Measurement of receptor-independent lipoprotein catabolism using 1,2 cyclohexanedione-modified low density lipoprotein

Daily melatonin supplementation in mice increases atherosclerosis in proximal aorta

Considerable evidence supports the hypothesis that LDL oxidation plays an important role in atherosclerosis. Even though high melatonin doses inhibit LDL oxidation in vitro, the effect of melatonin on atherosclerosis has never been studied. We have demonstrated that the feeding of hypercholesterolemic mice with an atherogenic diet supplemented with melatonin highly increases the surface of atherosclerotic lesions in the proximal aorta. These observations occur without detectable lipidic or glucidic phenotype alteration. Melatonin treatment increased highly the sensitivity of atherogenic lipoprotein to Cu(2+) and gamma-radiolysis generated oxyradical ex vivo oxidation during the fasting period. Moreover, these altered lipoproteins were less recognized by the LDL receptor metabolic pathway of murine fibroblasts while they transferred many more cholesteryl esters to murine macrophages. This study suggests that caution should be taken as regards high melatonin dosage in hypercholesterolemic patients.

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

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

Receptor-dependent and -independent catabolism of low-density lipoprotein in a kindred with familial hypobetalipoproteinemia

Three affected members of a kindred with asymptomatic hypobetalipoproteinemia (HBL) were injected intravenously with 125I-labeled native low-density lipoproteins (LDL) and 131I-labeled cyclohexanedione (CHD)-treated LDL. Plasma and urine radioactivity data were collected for 15 days at regular intervals. A compartmental model using the SAAM program was built to fit simultaneously 125I and 131I plasma radioactivity decay and urine excretion data. This model allows precise calculation of the kinetic parameters of both receptor-independent (NR) and receptor-dependent (R) pathways. Compared with normal subjects, HBL patients show a 90% increased fractional catabolic rate (FCR) of LDL by both routes, more marked for the R pathway (215% increase), and an approximately 50% reduced production rate (PR). Structural analysis did not show significant abnormalities of apolipoprotein (apo) B in HBL patients compared with normal. These data suggest that the very reduced, LDL-apo B plasma levels result from a combination of two processes: (1) an increased activity of all catabolic routes, and (2) a reduced "synthesis" rate. The latter may result from a decreased conversion of very-low-density lipoprotein (VLDL) to LDL secondary to an increased direct removal of large VLDL, suggested by apo C-II and C-III turnover studies previously reported.

Low-density lipoproteins interact with liposome-binding sites on the cell surface

Under physiological conditions significant amounts of low-density lipoprotein LDL particles ar taken up by cells independently of specific high-affinity LDL receptors (apo-B receptors). Previously it was established that some cells contain surface sites capable of binding liposomes. We proposed that liposome-binding sites could contribute to LDL interaction with the cell surface via phospholipid molecules of LDL particles. To check this hypothesis we studied the competitive interaction of human LDL and DPPC liposomes with mouse embryo fibroblasts depleted of apo-B receptors by preliminary incubation with LDL. We have found that after removal of the liposome-binding sites from cell lamellae these areas of the cell surface lose their ability to bind LDL.

Effect of combined therapy with bezafibrate and cholestyramine on low-density lipoprotein metabolism in type IIa hypercholesterolemia

This study was designed to examine the influence of combined therapy with bezafibrate and cholestyramine on plasma lipids and on the metabolism of low-density lipoprotein (LDL). Twenty-one type II hyperlipidemic subjects were treated with bezafibrate alone or in combination with cholestyramine. A 17% fall in plasma cholesterol was seen with bezafibrate, and addition of cholestyramine produced an additional 9% reduction in this lipid. The effectiveness of the combination therapy was mediated through a 47% decrement in very-low-density lipoprotein (VLDL) cholesterol, a 37% reduction in LDL cholesterol, and a 15% increase in the level of that lipid in high-density lipoprotein (HDL). Plasma triglyceride fell 43% when bezafibrate was given alone, and did not change further when cholestyramine was added. The metabolism of LDL was examined in nine individuals to determine the mechanism underlying these changes. No significant modification in LDL synthetic rate was incurred with either drug regimen, whereas the fractional catabolic rate of LDL via the receptor pathway rose by 66% with bezafibrate alone and by 79% (compared to baseline) following the addition of cholestyramine. Plasma HDL rose during bezafibrate therapy due to an increase in the HDL3 subfraction. Compositional analysis of LDL showed a reduction in cholesterol ester and an increase in triglyceride and phospholipid during combined drug therapy. These results demonstrate that combined therapy with bezafibrate and cholestyramine markedly improves the lipoprotein profile in type II hyperlipidemia. The drugs appear to be complementary in their actions upon the LDL receptor pathway.

Influence of etofibrate on low density lipoprotein metabolism

This study examined the effect of single dose etofibrate (1.0 g/day) on plasma lipids and lipoproteins in a group of eleven hypercholesterolemic individuals. The drug lowered plasma triglyceride and cholesterol by 32% and 14%, respectively (P less than 0.005). The cholesterol reduction came from a decrement in both VLDL and LDL. The cholesterol content of HDL did not change although its mass as determined by analytical ultracentrifugation rose by 29%. LDL metabolism was followed before and during drug therapy. Treatment increased catabolism of this lipoprotein by 14%, without affecting synthesis. The increased clearance resulted from activation (64%) of the LDL receptor pathway. There was a reciprocal decrease in the amount of lipoprotein channelled into the receptor-independent route.

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.

Metabolism of the apolipoprotein B-containing lipoproteins

This chapter was designed to describe the approaches one can take to study the metabolism of the apoB-containing particles in vivo. The focus has been to blend (1) what is the current tracer kinetics analysis methodology and (2) what are the current experimental protocols being used into a total picture so that the experimentalist wishing to perform such studies may have a better perspective of the strong points and pitfalls of this important experimental tool. Hence, these points have been summarized from the point of view of what caveats are associated with each methodology. Recognition of these is essential to avoid reaching potentially erroneous conclusions. More important, attention has been focused on the realization that certain methodologies can be chosen depending upon what questions are being asked. Finally, areas where future development is needed in order to proceed to the next level of understanding are pointed out in the context of using tracer kinetic analysis as an integral part of a total experimental design.

Fenofibrate reduces low density lipoprotein catabolism in hypertriglyceridemic subjects

This study examines the kinetic basis for the increment in plasma low density lipoprotein (LDL) levels that accompanies the fenofibrate treatment of severely hypertriglyceridemic (HTG) patients. Seven HTG men with a mean plasma triglyceride level of 1470 mg/dl were treated for 6 weeks. During treatment, their plasma triglyceride level fell by 77% and their cholesterol level by 41%. The fall in very low density lipoprotein (VLDL) cholesterol level was reciprocated by increments in the cholesterol level in both LDL and high density lipoproteins (HDL); the rise in HDL was confined to HDL3. LDL catabolism was examined before and during therapy using native and chemically modified tracers in an attempt to distinguish receptor-mediated from non-receptor-mediated clearance. In their basal state, the hypertriglyceridemic subjects overcatabolized both the native and the modified lipoprotein, implying that the non-receptor pathways were hyperactive. The mean fractional clearance rate of LDL via the receptor pathway was not significantly different from normal. Fenofibrate therapy corrected the patients' hypercatabolism, reducing the receptor-independent fractional clearance of apo LDL by 50% (from 0.48 to 0.24 pools/day; p less than 0.05). The mean fractional catabolic activity of the receptor route did not change, but when the increment in the plasma apo LDL concentration was taken into account, it was clear that the drug treatment was associated with an increase in the net amount cleared by the receptor pathway and with a reduction of lipoprotein uptake into receptor-independent routes.

Contribution of the receptor pathway to low density lipoprotein catabolism in humans. New methods for quantitation

Receptor-mediated catabolism of low density lipoprotein (LDL) by cultured cells depends on the presence of functionally significant arginine and lysine residues on the lipoprotein apoprotein. When these are blocked, the recognition process is abolished, and catabolism of the modified lipoprotein is restricted to other mechanisms. Accurate discrimination between the activities of the receptor and nonreceptor pathways in vivo depends critically on the metabolic properties of this chemically modified lipoprotein. Here we report our experiences with two lysine-modified LDL tracers, glucosylated LDL (GLC-LDL) and 2-hydroxyacetaldehyde-treated LDL (HOET-LDL). The fractional clearance rate of GLC-LDL (0.25 +/- 0.05 pools/day, n = 5) was 50% of that of control material (0.51 +/- 0.09 pools/day) injected simultaneously into normal subjects. The HOET-LDL was also retarded in its clearance. Here, however, the fractional clearances of the control (0.37 +/- 0.06 pools/day, n = 6) and modified lipoprotein (0.19 +/- 0.03 pools/day) were lower than those obtained by the glucosylation procedure. We suspect that the prolonged incubation required for glucosylation of LDL artifactually accelerated its catabolism. The HOET-LDL does not suffer from this defect and seems to be a better tracer of the receptor-independent pathway. In a group of 10 subjects, HOET-LDL was metabolically indistinguishable from 1,2 cyclohexanedione-treated, arginine-modified LDL.

In vivo characteristics of a specific recognition site for LDL on non-parenchymal rat liver cells which differs from the 17 alpha-ethinyl estradiol-induced LDL receptor on parenchymal liver cells

Chemical modification of lysine or arginine residues of apolipoprotein B-100 in human low-density lipoprotein (LDL) with respectively reductive methylation (Me-LDL) or cyclohexanedione treatment (CHD-LDL) was applied to determine the role of these amino acids in LDL recognition by the various liver cell types. The cell association of native human LDL, Me-LDL and CHD-LDL to parenchymal and non-parenchymal cells was determined in vivo by isolating the various cell types 30 min after intravenous injection of the lipoproteins. In order to prevent degradation or release of cell-bound apolipoproteins during cell dissociation and purification, a low-temperature (8 degrees C) liver perfusion and cell isolation procedure was performed. It was found that reductive methylation of LDL inhibits the association of LDL to both parenchymal and non-parenchymal cells, indicating that lysine residues are important for recognition of LDL by both these cell types. In contrast, cyclohexanedione treatment of LDL did not influence the cell association of LDL to non-parenchymal cells. 17 alpha-Ethinyl estradiol treatment selectively increases the cell association of LDL by parenchymal cells (16-fold), leaving the non-parenchymal cell association uninfluenced. The increased cell-association of LDL to parenchymal cells is almost completely blocked by cyclohexanedione treatment of LDL (by 81%) or by methylation of LDL (by 97%). These data indicate that the arginine residues in LDL are not important for the recognition of LDL by non-parenchymal cells, whereas for the cell association of LDL to the estrogen-stimulated binding site on parenchymal cells both arginine and lysine residues are essential. The in vivo cell association of CHD-LDL or native LDL to non-parenchymal cells was lowered to the level of Me-LDL by ethyl oleate treatment of the rats, while no effect of ethyl oleate on parenchymal cells was noticed. These data suggest that the specific site for LDL on non-parenchymal cells, which need lysine residues on LDL for recognition, can be down-regulated by ethyl oleate treatment. The LDL, internalized by non-parenchymal cells, is effectively degraded. This degradation occurs at least partly in the lysosomes. It is suggested that the unique recognition site for LDL on non-parenchymal cells may be quantitatively important for serum LDL catabolism.

Receptor-independent low-density lipoprotein catabolism. Evaluation of 2-hydroxyacetaldehyde-treated lipoprotein as a probe for its measurement

This study examines the protein modification procedures available for inhibiting receptor recognition of low-density lipoprotein (LDL). Glycosylation with glucose, idose or ribose blocks the interaction of the lipoprotein with the high-affinity LDL receptor on cultured fibroblast membranes and delays its clearance from the plasma of rabbits. However, the prolonged incubation required in the process also changes the metabolic properties of the lipoprotein. An alternative approach using 2-hydroxyacetaldehyde-treated LDL completely blocks receptor recognition. This modified tracer has the same metabolic properties as the reductively methylated lipoprotein in rabbits and appears to be a suitable probe for the measurement of the receptor-independent LDL catabolic pathway in humans.

Regulation of LDL receptors in vivo

The information reviewed here indicates that LDL receptor activity in vivo is regulated in normal animals and probably in normal humans. Furthermore, LDL receptor activity is also regulated in patients with heterozygous Familial Hypercholesterolemia. Our experiments in patients with this disease indicate that one can exploit the normal regulation of receptor synthesis to stimulate the single normal gene in these patients to produce an increased number of LDL receptors. This stimulation can be accomplished by manipulating the intracellular cholesterol pools in the liver with drugs that inhibit HMG-CoA reductase and by maneuvers that result in bile acid depletion. These therapeutic measures are most effective when combined. Such therapy is likely to be beneficial, particularly since the results of the Lipid Research Clinics Primary Intervention Trial using the bile acid sequestrant, cholestyramine, have demonstrated that lowering LDL-cholesterol levels with this drug also lowers the incidence of coronary heart disease. In a broader sense, the success of this regulatory manipulation suggests that other genetic diseases may be treated by manipulation of regulatory signals that control the rates of synthesis of gene products.

Pathophysiology of human lipoprotein receptors: clinical consequences of a cellular defect

The function of the low density lipoprotein receptor has been reviewed at the cell level and in man. Its key role in cholesterol metabolism is unquestioned and it is central to the actions of a number of important hypocholesterolaemic agents. Clearly it must be involved in atherogenesis since its dysfunction leads to premature and severe atherosclerosis in both animals and man. The tools are now available to address this question and answers should be forthcoming in the near future.

Measurement of receptor-independent metabolism of low-density lipoprotein. An application of glycosylated low-density lipoprotein

Incubation of human low-density lipoprotein (LDL) with glucose results in a nonenzymatic formation of a Schiff base between the monosaccharide and lysyl residues of apolipoprotein B. Increasing the percentage of lysyl residues of apolipoprotein B modified by glycosylation decreases the fractional catabolic rate of the glycosylated LDL, and decreases the metabolism of the glycosylated LDL by human skin fibroblasts. The glycosylated LDL, containing 20-40% of total lysyl residues of apoprotein B modified, was metabolized at a slow rate by both human skin fibroblasts and mouse peritoneal macrophages. These results led to the suggestion that glycosylated LDL is primarily catabolized via a receptor-independent process. Assuming LDL catabolism occurs via receptor-dependent and receptor-independent processes, the ratio of (fractional catabolic rate of glycosylated LDL)/(fractional catabolic rate of native LDL) should be an estimate of the percentage of LDL catabolism via the receptor-independent process. From the fractional catabolic rates of glucose-LDL (20-40% of lysyl residues modified) and galactose-LDL (30-60% of lysyl residues modified) 41% and 30% respectively, of LDL catabolism occurred by a receptor-independent process.

Receptor-mediated catabolism and tissue uptake of human low density lipoprotein in the cholesterol-fed, atherosclerotic rabbit

The LDL receptor pathway, which was delineated in cultured cells, is now known to operate in vivo. In this study we have measured the plasma clearances and tissue uptakes of native and chemically modified (1,2-cyclohexanedione-treated or reductively methylated) LDL in rabbits in order to determine the response of the pathway to a high-cholesterol diet. 1 week on the diet increased circulating LDL and suppressed its receptor-mediated plasma clearance and uptake into all tissues. The fractional catabolic rate of the lipoprotein via the receptor-independent route also fell. Continuation of the feeding program for 12 weeks accentuated these changes and virtually eliminated receptor uptake into all tissues so that the plasma decay curves of native and cyclohexanedione-treated LDL were superimposable. Lipoprotein assimilation by the aorta, however, did not follow this general trend. This tissue, after 12 weeks, was variably infiltrated by atheromatous deposits and the appearance of these lesions was associated with a substantial increase in the relative uptakes of both native and chemically modified (cyclohexanedione-treated and reductively methylated) LDL. We concluded (a) that expansion of tissue cholesterol pools virtually abolishes LDL receptor activity in rabbits; and (b) that LDL assimilation (both apparently receptor-mediated and receptor-independent) paradoxically increases at sites where the aorta is affected by atheromatous lesions.

Effects of bezafibrate on receptor-mediated and receptor-independent low density lipoprotein catabolism in type II hyperlipoproteinaemic subjects

This study examines the effects of bezafibrate (200 mg t.i.d.) on LDL metabolism in 7 type II hyperlipoproteinaemic subjects. Eight weeks of treatment lowered plasma cholesterol and triglyceride by 10% and 30%, respectively (P less than 0.02). These reductions were associated with a fall in circulating VLDL (31%, P less than 0.02) and LDL (11%, P less than 0.05), while HDL cholesterol stayed the same. LDL metabolism changed during therapy. The plasma fractional clearance rate (FCR) of autologous [125I]LDL normalized from a low value of 0.256 +/- 0.048 (mean +/- SD) to 0.298 +/- 0.040 pools/day (P less than 0.001). This was attributable to a 65% increase (P less than 0.01) in receptor-mediated LDL catabolism since the clearance of simultaneously injected 1,2-cyclohexanedione-modified [131I]LDL, which measures the receptor-independent pathway, was unaltered (FCR of [131I]cyclohexanedione/LDL in control phase = 0.194 +/- 0.030 pools/day; during drug treatment = 0.194 +/- 0.024 pools/day). We conclude that bezafibrate lowers plasma LDL in type II hyperlipoproteinaemia by promoting its degradation via high affinity receptors.