alternative catabolism can be studied in vivo using a preparation of chemically modified LDL that are reductively methylated. 3. Recently we showed that a calf thymus protein extract affects the cholesterol metabolism via activation of LDL catabolism. 4. The aim of this study was to investigate whether in vivo the specific LDL receptor pathway and the independent LDL receptor pathway are affected by thymus treatment. 5. The results obtained injecting in rats native and chemically modified 125I-LDL to probe the receptor independent pathway, show that the thymus gland decreases serum cholesterol by activation of the specific LDL receptor pathway. 6. This effect is mainly evident in liver and kidney that represent organs in which the specific LDL receptors are widely present.

Dyslipoproteinaemia of liver disease

Hepatic diseases differ from most other causes of secondary dyslipidaemia in that the circulating lipoproteins are not only present in abnormal amounts but they frequently also have abnormal composition, electrophoretic mobility and appearance. Pre-beta and alpha bands can be absent on electrophoresis in all types of liver disease although material in the VLDL and HDL ranges can be isolated in the ultracentrifuge. Cholestatic liver disease has been the most extensively studied and the hyperlipidaemia can be extreme with marked elevations of free cholesterol and phospholipids. This results largely from the presence of LP-X, an abnormal LDL, with a vesicular structure that appears in rouleaux formation under the electron microscope. It is virtually specific for cholestasis and familial LCAT deficiency. The LDL, however, is heterogeneous and may also contain a large triglyceride-rich particle (LP-Y) as well as more normal-looking particles, which are none the less depleted in cholesteryl esters and rich in triglycerides. Indeed, when patients with cholestasis are hypertriglyceridaemic the excess triglyceride is to be found predominantly in these two LDL fractions rather than in VLDL. HDL in cholestasis may contain disc-like particles, similar to those newly secreted by the liver and intestine, as well as more normal-looking spherical particles. In extrahepatic obstruction concentrations of HDL and its major apolipoproteins, apoAI and apoAII, are frequently reduced, although a subfraction rich in apoE is often found. In all but the latest stages of chronic intrahepatic cholestasis due to primary biliary cirrhosis, however, HDL, especially HDL2, concentrations are increased, probably due to the presence of a circulating inhibitor of HL. Many of these lipoprotein changes found in cholestasis resemble those of familial LCAT deficiency, although the hyperlipidaemia is not usually so severe in the latter condition. Indeed, in patients with cholestasis but well-preserved LCAT activity many of the characteristic lipoprotein changes, such as LP-X, LP-Y and discoidal HDL, may not be seen. In acute hepatocellular disease, such as alcoholic or viral hepatitis, it is not unusual for the patient to go through a cholestatic phase and many of the same lipoprotein changes may be seen. In cirrhosis without cholestasis the patients are not usually significantly hyperlipidaemic and in advanced cases cholesterol and apoB levels may be reduced. Although LCAT activity and the proportion of plasma cholesterol esterified may also be markedly reduced, LP-X is not usually seen, possibly because the flux of free cholesterol and phospholipid (lecithin), the LCAT substrates, is relatively low. Discoidal HDLs are often present.(ABSTRACT TRUNCATED AT 400 WORDS)

Low density lipoprotein receptor-binding activity in human tissues: quantitative importance of hepatic receptors and evidence for regulation of their expression in vivo

The heparin-sensitive binding of 125I-labeled low-density lipoprotein (LDL) to homogenates from 18 different normal human tissues and some solid tumors was determined. The binding to adrenal and liver homogenates fulfilled criteria established for the binding of LDL to its receptor--namely, (i) saturability, (ii) sensitivity to proteolytic destruction, (iii) inhibition by EDTA, and (iv) heat sensitivity. When the binding of 125I-labeled LDL was assayed at a constant concentration (50 micrograms/ml), the adrenal gland and the ovary had the highest binding of normal tissues. The highest binding per g of tissue overall was obtained in homogenates of a gastric carcinoma and a parotid adenoma. When the weights of the parenchymatous organs were considered, the major amount of LDL receptors was contained in the liver. To study the possible regulation of hepatic LDL-receptor expression, 11 patients were pretreated with cholestyramine (8 g twice a day for 3 weeks). Increased binding activity (+105%, P less than 0.001) was obtained in homogenates from liver biopsies from the cholestyramine-treated patients as compared with 12 untreated controls. It is concluded that the liver is the most important organ for LDL catabolism in humans and that the receptor activity in this organ can be regulated upon pharmacologic intervention. Further studies are needed to confirm the possibility that certain solid tumors can exhibit high numbers of LDL receptors.

The genetic dyslipoproteinemias--nosology update 1990

The number of discrete disorders of lipid transport is growing. Concomitantly, the classification of the disorders is changing, from one based on altered concentrations of lipoproteins, to one based on current understanding of the genetics of the disorders and of lipoprotein biochemistry and physiology. Many disorders are now traceable to deficiencies of essential proteins such as apolipoproteins, enzymes, lipid transfer proteins and cellular receptors.

Binding and uptake of native and modified low-density lipoproteins by human hepatocytes in primary culture

The binding and uptake of native low-density lipoproteins and malondialdehyde-treated low density lipoproteins by human hepatocytes in primary culture has been analyzed. Experiments with 125I-labeled malondialdehyde-treated low-density lipoproteins showed that cultured liver cells took up and degraded malondialdehyde-treated low-density lipoproteins, but the cell type(s) responsible for this action remain unclear. Immunofluorescent visualization of receptor-bound low-density lipoproteins revealed that low-density lipoprotein binding sites were distributed on the surface of nearly all cells of the culture. Binding sites for malondialdehyde-treated low-density lipoproteins were found in only 5% of the cultured cells, and these cells differed from hepatocytes in shape and size. Cultured hepatocytes internalized and native low-density lipoproteins, but not malondialdehyde-treated low-density lipoproteins, labeled with the fluorescent dye 3',3'-dioctadecylindocarbocyanine. About 15% of the cells that take up 3',3'-dioctadecylindocarbocyanine-labeled malondialdehyde-treated low-density lipoproteins could be identified as liver endothelial cells and macrophages, since they internalized formaldehyde-treated human albumin and fluorescent carboxylated microspheres. Our results indicate that human hepatocytes in primary culture express surface receptors for native low-density lipoproteins but not for modified low-density lipoproteins.

Processing of cholesteryl ester from low-density lipoproteins in the rat. Hepatic metabolism and biliary secretion after uptake by different hepatic cell types

Biliary secretion of the cholesteryl ester moiety of (modified) low-density lipoprotein (LDL) was examined under various experimental conditions in the rat. Human LDL or acetylated LDL (acetyl-LDL), radiolabelled with [3H]cholesteryl oleate, was administered intravenously to unanesthetized rats equipped with permanent catheters in the bile duct, duodenum and heart. LDL was cleared relatively slowly from plasma, mainly by Kupffer cells. At 3 h after injection, only 0.9% of the radioactivity was found in bile; after 12 h this value was 4.5%. Uptake of LDL by hepatocytes was stimulated by treatment of the rats with 17 alpha-ethinyloestradiol (EE; 5 mg/kg for 3 successive days); this resulted in a more rapid secretion of radioactivity into bile, 3.9% and 12.4% after 3 h and 12 h respectively. The extremely rapid uptake of acetyl-LDL via the scavenger pathway, mainly by endothelial cells, resulted in the secretion of only 2.1% of its 3H label into bile within 3 h, and 9.5% within 12 h. Radioactivity in bile was predominantly in the form of bile acids; only a small part was secreted as free cholesterol. However, the specific radioactivity of biliary cholesterol was higher than that of bile acids in all three experimental conditions. EE-treated animals did not form cholic acid from [3H]cholesteryl oleate, which was a major product of the cholesteryl oleate from LDL and acetyl-LDL in untreated rats, but formed predominantly very polar bile acids, i.e. muricholic acids. It is concluded that uptake of human LDL or acetyl-LDL by the liver of untreated rats is not efficiently coupled to biliary secretion of cholesterol (bile acids). This might be due to the anatomical localization of their principal uptake sites, the Kupffer cells and the endothelial cells respectively. Induction of LDL uptake by hepatocytes by EE treatment warrants a more efficient disposition of cholesterol from the body via bile.

Some metabolic characteristics of low-density lipoprotein subfractions, LDL-1 and LDL-2: in vitro and in vivo studies

Two low-density lipoprotein subfractions, LDL-1 and LDL-2, with density ranges of respectively 1.023-1.034 and 1.036-1.041 g/ml, were isolated by aspiration after density gradient ultracentrifugation of human pooled serum. In vitro interactions of both LDL subfractions with the LDL receptor of human cultured fibroblasts, human hepatoma cell line Hep G2 and human hepatocytes were compared. No difference in association (binding and internalization) nor in degradation between LDL-1 and LDL-2 by these cells was found. However, kinetic studies in guinea pigs showed that LDL-2 disappeared faster from the circulation and accumulated to a greater extent in the liver, compared to LDL-1. Thus, we were unable to show a difference in the LDL receptor-mediated uptake of both LDL subfractions by various cells in vitro. The results obtained in vivo suggest that LDL-1 is more atherogenic than LDL-2, because its longer half-life renders the particle more susceptible to uptake by the scavenger LDL receptor on macrophages.

Stimulation of LDL receptor activity in Hep-G2 cells by a serum factor(s)

The regulation of low-density lipoprotein (LDL) receptor activity in the human hepatoma cell line Hep-G2 by serum components was examined. Incubation of dense monolayers of Hep-G2 cells with fresh medium containing 10% fetal calf serum (FM) produced a time-dependent increase in LDL receptor activity. Uptake and degradation of 125I-LDL was stimulated two- to four-fold, as compared with that of Hep-G2 cells cultured in the same media in which they had been grown to confluence (CM); the maximal 125I-LDL uptake plus degradation increased from 0.2 microgram/mg cell protein/4 h to 0.8 microgram/mg cell protein/4 h. In addition, a two-fold increase in cell surface binding of 125I-LDL to Hep-G2 cells was observed when binding was measured at 4 degrees C. There was no change in the "apparent" Kd. The stimulation of LDL receptor activity was suppressed in a concentration-dependent manner by the addition of cholesterol, as LDL, to the cell medium. In contrast to the stimulation of LDL receptor activity, FM did not affect the uptake or degradation of 125I-asialoorosomucoid. Addition of FM increased the protein content per dish, and DNA synthesis was stimulated approximately five-fold, as measured by [3H]thymidine incorporation into DNA; however, the cell number did not change. Cellular cholesterol biosynthesis was also stimulated by FM; [14C]acetate incorporation into unesterified and esterified cholesterol was increased approximately five-fold. Incubation of Hep-G2 cells with high-density lipoproteins (200 micrograms protein/ml) or albumin (8.0 mg/ml) in the absence of the serum factor did not significantly increase the total processed 125I-LDL. Stimulation of LDL receptor activity was dependent on a heat-stable, nondialyzable serum component that eluted in the inclusion volume of a Sephadex G-75 column. Uptake of 125I-LDL by confluent monolayers of human skin fibroblasts was not changed by incubation with FM or by incubation with Hep-G2 conditioned medium. Taken together, these data demonstrate that LDL receptor activity in Hep-G2 cells is stimulated by a serum component. Furthermore, this serum factor shows some specificity for the LDL receptor pathway in liver-derived Hep-G2 cells.

Modulation of bile secretion by hepatic low-density lipoprotein uptake and by chenodeoxycholic acid and ursodeoxycholic acid treatment in the hamster

The effects of both apolipoprotein B,E receptor-dependent and receptor-independent uptake of low-density lipoprotein (LDL) in the liver on bile secretion were studied in bile fistula hamsters. Three groups of animals were studied after 4 wk of feeding either a control, chenodeoxycholic acid-, or ursodeoxycholic acid-containing diet. The hepatic receptor-dependent and receptor-independent uptake of LDL was related to both bile flow and biliary lipid secretion. The correlation with bile flow and biliary lipid secretion was positive for the receptor-dependent, but negative for the receptor-independent uptake of LDL. Although the receptor-mediated LDL uptake appeared to exert a strong influence on bile acid-independent bile flow, the receptor-independent uptake showed a significant relation with biliary bile acid excretion. Differences between the two mechanisms of LDL uptake were also evident in the biliary bile acid-cholesterol coupling, which was significantly stronger during receptor-independent than during receptor-dependent uptake of LDL. The effects of LDL uptake on bile secretion were modulated by the experimentally induced changes in both the content and composition of bile acids in the enterohepatic circulation.

Role of the liver for the receptor-mediated catabolism of low-density lipoprotein in the 17 alpha-ethinylestradiol-treated rat

Effects of ethinylestradiol on LDL catabolism and plasma cholesterol were studied in the rat. Hormone treatment led to a reduction in plasma cholesterol and to an increased receptor-mediated catabolism of LDL in the liver and the adrenals. The liver was the most important organ accounting for more than 95% of the receptor-mediated tissue accumulation of LDL following estrogen treatment. It is concluded that the reduction in plasma cholesterol during treatment with ethinylestradiol is due to the stimulation of LDL receptors in the liver.

Acetaldehyde modification of low density lipoprotein accelerates its catabolism in man

Acetaldehyde (AcA), the first metabolite in ethanol oxidation, is chemically highly reactive and binds covalently to the free amino groups of various proteins. In this study, we examined the metabolism of acetaldehyde-modified LDL (AcA-LDL) in man. LDL was isolated from human volunteers, radiolabelled with either 125I or 131I, incubated in various AcA concentrations (Aca-LDL) and injected back into the donors simultaneously with LDL incubated in identical conditions but omitting AcA (C-LDL). Acetaldehyde treatment did not change the chemical composition, electrophoretic mobility or the flotation characteristics of LDL. The proportion of free amino groups of AcA-LDL, ranging from 97 to 54.5%, was negatively correlated with the final concentration of AcA used in the incubation medium (r = -0.99, P less than 0.001). AcA modification of LDL accelerated its in vivo catabolism in man in such a way that the fractional catabolic rate (FCR) for AcA-LDL was negatively correlated with the percentage of free amino groups in AcA-LDL (r = -0.87, P less than 0.01). The clearance of AcA-LDL modified in 0.4, 2.0, 4.0 and 8.0 mM AcA was 0.9, 1.4, 2.5 and 3.7 times faster than the clearance of C-LDL, respectively. If AcA-LDL is formed in man after ethanol ingestion, its rapid clearance may be one possible mechanism for the low LDL levels observed in chronic alcohol users.

Impaired hepatocyte binding, uptake and degradation of glucosylated low-density lipoproteins

The catabolism of low-density lipoproteins (LDL), the major cholesterol-carrying lipoproteins in plasma, is mediated in part via a high-affinity uptake pathway in the liver. Non-enzymatic glucosylation of lysine residues of apolipoprotein B, the major protein of LDL, blocks receptor-mediated uptake of LDL by fibroblasts and endothelial cells. We investigated the effect of the degree of glucosylation on the binding, uptake and degradation of radioiodinated LDL by the human hepatoma cell line Hep G2. Human LDL was glucosylated with 250 mM glucose and 30 mM cyanoborohydride at 37 degrees C. Incubations ranging from 3 to 48 h in duration resulted in the formation of 6-27% of glucitol-lysine adducts as demonstrated by coincubation with [14C]glucose. The degree of glucose incorporation corresponded to the extent of inhibition of binding, uptake and degradation of LDL (10-90%). The data are consistent with the view that glucosylation of LDL markedly impairs their catabolism. This phenomenon may be related to the pathophysiology of the premature atherosclerosis observed in diabetes mellitus.

Metabolism of low-density lipoproteins by cultured hepatocytes from normal and homozygous familial hypercholesterolemic subjects

The profoundly elevated concentrations of low-density lipoproteins (LDL) present in homozygous familial hypercholesterolemia lead to symptomatic cardiovascular disease and death by early adulthood. Studies conducted in nonhepatic tissues demonstrated defective cellular recognition and metabolism of LDL in these patients. Since mammalian liver removes at least half of the LDL in the circulation, the metabolism of LDL by cultured hepatocytes isolated from familial hypercholesterolemic homozygotes was compared to hepatocytes from normal individuals. Fibroblast studies demonstrated that the familial hypercholesterolemic subjects studied were LDL receptor-negative (less than 1% normal receptor activity) and LDL receptor-defective (18% normal receptor activity). Cholesterol-depleted hepatocytes from normal subjects bound and internalized 125I-labeled LDL (Bmax = 2.2 micrograms LDL/mg cell protein). Preincubation of normal hepatocytes with 200 micrograms/ml LDL reduced binding and internalization by approx. 40%. In contrast, 125I-labeled LDL binding and internalization by receptor-negative familial hypercholesterolemic hepatocytes was unaffected by cholesterol loading and considerably lower than normal. This residual LDL uptake could not be ascribed to fluid phase endocytosis as determined by [14C]sucrose uptake. The residual LDL binding by familial hypercholesterolemia hepatocytes led to a small increase in hepatocyte cholesterol content which was relatively ineffective in reducing hepatocyte 3-hydroxy-3-methylglutaryl-CoA reductase activity. Receptor-defective familial hypercholesterolemia hepatocytes retained some degree of regulatable 125I-labeled LDL uptake, but LDL uptake did not lead to normal hepatocyte cholesterol content or 3-hydroxy-3-methylglutaryl-CoA reductase activity. These combined results indicate that the LDL receptor abnormality present in familial hypercholesterolemia fibroblasts reflects deranged hepatocyte LDL recognition and metabolism. In addition, a low-affinity, nonsaturable uptake process for LDL is present in human liver which does not efficiently modulate hepatocyte cholesterol content or synthesis.

The expressed human hepatic receptor for low-density lipoproteins differs from the fibroblast low-density lipoprotein receptor

The role of the cellular receptor for the low-density lipoproteins (LDL) in cholesterol transport was initially defined through the study of nonhepatic cells in vitro. Since the liver is central in plasma lipoprotein metabolism, an investigation of hepatic lipoprotein receptors is important for understanding normal lipoprotein transport. Utilizing human hepatic and fibroblast membranes, the characteristics of receptors for LDL from hepatic and nonhepatic tissues were directly compared. Human hepatic membranes reversibly bound LDL within 5 min. Although both fibroblast and hepatic membranes saturably bound LDL at 37 degrees C, the fibroblast LDL receptor affinity (Kd = 2.5 X 10(-8) M) and number (5.5 X 10(12) sites/mg membrane protein) were greater than the hepatic receptor affinity (Kd = 10.8 X 10(-8) M) and number (0.5 X 10(12) sites/mg membrane protein). In contrast to the fibroblast LDL receptor which was unable to bind LDL in the presence of EDTA, the hepatic LDL receptor binding of LDL was only partially blocked by EDTA. The binding of LDL to its hepatic receptor is highly temperature-dependent, and studies utilizing both radiolabeled LDL and colloidal gold-labeled LDL indicate that little, if any, binding of LDL hepatic membranes occur at 0-4 degrees C. The hepatic membrane receptor(s) (Mr approximately equal to 270 000 and 330 000) differ from that of the fibroblast LDL receptor (Mr approximately equal to 130 000) and these proteins are present in hepatic membranes from a patient lacking the fibroblast LDL receptor. These data indicate that an expressed hepatic LDL receptor has unique properties different from those of the fibroblast LDL receptor and that the expressed protein(s) is genetically distinct from the fibroblast receptor.

Receptor-independent low-density lipoprotein catabolism

Cultured cells, animal models, and man all possess mechanisms for LDL degradation that do not require the agency of the high-affinity receptor. In healthy individuals their functional significance can be determined using LDL which has been modified by specific chemical reactions designed to inhibit it receptor binding. Although the precise nature of the pathways has not been defined, there is evidence to implicate the monocyte-macrophage system in the process.

Receptor-independent low density lipoprotein transport in the rat in vivo. Quantitation, characterization, and metabolic consequences

Receptor-independent low density lipoprotein (LDL) transport plays a critical role in the regulation of plasma cholesterol levels; hence, these studies were done to characterize this process in the tissues of the rat. High rates of receptor-independent clearance were found in the spleen, but other organs, like liver, gastrointestinal tract, and endocrine glands manifested lower clearance rates that varied from 3 to 9 microliter/h per g, while the rates in nervous tissue, muscle, and adipose tissue were less than 1 microliter/h per g. Receptor-dependent uptake was much higher in liver (85 microliter/h per g) and adrenal gland (219 microliter/h per g), but was also low in most other tissues. At normal plasma LDL concentrations, 67% of the receptor-dependent transport in the whole animal was accounted for by LDL uptake in the liver. In contrast, the receptor-independent uptake found in the whole animal took place in many organs, including skeletal muscle (20%), liver (16%), small bowel (15%), skin (10%), and spleen (7%). Furthermore, in liver, the rate of cholesterol synthesis could be varied 11-fold, yet the rate of receptor-independent LDL clearance remained constant at approximately 8 microliter/h per g. When the circulating levels of LDL were systematically increased, receptor-independent LDL clearance also remained constant, so that hepatic LDL-cholesterol uptake by this mechanism increased linearly, from 1 to 20 micrograms/h per g, as the plasma LDL-cholesterol level was increased from 10 to 250 mg/dl. Finally, when equal amounts of LDL-cholesterol were delivered into the liver by either the receptor-dependent or receptor-independent mechanism, there was significant suppression of cholesterol synthesis and an increase in cholesteryl esters. Thus, in any situation in which receptor-dependent LDL degradation is lost, cholesterol balance in the whole animal and across individual organs is maintained by receptor-independent mechanisms, although when the new steady state is achieved, circulating levels of LDL must necessarily be very much increased.

LDL receptors in bovine tissues assayed as the heparin-sensitive binding of 125I-labeled LDL in homogenates: relation between liver LDL receptors and serum cholesterol in the fetus and post term

The heparin-sensitive binding of 125I-labeled LDL in homogenates of bovine tissues was determined using a membrane filter assay. The binding fulfilled several criteria which have been established for the binding of LDL to its receptor, namely: saturability, dependence on Ca2+, sensitivity to proteolytic destruction and heat sensitivity. The adrenal cortex and the active corpus luteum exhibited the highest binding activity of the 22 different tissues assayed. Tissues from the central nervous system had low binding activity. Livers from fetal animals had higher binding than livers from young and adult animals and the binding of 125I-LDL to fetal liver homogenates showed an inverse correlation to the serum cholesterol levels, indicating that the LDL receptors in fetal liver may play a role in the regulation of the serum cholesterol level in the fetus during gestation. After birth, the binding of 125I-LDL to calf liver homogenates decreased to levels found in adult animals and this was paralleled by an increase of total serum cholesterol, suggesting that the rapid rise in serum cholesterol in mammals observed soon after birth may be caused by a decrease of the receptor-mediated catabolism of LDL in the liver.

Internalization of insulin: structures involved and significance

The binding of insulin to its receptor is followed by aggregation of hormone-receptor complexes and their internalization into the cell. Internalized hormone is concentrated in Golgi-enriched not lysosomal endocytotic structures which, in rat liver, contain lipoprotein particles and can be resolved by centrifugation techniques into three different entities. Recent work has shown that the bulk of endocytotic structures can be resolved from biochemically defined (i.e., galactosyltransferase-containing) Golgi elements. The endosomal apparatus or endosomes appear to function as a sorting center wherein internalized hormone-receptor complexes are concentrated and dissociated prior to directing hormone to lysosomes and receptor back to the cell surface for reutilization. Endosomes are heterogeneous and different functions might be subserved by different endosomal structures. Since an insulin stimulable receptor kinase activity can be identified in endosomes certain aspects of insulin action might be initiated herein.

Biliary proteins

The study of biliary proteins has grown enormously in the last 10 years. Although much has been recently learned about the nature, origin and hepatobiliary transport of these proteins, little is known of their function in bile or their effect on its physical state. This review will focus on description of the proteins and mechanisms by which they are secreted into bile.

Animal models of cholesterol gallstone disease

There is no genetically susceptible, spontaneous and naturally occurring animal model for human cholesterol cholelithiasis. The disease has been reported to occur spontaneously only rarely in some primates. The human disease is probably multifactorial; therefore, the finding or development through inbreeding of a spontaneous genetic model is unlikely. The two most popular animals in use today as models are rodent species: hamster and prairie dog. Despite widely different means of dietary induction of the disease in the two, the feature common to both is cholesterol overload. In the case of the essential fatty acid-deficient hamster, the predominant defect is a unique endogenous overproduction through increased total body synthesis of cholesterol. In the prairie dog, the cholesterol overload is simply exogenous due to massive and rapid intestinal absorption. Neither model is remotely physiological. Although a number of useful aspects of the lithogenic process can be studied using these models, certain changes apparently associated with the formation of gallstones under these conditions may in part be a function of the unphysiological dietary requirements for induction of the disease.