Stimulation of esterified cholesterol accumulation in tissue culture cells exposed to high density lipoproteins enriched in free cholesterol

PCSK9 Affects Astrocyte Cholesterol Metabolism and Reduces Neuron Cholesterol Supplying In Vitro: Potential Implications in Alzheimer's Disease

The Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) involvement in Alzheimer’s disease (AD) is poorly investigated. We evaluated the in vitro PCSK9 modulation of astrocyte cholesterol metabolism and neuronal cholesterol supplying, which is fundamental for neuronal functions. Moreover, we investigated PCSK9 neurotoxic effects. In human astrocytoma cells, PCSK9 reduced cholesterol content (−20%; p < 0.05), with a greater effect in presence of beta amyloid peptide (Aβ) (−37%; p < 0.01). PCSK9 increased cholesterol synthesis and reduced the uptake of apoE-HDL-derived cholesterol (−36%; p < 0.0001), as well as the LDL receptor (LDLR) and the apoE receptor 2 (ApoER2) expression (−66% and −31%, respectively; p < 0.01). PCSK9 did not modulate ABCA1- and ABCG1-cholesterol efflux, ABCA1 levels, or membrane cholesterol. Conversely, ABCA1 expression and activity, as well as membrane cholesterol, were reduced by Aβ (p < 0.05). In human neuronal cells, PCSK9 reduced apoE-HDL-derived cholesterol uptake (−41%; p < 0.001) and LDLR/apoER2 expression (p < 0.05). Reduced cholesterol internalization occurred also in PCSK9-overexpressing neurons exposed to an astrocyte-conditioned medium (−39%; p < 0.001). PCSK9 reduced neuronal cholesterol content overall (−29%; p < 0.05) and increased the Aβ-induced neurotoxicity (p < 0.0001). Our data revealed an interfering effect of PCSK9, in cooperation with Aβ, on brain cholesterol metabolism leading to neuronal cholesterol reduction, a potentially deleterious effect. PCSK9 also exerted a neurotoxic effect, and thus represents a potential pharmacological target in AD.

The endosomal lipid bis(monoacylglycero) phosphate as a potential key player in the mechanism of action of chloroquine against SARS-COV-2 and other enveloped viruses hijacking the endocytic pathway

The anti-malarial drug Chloroquine (CQ) and its derivative hydroxychloroquine have shown antiviral activities in vitro against many viruses, including coronaviruses, dengue virus and the biosafety level 4 Nipah and Hendra paramyxoviruses. The in vivo efficacy of CQ in the treatment of COVID-19 is currently a matter of debate. CQ is a lysosomotrophic compound that accumulates in lysosomes, as well as in food vacuoles of Plasmodium falciparum. In the treatment of malaria, CQ impairs the digestion and growth of the parasite by increasing the pH of the food vacuole. Similarly, it is assumed that the antiviral effects of CQ results from the increase of lysosome pH and the inhibition of acidic proteases involved in the maturation of virus fusion protein. CQ has however other effects, among which phospholipidosis, characterized by the accumulation of multivesicular bodies within the cell. The increase in phospholipid species particularly concerns bis(monoacylglycero)phosphate (BMP), a specific lipid of late endosomes involved in vesicular trafficking and pH-dependent vesicle budding. It was shown previously that drugs like progesterone, the cationic amphiphile U18666A and the phospholipase inhibitor methyl arachidonyl fluoro phosphonate (MAFP) induce the accumulation of BMP in THP-1 cells and decrease cell infection by human immunodeficiency virus. HIV viral particles were found to be retained into large endosomal-type vesicles, preventing virus spreading. Since BMP was also reported to favour virus entry through hijacking of the endocytic pathway, we propose here that BMP could play a dual role in viral infection, with its antiviral effects triggered by lysosomotropic drugs like CQ.

Cholesterol composition of erythrocyte membranes and its association with clinical presentation of coronary artery disease

OBJECTIVES

Presence of free cholesterol in atherosclerotic plaques is a major determinant of plaque instability. It is hypothesized that extravasated erythrocytes may contribute to free cholesterol accumulation in atherosclerotic plaques through their rich in cholesterol membrane. In this study we assessed whether cholesterol in erythrocyte membranes (CEMs), that is, free (FCEM) versus esterified (ECEM), differs in patients with chronic stable angina (CSA) compared with patients presenting with acute coronary syndromes (ACSs).

METHODS

Consecutive angina patients were prospectively assessed; 154 had CSA (118 men, 63 years, 56-69 years) and 164 ACS (124 men, 63 years, 55-71 years). FCEM and ECEM were measured using an enzymatic assay, and protein content was assessed by the Bradford method.

RESULTS

FCEM was significantly higher (P<0.001) in the ACS patients group (94.1 microg/mg, IQ 71-116.5 microg/mg) compared with patients with CSA (61.9 microg/mg, IQ 49.3-73.1 microg/mg). ECEM levels were also significantly higher (P<0.001) in ACS patients (23.3 microg/mg, IQ 14.9-47.7 microg/mg) compared with CSA patients (10.8 microg/mg, IQ 8-22.3 microg/mg). In contrast, ratio of free-to-esterified cholesterol (P=0.110) as well as ratio of free-to-total CEM (P=0.109) were not different among CSA and ACS patients.

CONCLUSION

Findings of this study show that although free cholesterol is the prevailing form of CEMs, both FCEM and ECEM levels are increased in patients with ACS compared with CSA patients. These findings suggest that it is the quantity of CEM rather than the type of cholesterol present in the erythrocyte membrane that determines plaque progression.

Expression of scavenger receptor BI in COS-7 cells alters cholesterol content and distribution

Previous studies have shown that scavenger receptor BI (SR-BI) stimulates the bidirectional flux of free cholesterol (FC) between HDL and SR-BI-expressing cells. A major component of the enhanced FC flux appears to occur independently of HDL binding to SR-BI and may be due to changes in membrane lipid domains resulting from SR-BI expression (1). In the present study, the impact of SR-BI on cellular cholesterol metabolism was determined by examining SR-BI-mediated changes in cellular cholesterol mass, the esterification of HDL-derived FC, and changes in membrane lipid pools. Growth of SR-BI-expressing cells in medium containing HDL led to increased cellular cholesterol mass, most of which accumulated as ester. The esterification of HDL-derived FC was enhanced by SR-BI-expression to a far greater extent than the SR-BI mediated increase in FC uptake, suggesting an SR-BI-mediated effect on cholesterol utilization in the cell. This observation was tested by comparing FC esterification rates in SR-BI positive and negative cells when equivalent amounts of extracellular FC were taken up via cyclodextrins or apolipoprotein AI/phospholipid disks, neither of which contained cholesteryl ester. Under these conditions, SR-BI did not preferentially stimulate cholesterol esterification. These results indicate that the enhanced esterification of HDL-derived FC in SR-BI-expressing cells is due to the expanded pool of cellular FC and not to a specific effect of SR-BI on cholesterol utilization. Two approaches were used to test the effects of SR-BI expression on membrane lipid organization. In the first, the sensitivity of cellular FC to exogenous cholesterol oxidase was tested under conditions in which there is a preferential oxidation of caveolar cholesterol. SR-BI-expression was found to greatly increase the fraction of cellular cholesterol available to the oxidase as compared to either vector-transfected cells or cells expressing the related class B scavenger receptor CD36. These results suggest that SR-BI expression alters the distribution of membrane-free cholesterol to a caveolar fraction or alters the accessibility of this membrane fraction to exogenous cholesterol oxidase. In the second approach, the efflux of cellular FC to high concentrations of cyclodextrins was monitored under conditions where desorption of FC from the plasma membrane is rate limiting for efflux. SR-BI-expressing cells showed a shift in the distribution of FC between two kinetic pools with more FC in the fast pool and less in the slow pool. These data support a model in which SR-BI expression leads to a redistribution of cholesterol to membrane domains that serve to facilitate the flux of FC between cells and lipoproteins.

Inhibition of ADP-induced platelet aggregation by apoE is not mediated by membrane cholesterol depletion

We have previously shown that plasma HDL-E, a minor subclass of high-density lipoproteins (HDL) containing apolipoprotein (apo) E, has a potent anti-platelet effect and implicated apoE as the active constituent. Recently, apoE complexes with phospholipids (DMPC) were reported to inhibit thrombin-induced aggregation by sequestering platelet membrane cholesterol. Here we demonstrate that platelet cholesterol depletion is an improbable explanation for the suppressive effect of apoE:DMPC on ADP-mediated platelet aggregation; only 0.5% of cholesterol was released prior to addition of ADP to initiate aggregation while lactoferrin, which does not accept cellular cholesterol, was also inhibitory. Previous studies have shown that apoE and lactoferrin are both bound by platelets but whether this provides the initial stimulus for suppression of aggregation remains to be established.

Inhibition of ADP-induced platelet aggregation by apoE is not mediated by membrane cholesterol depletion

We have previously shown that plasma HDL-E, a minor subclass of high-density lipoproteins (HDL) containing apolipoprotein (apo) E, has a potent anti-platelet effect and implicated apoE as the active constituent. Recently, apoE complexes with phospholipids (DMPC) were reported to inhibit thrombin-induced aggregation by sequestering platelet membrane cholesterol. Here we demonstrate that platelet cholesterol depletion is an improbable explanation for the suppressive effect of apoE:DMPC on ADP-mediated platelet aggregation; only 0.5% of cholesterol was released prior to addition of ADP to initiate aggregation while lactoferrin, which does not accept cellular cholesterol, was also inhibitory. Previous studies have shown that apoE and lactoferrin are both bound by platelets but whether this provides the initial stimulus for suppression of aggregation remains to be established.

Comparison of the effect of hyperinsulinemia on acyl-CoA:cholesterol acyltransferase activity in the liver and intestine of the rat

Recent studies have suggested that cholesteryl ester synthesis plays a critical role in the assembly of VLDL apo B and triacylglycerol in the liver. Chronic hyperinsulinemia is associated with increased TG production and since cholesteryl ester synthesis depends on acyl CoA:cholesterol acyltransferase (ACAT), we investigated the possibility that chronic hyperinsulinemia might increase ACAT activity. We also measured ACAT activity in the intestinal mucosa, where it has been suggested to play a role in induction of diabetes-associated hypercholesterolemia. Chronically hyperinsulinemic rats were produced by injecting insulin (2 weeks, 6U/day). To prevent profound hypoglycemia, these rats were given 10% sucrose in place of drinking water. Acute hyperinsulinemia was produced by injecting a single dose of 0.5 U insulin ip. Chronic hyperinsulinemia led to a significant increase in free cholesterol, cholesteryl esters, triacylglycerols and phospholipids in the whole liver (27%, P < 0.05; 60%, P < 0.05; 70%, P < 0.01; 37%, P < 0.01, respectively) and an increase in hepatic microsomal triacylglycerol (P < 0.05). In contrast, the microsomal lipids of the intestinal mucosa decreased significantly. In chronically hyperinsulinemic rats there was no change in hepatic ACAT, while ACAT in the intestine actually decreased (26%-50%, P < 0.01).

CONCLUSIONS

since the effect of chronic hyperinsulinemia on hepatic ACAT did not parallel that seen previously on VLDL secretion, the effect of chronic hyperinsulinemia on VLDL production cannot be explained by its effect on hepatic ACAT. However, chronic hyperinsulinemia was associated with reduced ACAT activity in the intestine and this might result in decreased lipoprotein production in the hyperinsulinemic intestine.

Effect of ionic strength on the transfer of 1-pyrenemethyl-3 beta-hydroxy-22,23-bisnor-5-cholenate between bilayer vesicles containing phosphatidylserine

The influence of ionic strength or the concentration of K+ ([K+]) of the aqueous phase on the spontaneous transfer of cholesterol between negatively charged bilayer vesicles composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylserine (DPPS) (1:1, mole:mole) was studied using a pyrene-labelled cholesterol analogue, 1-pyrenemethyl-3 beta-hydroxy-22,23-bisnor-5-cholenate (PMC), as the probe. The decrease in PMC excimer fluorescence was best fitted to a bi-exponential function. Increasing [K+] from 0.1 M to 0.3 M had little effect on the shorter half-time (1.4 +/- 0.2 min) but increased the longer half-time from 16.3 +/- 1.9 min to 26.7 +/- 2.1 min. Fluorescence quenching and titration of an interface-located fluorophore, 1-anilinonaphthalene-8-sulfonic acid (ANS) revealed an increase in interfacial hydrophobicity upon increasing in ionic strength. The physical state of the acyl chains was not affected by ionic strength as indicated by a constant PMC excimer:monomer fluorescence intensity ratio. However, an increase in enthalpy change of the lipid phase transition from 15.7 kJ/mol ([K+] = 0.1 M) to 21.3 kJ/mol ([K+] = 0.3 M), together with a slight increase in the transition temperature, implies that interactions between adjacent molecules in the charged lipid bilayer vesicles became stronger at higher ionic strength. Our results suggest that the van der Waals attraction between PMC and phospholipid molecules could be affected by conformation changes in the charged head group region brought about by changes of ionic strength in the aqueous phase, with consequent effects on the desorption of cholesterol from the bilayer surface.

Cholesterol transport between cells and high-density lipoproteins

Various types of studies in humans and animals suggest strongly that HDL is anti-atherogenic. The anti-atherogenic potential of HDL is thought to be due to its participation in reverse cholesterol transport, the process by which cholesterol is removed from non-hepatic cells and transported to the liver for elimination from the body. Extensive studies in cell culture systems have demonstrated that HDL is an important mediator of sterol transport between cells and the plasma compartment. The topic of this review is the mechanisms that account for sterol movement between HDL and cells. The most prominent and easily measured aspect of sterol movement between HDL and cells is the rapid bidirectional transfer of cholesterol between the lipoprotein and the plasma membrane. This movement occurs by unmediated diffusion, and in most situations its rate in each direction is limited by the rate of desorption of sterol molecules from the donor surface into the adjacent water phase. The net transfer of sterol mass out of cells occurs when there is either a relative enrichment of sterol within the plasma membrane or a depletion of sterol in HDL. Recent studies suggest that certain minor subfractions of HDL (with pre-beta mobility on agarose gel electrophoresis and containing apoprotein A-I but no apo A-II) are unusually efficient at promoting efflux of cell sterol. To what extent efflux to these HDL fractions is balanced by influx from the lipoprotein has not yet been established clearly. The prevention and reversal of atherosclerosis require the mobilization of cholesterol from internal (non-plasma membrane) cellular locations. To some extent, this may involve the retroendocytosis of HDL. However, most mobilization probably involves the transport of internal sterol to the plasma membrane, followed by desorption to extracellular HDL. Several laboratories are investigating the transport of sterol from intracellular locations to the plasma membrane. Studies on biosynthetic sterol (probably originating mostly in the smooth endoplasmic reticulum) suggest that there is rapid transport to the plasma membrane in lipid-rich vesicles. Important features of this transport are that it bypasses the Golgi apparatus and may be positively regulated by the specific binding of HDL to the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Lysosomal hydrolysis of lipids in a cell culture model of smooth muscle foam cells

Rabbit aortic smooth muscle cells take up lipid droplets when they are presented using an inverted culture technique. These droplets were localized in secondary lysosomes as demonstrated by staining for acid phosphatase. Initially, 69% of the cell volume was occupied by lipid, and 94% of the lipid was in lysosomes. After a 24-hr clearance period, the cell volume occupied by lipid decreased to 53%, although there was no change in the fraction of cell lipid that was in lysosomes. To confirm that hydrolysis of droplet lipid was occurring in lysosomes, cultures were exposed to medium containing Sandoz 58-035, an inhibitor of acyl CoA:cholesterol acyl transferase, for 24 hr in the presence and absence of chloroquine, ammonium chloride, or methylamine. Although the hydrolysis of cholesteryl oleate was sensitive to these lysosomotropic agents, the hydrolysis of triolein was not. Using reconstituted LDL containing cholesteryl oleate and triolein, we demonstrated that the hydrolyses of cholesteryl oleate and triolein were equally sensitive to the lysosomotropic agents when the cells were not loaded with droplet lipid. However, in cells loaded with lipid, hydrolysis of LDL cholesteryl ester was sensitive to the lysosomotropic agents but hydrolysis of triolein was not. We therefore conclude that both droplet lipids were hydrolyzed in lysosomes, and we attribute the failure of the lysosomotropic agents to inhibit fully the hydrolysis of droplet triolein to the presence of a large mass of free fatty acids in the lysosome that maintains a sufficiently low pH to sustain the triglyceridase activity, but not the cholesteryl esterase activity, of the lysosomal acid lipase.

Behaviour of phospholipase modified-HDL towards cultured hepatocytes. II. Increased cell cholesterol storage and bile acid synthesis

Human total HDL (hydrated density 1.070-1.210), HDL2 (1.070-1.125), HDL3 (1.125-1.210) or HDL separated by heparin affinity chromatography were treated with or without purified phospholipase A2 from Crotalus adamanteus. Control and treated HDL were reisolated and were then incubated with cultured hepatocytes. 1. Mass measurements evidenced a time-dependent cholesterol enrichment in hepatocytes cultured in the absence of lipoproteins. Addition of HDL2 still enhanced by 25% the cell cholesterol content and down-regulated endogenous sterol synthesis in similar proportions. Conversely, HDL3 slightly decreased the amount of free cholesterol in hepatocytes (-12%). 2. Incubations with phospholipase A2-treated HDL resulted in a 35%-50% increase of both the cellular cholesterol esterification and the cholesterylester accumulation, when compared to cells cultured in the presence of control-HDL. This effect was observed with HDL2, HDL3 and combining the data with all subfractions. 3. Cultured hepatocytes secreted cholic and beta-muricholic acids as major bile acids and HDL2 showed a tendency to stimulate their secretion. Phospholipase treatment of HDL again induced an increased production by hepatocytes of those two bile acids. Thus, whereas HDL2 and HDL3 display different behaviours with respect to cell cholesterol content, neosynthesis and bile acid secretion, their modifications by phospholipases always orientate the cell sterol metabolism in the same direction: increased cholesterylester accumulation and bile acid production.

Role of acidic phospholipids in intermembrane sterol transfer

A liposomal membrane model system was used to examine the effect of acidic phospholipids on spontaneous intermembrane cholesterol transfer. The spontaneous exchange of sterol between small unilamellar vesicles (SUV) containing 35 mol% sterol was monitored with a recently developed assay (Nemecz, G., Fontaine, R.N. and Schroeder, F. (1988) Biochim. Biophys. Acta 943, 511-541), not requiring separation of donor and acceptor membrane vesicles. Acidic phospholipids (2.5-30 mol%) increased the initial rate of spontaneous exchange of sterol by 5-89%, depending on the specific phospholipid. The stimulation of spontaneous sterol transfer by acidic phospholipids was suppressed by high ionic strength, CaCl2 and low pH. The results suggest that negatively charged phospholipids may fluidize sterol-poor domains in SUV membranes and thereby play an important role in the mechanism whereby sterols desorb from membranes into the aqueous medium.

Weight set-point theory and the high-density lipoprotein concentrations of long-distance runners

Long-distance runners have higher high-density lipoprotein (HDL)-cholesterol concentrations and lower adiposity than sedentary men. Most cross-sectional studies claim that the runners' elevated HDL-cholesterol is not due to the runners' leanness. However, when cross-sectional studies use analysis of covariance (ANCOVA) to adjust for adiposity, or when they compare runners with lean sedentary men, they make an incorrect tacit assumption. They assume that the relationship between change in adiposity and change in HDL-cholesterol in men who have lost fat by running is the same as the cross-sectional difference in HDL-cholesterol between naturally fat and lean sedentary men. Regression slopes for HDL-cholesterol versus adiposity during and at the end of 1 year of running in 35 initially sedentary men suggest this assumption is incorrect; the increase in HDL-cholesterol that accompanies weight loss (-4.28 +/- 1.01 mg/100 mL per kg/m2) is considerably greater than the increase in HDL-cholesterol that is associated with lower adiposity cross-sectionally (-0.78 +/- 0.46 mg/100 mL per kg/m2). These results suggest the following theory: long-distance runners have the HDL metabolism of men who are below their sedentary set-point weight rather than the HDL metabolism of men who are naturally lean without exercising or dieting. This theory was applied to data from 23 published comparisons between long-distance runners and sedentary men.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in lipoprotein subfractions during diet-induced and exercise-induced weight loss in moderately overweight men

We studied separately the effects of weight loss by calorie restriction (dieting) and by calorie expenditure (primarily, running) on lipoprotein subfraction concentrations in sedentary, moderately overweight men assigned at random into three groups as follows: exercise without calorie restriction (n = 46), calorie restriction without exercise (n = 42), and control (n = 42). Plasma lipoprotein mass concentrations were measured by analytic ultracentrifugation for flotation rates (F0(1.20), S0f) within high density lipoprotein (HDL) (F0(1.20) 0-9), low density lipoprotein (LDL) (S0f 0-12), intermediate density lipoprotein (IDL) (S0f 12-20), and very low density lipoprotein (VLDL) (S0f 20-400) particle distributions. Particle diameter and flotation rate of the most abundant LDL species were determined by nondenaturing polyacrylamide gradient gel electrophoresis and analytic ultracentrifugation, respectively. During the 1-year trial, the exercisers ran (mean +/- SD) 15.6 +/- 9.1 km/wk, and the dieters ate 340 +/- 71 fewer kilocalories per day than at baseline. Total body weight was reduced significantly more in dieters (-7.2 +/- 4.1 kg) and exercisers (-4.0 +/- 3.9 kg) than controls (0.6 +/- 3.7 kg). As compared with mean changes in controls, the exercisers and dieters significantly increased HDL2 mass (48.6% and 47.1%, respectively), decreased VLDL mass (-23.9% and -25.5%), and increased LDL peak particle diameter (2.4 and 3.2 A). When adjusted to an equivalent change in body mass index by analysis of covariance, 1) exercise-induced and diet-induced weight loss produced comparable mean changes in the mass of small LDL and VLDL, and in LDL peak particle diameter; 2) the exercisers versus control group difference in HDL2 was attributed to the exercisers' reduced body mass index; and 3) HDL2 increased significantly less in dieters than in exercisers. In dieters, low calorie intake might mitigate the effects of weight loss on HDL2.

Macrophages modify beta-VLDL by proteolysis and enhance subsequent lipid accumulation in arterial smooth muscle cells

Murine resident peritoneal macrophages (MRPM), incubated with beta-very low density lipoprotein (beta-VLDL), modify the beta-VLDL, producing an increase in the mobility of the lipoprotein. The modification does not result in an increase of thiobarbituric acid-reactive substances (TBARS) in the lipoprotein, and is not inhibited by butylated hydroxyanisole (BHA), EDTA, removal of copper and iron from the medium, or by diphenyliodonium (DPI), suggesting that the mechanism of modification is independent of oxidation. Macrophage conditioned medium performed the modification in the absence of cells, and phenylmethylsulphonyl fluoride (PMSF) inhibited beta-VLDL modification, whereas other protease inhibitors did not, suggesting that a secreted neutral serine protease may possibly be involved in the mechanism. The modified beta-VLDL enhanced the accumulation of cholesterol esters by smooth muscle cells (SMC).

Phosphatidylcholine and triacylglycerol hydrolysis in HDL as induced by hepatic lipase: modulation of the phospholipase activity by changes in the particle surface or in the lipid core

(1) Human HDL2 (d 1.063-1.125) and HDL3 (d 1.125-1.210), labelled with 2-[14C]oleoylphosphatidylcholine (PC), and with/without tri[3H]oleoylglycerol, were incubated with a partially purified human hepatic triacylglycerol lipase, at pH 8.5. PC hydrolysis was linear up to 90-120 min incubation and within a range of lipase activities, from 50 to 500 mIU/ml. At low degrees of lipolysis, the hydrolysis of triacylglycerol was linearly related to that of PC, but the relative degradation rate was 10-fold higher for the former, which was thus very rapidly consumed. HDL subfractions were then differentiated in terms of PC hydrolysis. Km values were 0.32 and 0.43 mM for HDL2 PC and HDL3 PC, respectively. The corresponding Vmax values expressed for 200 mIU/ml hepatic lipase activity were 41.0 nmol PC hydrolysed/ml per h (HDL2) and 28.6 nmol PC/ml per h (HDL3). (2) HDL3 were modified in the presence of VLDL by inducing triacylglycerol lipolysis in VLDL with a semi-purified human plasma or bovine milk lipoprotein lipase (LPL). Lipolysis-modified HDL3 (LIP-HDL3) were mostly enriched in free cholesterol (+80%, P less than 0.05) and to a lesser extent in triacylglycerol (+33%). As a consequence, 45% of the LIP-HDL3 was reisolated in the HDL2-density interval, and is referred to as light LIP-HDL3. LIP-HDL3 displayed a 65% increase in its reactivity towards hepatic lipase compared to control HDL3. The light LIP-HDL3 showed the lowest Km (0.19 mM PC) and the highest Vmax (69 nmol/ml per h) of all HDL tested. Coincubation of HDL3 with VLDL and albumin did not alter the further reactivity of HDL3 towards hepatic lipase. Cholesterol loading of HDL3 by celite-cholesterol dispersions also led to an enhanced reactivity, though less important than with the lipolysis modification. (3) HDL3 were also modified by coincubation with VLDL and the lecithin-cholesterol acyltransferase-inhibited plasma fraction of d greater than 1.21 g/ml, thus allowing the cholesteryl ester transfer reaction to occur. The modified HDL3 (CET-HDL3) were depleted in esterified cholesterol (-25%, P less than 0.05) and enriched in triacylglycerol (+70%, P less than 0.05). However, these particles behaved like control HDL3 in their reactivity towards hepatic triacylglycerol lipase. Thus, the hydrolysis of HDL PC mediated by hepatic triacylglycerol lipase appears to be influenced by changes occurring in the particle's surface rather than in the lipid core.

Differential uptake and metabolism of free and esterified cholesterol from high-density lipoproteins in the ovary

Rat luteal cells utilize high-density lipoproteins (HDL) as a source of cholesterol for steroid synthesis. Both the free and esterified cholesterol of HDL are utilized by these cells. In this report, we have examined the relative uptake of free and esterified cholesterol of HDL by cultured rat luteal cells. Incubation of the cells with HDL labeled with [3H]cholesterol or [3H]cholesteryl linoleate resulted in 4-6-fold greater uptake of the free cholesterol compared to esterified cholesterol. The increased uptake of free cholesterol correlated with its utilization for progestin synthesis: utilization of HDL-derived free cholesterol was 3-6-fold higher than would be expected from its concentration in HDL. The differential uptake and utilization of free and esterified cholesterol was further examined using egg phosphatidylcholine liposomes containing cholesterol or cholesteryl linoleate as a probe. Liposomes containing free cholesterol were able to deliver cholesterol to luteal cells and support steroid synthesis in the absence of apolipoproteins, and the addition of apolipoprotein A-I (apo A-I) moderately increased the uptake and steroidogenesis. Similar experiments using cholesteryl linoleate/egg phosphatidylcholine liposomes showed that inclusion of apo A-I resulted in a pronounced increase in the uptake of cholesteryl linoleate and progestin synthesis. These experiments suggest that free cholesterol from HDL may be taken up by receptor-dependent and receptor-independent processes, whereas esterified cholesterol uptake requires a receptor-dependent process mediated by apolipoproteins.

The effect of variations in lipid composition of high-density lipoprotein on its interaction with receptors on human fibroblasts

To test whether the altered lipid composition of high-density lipoprotein (HDL) particles influences their ability to interact with the HDL receptor on cultured fibroblasts, HDL3 isolated from normal and diabetic donors with different degrees of hypertriglyceridemia was subjected to binding competition, cholesterol efflux, and net cholesterol transport assays. When HDL3 particles from different subjects were incubated with cholesterol-loaded fibroblasts, the initial rates of cholesterol efflux from cells to HDL3 particles appeared to be an exclusive function of the relative ability of HDL3 to interact with the HDL receptor. Variation in lipid composition of HDL3 particles did not appear to have any significant influence on either the receptor-binding or the efflux-promoting abilities of HDL3. When the movement of cholesterol between cells and HDL3 particles was allowed to approach equilibrium, the lipid composition of HDL3 became an important factor in determining the net amount of cholesterol removed from cells, with cholesterol-deficient triacylglycerol-rich HDL3 particles having the best capacity to promote net transport of cholesterol from cells. These results suggest that the ability of HDL to bind to its cell-surface receptor, rather than variations in the lipid composition of the HDL particle, is the major determinant of cholesterol efflux from cells to HDL particles. However, the lipid composition of HDL as well as its receptor-binding activity determine the net amount of cholesterol transported from cells over long-term incubation.

Mechanisms and consequences of cellular cholesterol exchange and transfer

It is apparent from consideration of the reactions involved in cellular cholesterol homeostasis that passive transfer of unesterified cholesterol molecules plays a role in cholesterol transport in vivo. Studies in model systems have established that free cholesterol molecules can transfer between membranes by diffusion through the intervening aqueous layer. Desorption of free cholesterol molecules from the donor lipid-water interface is rate-limiting for the overall transfer process and the rate of this step is influenced by interactions of free cholesterol molecules with neighboring phospholipid molecules. The influence of phospholipid unsaturation and sphingomyelin content on the rate of free cholesterol exchange are known in pure phospholipid bilayers and similar effects probably occur in cell membranes. The rate of free cholesterol clearance from cells is determined by the structure of the plasma membrane. It follows that the physical state of free cholesterol in the plasma membrane is important for the kinetics of cholesterol clearance and cell cholesterol homeostasis, as well as the structure of the plasma membrane. Bidirectional flux of free cholesterol between cells and lipoproteins occurs and rate constants characteristic of influx and efflux can be measured. The direction of any net transfer of free cholesterol is determined by the relative free cholesterol/phospholipid molar ratios of the donor and acceptor particles. Cholesterol diffuses down its gradient of chemical potential generally partitioning to the phospholipid-rich particle. Such a surface transfer process can lead to delivery of cholesterol to cells. This mechanism operates independently of any lipoprotein internalization by receptor-mediated endocytosis. The influence of enzymes such as lecithin-cholesterol acyltransferase and hepatic lipase on the direction of net transfer of free cholesterol between lipoproteins and cells can be understood in terms of their effects on the pool sizes and the rate constants for influx and efflux. Excess accumulation of free cholesterol in cells stimulates the rate of cholesteryl ester formation and induces deposition of cholesteryl ester inclusions in the cytoplasm similar to the situation in the 'foam' cells of atherosclerotic plaque. Clearance of cellular cholesteryl ester requires initial hydrolysis to free cholesterol followed by efflux of this free cholesterol. The rate of clearance of cholesteryl ester from cytoplasmic droplets is influenced by the physical state of the cholesteryl ester; liquid-crystalline cholesteryl ester is removed more slowly than cholesteryl ester in a liquid state.(ABSTRACT TRUNCATED AT 400 WORDS)

[Influence of calcic and magnesic sulphurous thermal water on the metabolism of lipoproteins in the rat]

We have studied in rats fed hypercholesterolemic diet the action of calcic and magnesic sulphurous water from Capvern on the modification of the lipoproteins metabolism caused by hypercholesterolemia. The rats subjected to a hypercholesterolemic diet with thermal water of Capvern was found to have a plasma level of cholesterol significantly less increased (P less than 0.01) compared to those subjected to the same diet with ordinary drinking water (25%). We demonstrated after 105 days of experimentation on tested rats that thermal water may affect the cholesterol catabolism by increased level of cholesterol HDL (52%) and stabilizing level of cholesterol LDL comparatively to the controls. These data suggest that the thermal water from Capvern enhanced the transformation of cholesterol to biliary acids and their biliary secretion. A possible relationship between the influence of the thermal water and the metabolism of lipoproteins would be explained by a possible increase of hepatic receptors which identify apolipoproteins B (LDL) and E (HDLc) on cholesterol fed rats, suggesting a great synthesis of nascent apolipoproteins HDL which are antiatherogenic.

The synthesis and esterification of cholesterol by hepatocytes and H35 hepatoma cells are independent of the level of nonspecific lipid transfer protein

The level of the nonspecific lipid transfer protein (i.e., sterol carrier protein 2) is 16-fold lower in the Reuber H35 hepatoma cells as compared to the hepatocytes in culture (49 and 810 ng of protein per mg of 105000 X g supernatant protein, respectively). In order to establish whether there is a relationship between the level of nonspecific transfer protein and intracellular cholesterol metabolism, we have determined the biosynthesis and esterification of cholesterol in these hepatoma cells and hepatocytes. Both types of cells incorporated [3H]mevalonate into cholesterol and cholesterol ester. Incubation of both cell types with [3H]cholesterol in the medium resulted in a time-dependent uptake and subsequent conversion into cholesterol ester. In both instances, the amount of 3H label incorporated into cholesterol per mg of cellular protein was about 2-fold higher for the hepatoma cells. The kinetics of esterification of endogenously synthesized cholesterol were similar for both hepatoma cells and hepatocytes. Esterification of cholesterol derived from the medium proceeded 2-times faster in the hepatoma cells than in the hepatocytes. From the kinetics of cholesterol esterification we conclude that cells do not discriminate between cholesterol synthesized de novo and cholesterol derived from the medium. In addition, the proposition that the nonspecific lipid transfer protein is involved in cholesterol synthesis and esterification is not substantiated by this study.

Cholesterol exchange in platelets, erythrocytes and megakaryocytes

Cholesterol exchange between plasma and human platelets and erythrocytes and guinea pig platelets, erythrocytes and megakaryocytes was studied. The characteristics of exchange of cholesterol between [3H]cholesterol-labeled plasma and human platelets and erythrocytes were similar: exchange per cell was independent of cell concentration in whole plasma, decreased only 2-fold over a wide range of cell concentrations in low concentrations of plasma and approached a plateau at 1/3 normal plasma cholesterol concentration, and there was no net change in the cholesterol content of either cell. The activation energy for exchange for both cells was 47 kJ/mol. In all experiments, erythrocyte cholesterol was labeled to approximately twice the specific activity of platelet cholesterol. Guinea pig megakaryocyte cholesterol exchanged at 25-33% of the rate of guinea pig platelet cholesterol in vitro. Similarly, when guinea pigs were fed [3H]cholesterol, erythrocyte cholesterol specific activity after 24 h was 90%, platelet 50-65%, and megakaryocyte 20-26% that of plasma. Guinea pig platelets incubated with plasma radiolabeled in free and esterified cholesterol incorporated radioactivity from free but not esterified cholesterol. The similarity of free cholesterol exchange in platelets and erythrocytes in vitro and in vivo and the apparent inability of platelets to take up cholesterol esters from lipoproteins suggest that the interaction between normal platelets and normocholesterolemic plasma is limited to cholesterol exchange.

The effect of hypercholesterolemia on guinea pig platelets, erythrocytes and megakaryocytes

This study has examined the effect of diet-induced hypercholesterolemia on guinea pig platelets, erythrocytes, megakaryocytes and plasma. The cholesterol/phospholipid ratios of plasma and erythrocytes began to increase after one day on the diet and increased steadily for two weeks and more slowly thereafter until 30 days. In contrast, the cholesterol/phospholipid ratio of platelets remained constant for 4-5 days, then increased until reaching a maximum of about 0.85 in two weeks. Thus, the time-course for increase of the cholesterol/phospholipid ratio is different for platelets than for erythrocytes and plasma. The increase in the cholesterol/phospholipid ratio of megakaryocytes was small and not dependent on the degree of increase in the plasma cholesterol/phospholipid ratio. The cholesterol esters of both platelets and megakaryocytes increased with time for two weeks. The increase in megakaryocyte cholesterol esters appeared to precede that of platelets. The protein content of platelets and megakaryocytes and average megakaryocyte size were increased. Normal platelets incubated in plasma from hypercholesterolemic guinea pigs did not accumulate excess cholesterol, but erythrocyte cholesterol increased 45% in 6 h under the same conditions. Cholesterol synthesis in megakaryocytes was depressed 50-80% by cholesterol feeding and by in vitro incubation of the cells in hypercholesterolemic plasma. The data suggest that the platelets and erythrocytes may accumulate excess cholesterol by different mechanisms. The effects of cholesterol feeding on megakaryocytes and the lag in accumulation of cholesterol in platelets relative to erythrocytes and plasma suggest that a defect in the megakaryocyte may be a primary determinant of accumulation of cholesterol in platelets.

Binding and degradation of human high-density lipoproteins by human hepatoma cell line HepG2

The catabolism of human HDL was studied in human hepatoma cell line HepG2. The binding of 125I-labeled HDL at 4 degrees C was time-dependent and reached completion within 2 h. The observed rates of binding of 125I-labeled HDL at 4 degrees C and uptake and degradation at 37 degrees C indicated the presence of both high-affinity and low-affinity binding sites for this lipoprotein density class. The specific binding of 125I-labeled HDL accounted for 55% of the total binding capacity. The lysosomal degradation of 125I-labeled HDL was inhibited 25 and 60% by chloroquine at 50 and 100 microM, respectively. Depolymerization of microtubules by colchicine (1 microM) inhibited the degradation of 125I-labeled HDL by 36%. Incubation of cells with HDL caused no significant change in the cellular cholesterol content or in the de novo sterol synthesis and cholesterol esterification. Binding and degradation of 125I-labeled HDL was not affected by prior incubation of cells with HDL. When added at the same protein concentration, unlabeled VLDL, LDL and HDL had similar inhibitory effects on the degradation of 125I-labeled HDL, irrespective of a short or prolonged incubation time. Reductive methylation of unlabeled HDL had no significant effect on its capacity to inhibit the 125I-labeled HDL degradation. The competition study indicated no correlation between the concentrations of apolipoproteins A-I, A-II, B, C-II, C-III, E and F in VLDL, LDL and HDL and the inhibitory effect of these lipoprotein density classes on the degradation of 125I-labeled HDL. There was, however, some association between the inhibitory effect and the levels of apolipoprotein D and C-I.

Intracellular transport and esterification of exchangeable cholesterol in cultured human lung fibroblasts

We have studied the intracellular fate of exchangeable cholesterol in a model system with lipid vesicles (cholesterol/phospholipid mole ratio 1:1) and cultured human lung fibroblasts. Exchangeable [3H]cholesterol in lipid vesicles was readily incorporated into cellular plasma membranes and transported to intracellular esterification sites. The formation of [3H]cholesteryl esters was not affected by cytoskeleton-disrupting drugs. A reduction of cellular pinocytosis by 75% did not reduce the formation of tracer-labelled esters, suggesting that membrane flow due to the energy-dependent pinocytosis is no major contributor to the intracellular transport of molecular cholesterol between plasma membranes and esterification sites. The formation of [3H]cholesteryl esters was not significantly affected by energy poisons (NaF and KCN) but was inhibited (to 50%) by chloroquine at 50 microM. This may indicate that membrane-derived cholesterol passes through the lysosomal compartment on its way to intracellular esterification sites.

Early indicators for the risk of atherogenesis. A field test of a new methodology on kindergarten children in Düsseldorf

Parameters related to the risk of atherogenesis in adults were investigated in kindergarten children living in an industrial city. Height, weight and blood pressure were measured and capillary blood obtained. This study is a field test of methods for the analysis of 11 metabolic variables in capillary serum [glucose (GLUC), uric acid (UA), total, free and esterified cholesterol (TC, C, CE), triacylglycerols (TG), phosphatidylethanolamine (PE), phosphatidylcholine (PC); beta-, pre-beta- and alpha-lipoproteins (LPs)]. For lipid and LP analysis, new micro- and ultramicroprocedures were used, which have proved to be adequate for large-scale studies outside the clinic, including young children. Among 497 German children, a reference group, an overweight group and a socially underprivileged group were distinguished. In the reference group, several sub-groups with parameters lying in the tails of the concentration distributions were considered separately, thus providing specific metabolic patterns for these groups and, also, for any individual child. The children in the overweight group exhibited significantly reduced TC and beta-LP levels. In socially underprivileged children with anthropometric data within the normal range, a reduction of several blood constituents was seen: GLUC, TG, TC, PC, pre-beta- and alpha-LP; the PC: PE ratio was diminished. The beta: alpha-LP ratio in these children was moderately increased. This effect was more pronounced in growth-retarded children, who also showed negative correlation of TG: alpha-LP.

Transfer of [3H]cholesterol between lipid vesicles and rat arterial smooth muscle cells in vitro

Unesterified [3H]cholesterol is rapidly transferred between cholesterol-phosphatidylcholine vesicles and rat arterial smooth muscle cells in vitro. Exchange rate is influenced by the vesicle/cell ratio in a saturable way. The maximal transfer of cholesterol, which is 3.76 micrograms per mg cell protein during 4 h, is achieved with a vesicle/cell ratio of 3.4 X 10(7). Bovine serum albumin enhances the exchange by a factor of 4.5 compared to a protein-free system. The activation energy for the process is + 38.5 kJ X mol-1 with vesicles of 1:1 mole ratio of cholesterol to phosphatidylcholine (C/P). A fraction of the incorporated free [3H]cholesterol is esterified within 4 h with donor vesicles of over 1:1 C/P. When cells were incubated with vesicles of low C/P mole ratio (1:2) a fraction of the incorporated free [3H]cholesterol was esterified within 16 h. Our results are compatible with the aqueous diffusion mechanism of cholesterol exchange. Furthermore, we suggest that, in rat smooth muscle cells, the cell membrane cholesterol pool is not metabolically isolated from internal cholesterol pools, at least as judged by the ability of the cells to esterify incorporated free cholesterol.

Increased cholesterol esterification in rat liver microsomes in purified non-specific phospholipid transfer protein

The effect of the non-specific phospholipid transfer protein purified from rat liver on the activity of acyl-CoA:cholesterol acyltransferase (EC 2.3.1.26) in rat liver microsomes was studied. The activity of cholesterol acyltransferase was measured from the rate of incorporation of [1-14C] oleoyl-CoA into cholesteryl oleate. Activity was stimulated by preincubation by the microsomes with the non-specific phospholipid transfer protein alone, but most effectively when vesicles consisting of phosphatidylcholine/cholesterol (molar ratio 1:1) also were present in the preincubation mixture. Preincubation with vesicles consisting of only phosphatidylcholine or phosphatidylcholine/phosphatidylethanolamine (molar ratio 1:1) had no effect. The stimulating effect is dependent on transfer protein and vesicle concentration and on the length of preincubation. Treatment of the transfer protein with N-ethylmaleimide abolished its effect on cholesterol ester formation. Preincubation of the microsomes with transfer protein and phosphatidylcholine/cholesterol vesicles containing radioactively labeled cholesterol shows that exogenous cholesterol is converted readily to cholesterol ester. The data are explained by the ability of non-specific phospholipid transfer protein to effect net transfer of cholesterol to those microsomes that contain cholesterol acyltransferase. Enlargement of the cholesterol substrate pool would then give rise to stimulation of the cholesterol acyltransferase activity. This study suggests a role for the transfer protein in modulating cholesterol metabolism by its ability to transport cholesterol between membranes.