Phosphatidylcholine metabolism after transfer from lipid emulsions injected intravenously in rats. Implications for high-density lipoprotein metabolism

Triglyceride increase in the core of high-density lipoproteins augments apolipoprotein dissociation from the surface: Potential implications for treatment of apolipoprotein deposition diseases

Lipids in the body are transported via lipoproteins that are nanoparticles comprised of lipids and amphipathic proteins termed apolipoproteins. This family of lipid surface-binding proteins is over-represented in human amyloid diseases. In particular, all major proteins of high-density lipoproteins (HDL), including apoA-I, apoA-II and serum amyloid A, can cause systemic amyloidoses in humans upon protein mutations, post-translational modifications or overproduction. Here, we begin to explore how the HDL lipid composition influences amyloid deposition by apoA-I and related proteins. First, we summarize the evidence that, in contrast to lipoproteins that are stabilized by kinetic barriers, free apolipoproteins are labile to misfolding and proteolysis. Next, we report original biochemical and biophysical studies showing that increase in triglyceride content in the core of plasma or reconstituted HDL destabilizes the lipoprotein assembly, making it more labile to various perturbations (oxidation, thermal and chemical denaturation and enzymatic hydrolysis), and promotes apoA-I release in a lipid-poor/free aggregation-prone form. Together, the results suggest that decreasing plasma levels of triglycerides will shift the dynamic equilibrium from the lipid-poor/free (labile) to the HDL-bound (protected) apolipoprotein state, thereby decreasing the generation of the protein precursor of amyloid. This prompts us to propose that triglyceride-lowering therapies may provide a promising strategy to alleviate amyloid diseases caused by the deposition of HDL proteins.

Consumption of Saturated Fat Impairs the Anti-Inflammatory Properties of High-Density Lipoproteins and Endothelial Function

OBJECTIVES

The purpose of this study was to investigate the influence of dietary fatty acids on the anti-inflammatory properties of high-density lipoproteins (HDL) and vascular function.

BACKGROUND

The effect of dietary fatty acids on atherogenesis remains uncertain.

METHODS

Fourteen adults consumed an isocaloric meal containing either a polyunsaturated or a saturated fat on 2 occasions. The effects of post-prandial HDL on endothelial cell expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) were determined. Flow-mediated dilation (FMD) and microvascular reactivity were assessed before and 3 and 6 h after the meal.

RESULTS

Plasma triglycerides, insulin, and nonesterified fatty acids rose after the meals. The HDL collected 6 h after the saturated meal were less effective than HDL isolated from fasting plasma in terms of their ability to inhibit expression of ICAM-1 and VCAM-1, whereas HDL collected 6 h after the polyunsaturated meal had an inhibitory activity that was greater than that of HDL collected from fasting plasma (p < 0.004 and p = 0.01 for comparison of effect of meals on ICAM-1 and VCAM-1, respectively). Post-hyperemic microvascular flow significantly increased at 3 h after the polyunsaturated meal by 45 +/- 14% and by 21 +/- 11% after the saturated meal. The FMD decreased 3 h after the saturated meal by 2.2 +/- 0.9% (p< 0.05 compared with baseline) and by 0.9 +/- 1% after the polyunsaturated meal.

CONCLUSIONS

Consumption of a saturated fat reduces the anti-inflammatory potential of HDL and impairs arterial endothelial function. In contrast, the anti-inflammatory activity of HDL improves after consumption of polyunsaturated fat. These findings highlight novel mechanisms by which different dietary fatty acids may influence key atherogenic processes.

Elevation of plasma high-density lipoprotein concentration reduces interleukin-1-induced expression of E-selectin in an in vivo model of acute inflammation

BACKGROUND

Although there is strong evidence that plasma HDL levels correlate inversely with the incidence of coronary artery disease, the precise mechanism(s) for the protective effect of HDLs remains unclear. We recently showed that HDLs inhibit endothelial cell expression of cytokine-induced leukocyte adhesion molecules in vitro. Our study therefore sought to test the hypothesis that elevating the level of circulating HDLs would inhibit endothelial cell activation in vivo.

METHODS AND RESULTS

We used a porcine model of inflammation previously established in our laboratory, in which the level of vascular endothelial cell expression of E-selectin in interleukin (IL)-1alpha-induced skin lesions was measured by the uptake of a radiolabeled anti-E-selectin antibody (1.2B6). Porcine plasma HDL levels were elevated by use of a bolus injection of reconstituted discoidal HDL (recHDL). These particles resemble nascent HDL particles in shape and contain apolipoprotein A-I as the sole protein and soybean phosphatidylcholine as the sole phospholipid. We found that recHDLs inhibited the expression of IL-1alpha-induced E-selectin by porcine aortic endothelial cells in vitro, confirming that the inhibitory effect is conserved with synthetic HDLs and demonstrating that the phenomenon is not restricted to human endothelial cells. In vivo, elevating the circulating level of HDLs approximately 2-fold led to significant inhibition of basal and IL-1alpha-induced E-selectin expression by porcine microvascular endothelial cells.

CONCLUSIONS

These observations demonstrate the potential anti-inflammatory action of HDLs and provide support for the further investigation of the mechanisms underlying the inhibitory effects of HDLs on endothelial cell activation.

Dietary soybean phosphatidylcholines lower lipidemia: mechanisms at the levels of intestine, endothelial cell, and hepato-biliary axis

The beneficial metabolic effects of dietary soybean lecithin on lipid metabolism are now more clearly established. The intestinal absorption of cholesterol is decreased by soybean phosphatidylcholine-enriched diet and results in a cholesterol-lowering effect. There is an enhancement of the cholesterol efflux by endothelial cells incubated with soybean phosphatidylcholines, and a stimulation of the reverse cholesterol transport by high density lipoprotein-phosphatidylcholines. As a result of all these processes, phosphatidylcholines provided by the soybean lecithin metabolism appear to be key molecules controlling the biodynamic exchanges of lipids. They regulate homeostasis of cholesterol and fatty acids by decreasing their synthesis and promoting cholesterol oxidation into bile salts. Finally, the outcome is the increase in bile secretion of these lipids and/or their metabolite forms. Such findings constitute promising goals in the field of nutritional effects of soybean lecithin in the treatment or prevention of hyperlipidemia and related atherosclerosis.

Cholesterol-lowering effect of soyabean lecithin in normolipidaemic rats by stimulation of biliary lipid secretion

The purpose of the present study was to assess the role of the liver in the plasma-cholesterol-lowering effect of soyabean lecithin. Normolipidaemic rats were fed on lecithin-enriched or control diets with the same amount of protein. The lecithin diets contained 200 g/kg high-fat commercial semi-purified soyabean lecithin (230 g/kg total lipids as soyabean phosphatidylcholine) or 200 g/kg high-fat purified soyabean lecithin (930 g/kg total lipids as soyabean phosphatidylcholine). The control diets were a lowfat diet (40 g fat/kg) and a high-fat triacylglycerol-rich diet (200 g fat/kg). The high-fat diets were isoenergetic. The cholesterol-lowering effect of the lecithin-enriched diets was associated with significantly lower levels of plasma total- and HDL-cholesterol and significantly higher levels of bile phosphatidylcholine (PC), bile salts and cholesterol. These findings suggest that the liver plays a major role in the reduction of plasma cholesterol, the increased biliary lipid being provided by both HDL and the hepatic microsomal pools of PC and cholesterol.

Effects of cholesterol and cholesteryl oleate on lipolysis and liver uptake of triglyceride/phosphatidylcholine emulsions in rats

Emulsions composed of soy bean triglyceride (TG), egg yolk phosphatidylcholine (PC), cholesterol (Chol) or cholesteryl-oleate (CO), labeled with a cholesteryl ether (3H-CHE) and a triglyceride (14C-TO), were injected into rats. 14C-TO was removed from plasma faster than 3H-CHE. The 14C-labeled moiety is cleaved by digestion of the TG in the emulsion in plasma and is removed to the endothelial cells (lipolysis). In contrast, the 3H-label remains stably associated and represents circulating emulsion particles. The majority (90%) of the 3H-label disappearing from the plasma accumulated in the liver for all types of emulsions. On the basis of these observations, the lipolysis and the removal of emulsion particles to organs (mainly liver) were determined: 30 mole percent of cholesterol (Chol) at the TG-PC emulsion surface markedly retarded organ uptake, but the effect on lipolysis was rather small; 20 mole percent of cholesteryl oleate (CO) in the TG-PC emulsion cores delayed both organ uptake and lipolysis, and induced a rapid increase in organ uptake rate after the initial delay accompanying the gradual progress of lipolysis. Lipolysis led to the enrichment of the cores with CO. Replacement of the core TG by CO, however, induced strong suppression of the liver uptake. These results show that the lipid composition at both surface and core of emulsion particles is a crucial factor in metabolism in the rat.

The effect of insulin deficiency on the plasma clearance and exchange of high-density-lipoprotein phosphatidylcholine in rats

Triolein/cholesteryl oleate/cholesterol/phosphatidylcholine emulsions designed to model the lipid composition of chylomicrons were injected intravenously into control and streptozotocin-treated insulin-deficient rats. As previously described for lymph chylomicrons, the emulsion triolein was hydrolysed and phosphatidylcholine was transferred to the plasma high-density lipoproteins (HDL). This mechanism was used to introduce a phospholipid label into HDL in vivo. The subsequent clearance of phospholipid radioactivity from the plasma of insulin-deficient rats was significantly slower than in controls (P less than 0.025). Plasma clearance was similarly slower in insulin-deficient rats after injection of HDL that was previously labelled with radioactive phospholipids. After injection, the phospholipid label redistributed rapidly between the large-particle fraction of plasma lipoproteins (very-low- and low-density lipoproteins), and the lighter and heavier fractions of HDL. Compared with control rats, in insulin-deficient rats less of the phospholipid label was distributed to the lighter HDL fraction and more to the heavier HDL fraction, and this difference was not due to changes in activity of lecithin: cholesterol acyltransferase or in the apparent activity of phospholipid transfer protein. In insulin-deficient rats the changes in HDL phospholipid clearance and exchange appeared to be secondary to the associated hypertriglyceridaemia and the related changes in distribution of phospholipids between classes of plasma lipoproteins.

Interactions between model triacylglycerol-rich lipoproteins and high-density lipoproteins in rat, rabbit and man

There are inverse relationships between HDL cholesterol and plasma triacylglycerol concentrations in normal and in hypertriglyceridemic individuals. To investigate the interactions between triacylglycerol-rich lipid particles and HDL, a lipid emulsion model of the triacylglycerol-rich lipoproteins was prepared. When emulsion particles were incubated with rat high-density lipoproteins (HDL) in the presence of lipid transfer activity (d greater than 1.21 g/ml fractions) from rabbit or human plasma there was a rapid bi-directional exchange of cholesteryl oleate (CO) and phospholipid (PL) labels between lighter and heavier fractions of HDL and emulsion particles. The transfers of CO and PL labels between both light and heavy fractions of HDL and the emulsion particles were increased with increasing amounts of emulsion added to the incubations. Incubation with the d greater than 1.21 g/ml fraction from rat plasma resulted in only a small exchange of CO whereas PL exchange was similar to rabbit and human plasma. Retinyl palmitate label was not transferred from emulsion particles to the HDL fractions even in the presence of lipid transfer activity from rabbit or human plasma. The present study shows that the transfer protein-mediated exchanges of surface and core lipids between HDL and the triacylglycerol-rich lipoproteins are affected by the quantity of triacylglycerol-rich particles in the system. This mechanism may contribute to the inverse relationships between plasma triacylglycerol concentrations and HDL concentrations in normal and hypertriglyceridemic individuals.

Effects of intravenous infusions of commercial fat emulsions (Intralipid 10 or 20%) on rat plasma lipoproteins: phospholipids in excess are the main precursors of lipoprotein-X-like particles

Like most commercial parenteral emulsions, Intralipid contains the same amount of phospholipids (12 mg/ml) to stabilize 100 or 200 mg of soybean oil (10 or 20% formula, respectively). By centrifugation, 10 or 20% Intralipid was separated into a supernatant, fat particles containing the bulk of triacylglycerols stabilized by a fraction of phospholipids and an infranatant--called mesophase--consisting mainly of phospholipids used in excess as emulsifier. We observed that the initial triacylglycerol/phospholipid ratio of the emulsion (100/12 and 200/12, respectively) determines the size of the triacylglycerol-rich particles (260 and 350 nm) as well as the phospholipid content of the mesophase (6.02 and 4.67 mg/ml). To understand the mechanism of the lipoprotein-X (LPX) accumulation generally reported after intravenous fat infusions, plasma lipid levels and lipoprotein profiles were first compared in the rats after infusion (at a constant rate of 0.5 or 1 ml/h for 43 h) of Intralipid 10 or 20%. For the same intravenous triacylglycerol load (100 mg/h), rats infused with Intralipid 10% at 1 ml/h displayed higher triacylglycerol levels than rats infused with the 20% emulsion at 0.5 ml/h, suggesting that the size of exogenous fat particles modulated the catabolic rate of their triacylglycerols. The plasma levels of LPX varied according to the infusion rate of phospholipids not associated with triacylglycerol-rich particles of the emulsion. Moreover, an apo E and apo B enrichment of plasma and an elevation of the apo B48/apo B100 ratio was always observed after Intralipid infusions. In order to confirm that phospholipids of the mesophase are the main LPX precursors, lipoprotein profiles were then compared in the rats after intravenous infusion, at a constant rate of 1 ml/h, of either the mesophase or a suspension of triacylglycerol-rich particles isolated from Intralipid 20%. As expected, significant LPX amounts were only detected in rats infused with the pure mesophase of the emulsion. It was concluded that products of the lipolysis of exogenous fat particles play only a minor role in the formation of LPX. In fact these abnormal lipoproteins are generated by phospholipids of the mesophase which, like infused liposomes, actively mobilize endogenous free cholesterol. Consequently, in order to be considered as true chylomicron models for safe fat delivery in parenteral nutrition and in order to prevent some detrimental effects on cholesterol metabolism, commercial emulsions should be cleared of phospholipid excess.