Suppression of 3-hydroxy-3-methylglutaryl-CoA reductase by low density lipoproteins produced in vitro by lipoprotein lipase action on nonsuppressive very low density lipoproteins

Sphingomyelinase activity associated with human plasma low density lipoprotein

Isolated human plasma low density lipoprotein (LDL) was observed to possess sphingomyelinase activity. Accordingly, the formation of ceramide was catalyzed by LDL at 37 degrees C using tertiary liposomes composed of sphingomyelin (mole fraction (x) = 0.2), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (x = 0.7), 1, 2-dimyristoyl-sn-glycero-3-phospho-rac-glycerol (x = 0.1), and either the fluorescent sphingomyelin analog Bodipy-sphingomyelin or [(14)C]sphingomyelin as substrates. However, this activity was not present in either very low density lipoprotein or the high density lipoprotein subfractions HDL(2) and HDL(3). Oxidation of LDL abrogated its sphingomyelinase activity. Aggregation of the liposomes upon incubation with LDL was evident from the light scattering measurements. Microinjection of LDL to the surface of giant liposomes composed of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), N-palmitoyl-d-sphingomyelin (C16:0-sphingomyelin), and Bodipy-sphingomyelin as a fluorescent tracer (0.75:- 0.20:0.05, respectively) revealed the induction of vectorial budding of vesicles, resembling endocytosis.

Aggregation of dimyristoylphosphatidylglycerol liposomes by human plasma low density lipoprotein

Turbidity (absorbance at 470 nm) measurements revealed human serum low density lipoprotein (LDL) to cause, within a few minutes and at physiological pH and [NaCl], the aggregation of liquid crystalline large unilamellar liposomes (LUVs) of dimyristoylphosphatidylglycerol (DMPG). No evidence for concomitant lipid or aqueous contents mixing was obtained with fluorescent assays for these processes, in keeping with the lack of fusion of LUVs. Involvement of apoB is implicated by the finding that tryptic digestion of LDL abrogates its ability to cause aggregation. Aggregation is not caused by VLDL, HDL2, or HDL3. Interestingly, also oxidised LDL failed to aggregate DMPG vesicles. Aggregation of DMPG LUVs by LDL did depend on the ionic strength of the medium as well as on the phase state of the lipid. More specifically, below the main transition temperature Tm maximal aggregation was seen in the presence of 25-100 mM NaCl, whereas slightly higher (up to 150 mM) [NaCl] were required when T>Tm. Aggregation due to LDL was also observed for dimyristoylphosphatidylserine as well as for dipalmitoylphosphatidylglycerol LUVs, whereas liposomes composed of either unsaturated acidic phospholipids or different phosphatidylcholines were not aggregated. Involvement of electrostatic attraction between the acidic phosphate of DMPG and cationic residues in apoB is suggested by the finding that increasing the content of dimyristoylphosphatidylcholine (DMPC) in DMPG liposomes reduced their aggregation and at XDMPC=0.50 no response was evident. Notably, increasing the mole fraction of 1-palmitoyl-2-oleyl-PG (POPG) in DMPG LUVs progressively reduced their aggregation by LDL and at XPOPG=0.50 there was complete inhibition. The latter effect of POPG is likely to be due to augmented hydration of the unsaturated lipid constituting a barrier for the contact between apoB and the vesicle surface. In keeping with this view, the presence of the strongly hygroscopic polymer, poly(ethylene glycol) at 1% (by weight) enhanced the aggregation and could partly reverse the inhibition by POPG.

The influence of apolipoprotein E on the interactions between normal human very low density lipoproteins and U937 human macrophages: heterogeneity among persons

Apolipoprotein E (apo E) can mediate the cell binding of normal human very low density lipoproteins (VLDL). However, the extent to which apo E is involved in the cell binding and uptake of VLDL from different normolipidemic persons is not well defined. The VLDL (d < 1.006 g/l) of eight subjects were fractionated into VLDL with apo E and without apo E using a monoclonal antibody that binds to the LDL receptor recognition region of apo E. VLDL particles that expressed the 1D7 binding region of apo E comprised an average of 34% (range 7-51%) of the VLDL particles. Anti-apo E blocked an average of 43% (range 8-63%) of the binding of unfractionated VLDL to U937 cells. Anti-apo E blocked a similar proportion of binding to U937 cells of three VLDL subfractions of different density ranges (Sf20-60, Sf60-100, Sf100-400). The proportion of the VLDL particles that contained apo E correlated with the extent of uptake of the total VLDL by U937 cells, but not with stimulation by total VLDL of cholesterol ester formation. The binding to cells of VLDL without apo E varied by six-fold among persons, and caused most of the binding of the total VLDL of some subjects. Therefore, normolipidemic VLDL contains particles across its density range that use apo E to bind to U937 macrophages. In some VLDL samples, apo E provides most of the cell binding activity, whereas in others the binding activity occurs by other means.

Lipoprotein lipase binds to low density lipoprotein receptors and induces receptor-mediated catabolism of very low density lipoproteins in vitro

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

Studies on the effect of storage of normolipidemic plasma on the metabolism of very-low-density lipoprotein cholesterol in human skin fibroblasts

Very-low-density lipoproteins (VLDL) isolated from fresh normolipidemic plasma were able to compete with labeled low-density lipoproteins (LDL) for receptor binding as efficiently as LDL in human skin fibroblasts. However, VLDL from fresh plasma failed to stimulate acyl-CoA: cholesterol acyltransferase (ACAT) (EC 2.3.1.26) activity significantly. When plasma was stored at 4 degrees C for 24-48 h before ultracentrifugation, the VLDL were able to stimulate ACAT activity significantly. Proteolytic and lipolytic enzymes do not appear to play a role in this phenomenon. The increased capacity of VLDL from stored plasma to stimulate ACAT activity appears to be due partly to the increase in the internalization and degradation processes. Both the larger and the smaller VLDL subclasses were able to stimulate ACAT activity in the cells. The actual changes that take place in the VLDL molecule during the storage of plasma are not known.

Studies on the binding and degradation of human very-low-density lipoproteins by human hepatoma cell line HepG2

The regulation of the hepatic catabolism of normal human very-low-density lipoproteins (VLDL) was studied in human-derived hepatoma cell line HepG2. Concentration-dependent binding, uptake and degradation of 125I-labeled VLDL demonstrated that the hepatic removal of these particles proceeds through both the saturable and non-saturable processes. In the presence of excess unlabeled VLDL, the specific binding of 125-labeled VLDL accounted for 72% of the total binding. The preincubation of cells with unlabeled VLDL had little effect on the expression of receptors, but reductive methylation of VLDL particles reduced their binding capacity. Chloroquine and colchicine inhibited the degradation of 125I-labeled VLDL and increased their accumulation in the cell, indicating the involvement of lysosomes and microtubuli in this process. Receptor-mediated degradation was associated with a slight (13%) reduction in de novo sterol synthesis and had no significant effect on the cellular cholesterol esterification. Competition studies demonstrated the ability of unlabeled VLDL, low-density lipoproteins (LDL) and high-density lipoproteins (HDL) to effectively compete with 125I-labeled VLDL for binding to cells. No correlation was observed between the concentrations of apolipoproteins A-I, A-II, C-I, C-II and C-III of unlabeled lipoproteins and their inhibitory effect on 125I-labeled VLDL binding. When unlabeled VLDL, LDL and HDL were added at equal contents of either apolipoprotein B or apolipoprotein E, their inhibitory effect on the binding and uptake of 125I-labeled VLDL only correlated with apolipoprotein E. Under similar conditions, the ability of unlabeled VLDL, LDL and HDL to compete with 125I-labeled LDL for binding was a direct function of only their apolipoprotein B. These results demonstrate that in HepG2 cells, apolipoprotein E is the main recognition signal for receptor-mediated binding and degradation of VLDL particles, while apolipoprotein B functions as the sole recognition signal for the catabolism of LDL. Furthermore, the lack of any substantial regulation of beta-hydroxy-beta-methylglutaryl-CoA reductase and acyl-CoA:cholesterol acyltransferase activities subsequent to VLDL degradation, in contrast to that observed for LDL catabolism, suggests that, in HepG2 cells, the receptor-mediated removal of VLDL proceeds through processes independent of those involved in LDL catabolism.

Receptor-mediated uptake of hypertriglyceridaemic VLDL remnants by mouse macrophages in vitro

Following incubation of the VLDL isolated from hypertriglyceridaemic patients with normal human postheparin plasma VLDL remnants were obtained. 125I-VLDL remnants were taken up and catabolized by mouse macrophages in vitro. This was a receptor-dependent process. Unlabelled VLDL remnants added to the incubation medium inhibited the degradation of 125I-VLDL remnants in the cells. This could not be produced by the presence of VLDL or acetyl-LDL in the medium. It shows that mouse macrophages bind VLDL remnants by means of an other receptor than that for acetyl-LDL. The cell cholesterol content increased after incubation of the cells with VLDL remnants or acetyl-LDL, but not with native VLDL or LDL.

Very low density lipoproteins promote triglyceride accumulation in macrophages

Incubation of mouse peritoneal macrophages with very low density lipoproteins (VLDL) from normal rats or rhesus monkeys markedly increased the levels of intracellular triglycerides by 10- to 56-fold and was accompanied by the production of oil red O positive vacuoles. The stimulation of triglyceride accumulation in macrophages was time- and concentration-dependent and was specific for VLDL. Three possible mechanisms for the VLDL-stimulated triglyceride accumulation in macrophages were explored: receptor-mediated uptake, action of lipoprotein lipase, and phagocytosis. Macrophage uptake and degradation of 125I-monkey and rat VLDL demonstrated saturable and nonsaturable components. Uptake of 125I-VLDL could be inhibited by unlabeled normal VLDL, although hyperlipemic VLDL was more effective. HDL did not compete to a significant extent. Heparin released lipoprotein lipase-like activity from peritoneal macrophages. Addition of heparin with VLDL resulted in a greater, more rapid elevation in intracellular triglycerides, which was partially inhibited by albumin. Free fatty acid and Intralipid also produced triglyceride accumulation in macrophages. The data showed that all three of the mechanisms examined could contribute to the metabolism of VLDL by macrophages and cause the production of triglyceride-rich cells with a "foamy" appearance, although the evidence suggested that the action of lipoprotein lipase was probably the most important in this process.

Plasma lipids, lipoproteins and apoproteins in a case of apo C-II deficiency

A new case of apo C-II deficiency is described. The patient had plasma triglyceride levels ranging from 10.2-30.5 mmol/l. Apo C-II deficiency was confirmed by gel electrophoresis, isoelectric focusing and immunochemistry. In this patient plasma lipoproteins were mainly chylomicrons and very low density lipoproteins, LDL and HDL levels being very low. Infusion of normal plasma effectively reduced plasma triglycerides and enhanced low density and high density lipoproteins cholesterol levels. These data suggest that in vivo a precursor-product relationship exists between triglyceride rich lipoproteins and LDL and HDL, and further stress the role of the lipoprotein lipase-apo C-II system in modulating these metabolic interconversions.

Hypertriglyceridemia: mechanisms, clinical significance, and treatment

The association between hypertriglyceridemia and coronary heart disease is explored followed by a discussion of the mechanisms of the disorder and guidelines on patient evaluation and treatment.

Intermediate-density lipoprotein and cholesterol-rich very low density lipoprotein in angiographically determined coronary artery disease

The relationship between the concentrations of intermediate-density lipoprotein (IDL) and other lipoproteins and the extent of coronary artery disease (CAD) was studied in 182 consecutive patients evaluated by selective coronary cineangiography. On univariate analysis, the extent of CAD correlated significantly and positively with very low density lipoprotein (VLDL) cholesterol, IDL cholesterol and low-density lipoprotein (LDL) cholesterol, and negatively with high-density lipoprotein (HDL) cholesterol. Analysis of four subgroups divided by IDL cholesterol and LDL cholesterol levels indicated that moderately increased levels of IDL cholesterol were closely associated with a high frequency of CAD. Moreover, multi-variate regression analysis demonstrated that IDL cholesterol for men, LDL cholesterol for men and women and HDL cholesterol for men were significant variables of use in the final weighting procedure. IDL cholesterol was closely associated with cholesterol-rich VLDL. This study shows that IDL and cholesterol-rich VLDL combine to contribute to the development of CAD.

The distribution of apo C-II and apo C-III in very low density lipoproteins of normal and type IV subjects

The apoC-II/apoC-III ratio and the distribution of C apoproteins as percentage of the total protein in VLDL obtained from normolipidemic and type IV patients was studied by analytical isoelectrofocusing. While the percentage concentration of apoCI-III was unchanged that of apoC-II was 9.2% of total protein in normal VLDL and only 7.4% in type IV VLDL (P less than 0.05). The apoC-II/apo C-III ratio significantly decreased in type IV patients. The analysis of VLDL subfractions obtained by density gradient ultracentrifugation gave essentially the same results. These data confirm the decrease of apoC-II as % of the protein in type IV VLDL and suggest that the decrease of the apoC-II/apoC-iii ratio in type IV patients is related only to the decreased content of apoC-II. Whether the reduced apoC-II percentage amount or the decreased apoC-II/apo C-III ratio are related to the hypertriglyceridemia in type IV subjects is still unclear.