Interactions of endogenous lipoproteins with capillary endothelium in spontaneously hyperlipoproteinemic rats

Effects of Long-Term High-Fat Diet and Its Reversal on Lipids and Lipoproteins Composition in Thoracic Duct Lymph in Pigs

Background

This study was carried out to evaluate the effects of a long-term high-fat diet on lipids and lipoproteins composition in thoracic duct lymph in pigs.

Material/Methods

We examined lymph taken from the thoracic duct from 24 female white sharp-ear pigs, divided into 3 experimental groups fed different diets for 12 months: (a) the control group, fed the standard balanced diet; (b) the HFD group, fed an unbalanced, high-fat diet, and (c) the reversal diet group (RD), fed an unbalanced, high-fat diet for 9 months and then a standard balanced diet for 3 months.

Results

Lymph analysis after 12 months of fixed diets revealed significantly higher concentration of proteins in the HFD group in comparison to the control and RD groups. Examination of lymph lipoproteins fractions showed that the high-fat diet in the HFD group in comparison to control group caused an increase in cholesterol, phospholipids, and proteins content within HDL and chylomicrons. There were also more proteins within HDL in the HFD group in comparison to the RD group and more triglycerides within chylomicrons in the HFD group in comparison to the control group.

Conclusion

A long-term high-fat diet resulted in changed structure of HDL and chylomicrons in the thoracic duct lymph. Alterations in HDL composition suggest that a high-fat diet enhances reverses cholesterol transport. Changes in chylomicrons structure show the adaptation to more intense transport of dietary fat from the intestine to the liver under the influence of a high-fat diet. Reversal to a standard balanced diet had the opposite effects.

Concentrations of electrophoretic and size subclasses of apolipoprotein A-I-containing particles in human peripheral lymph

When cultured cells are exposed to plasma, the initial acceptors of unesterified cholesterol are small lipid-poor apolipoprotein A-I (apoA-I)-containing high density lipoproteins (HDLs) with pre-beta electrophoretic mobility. These are converted by lecithin:cholesterol acyltransferase into larger spheroidal cholesteryl ester-rich HDLs with alpha mobility. To study the determinants of the concentration of small pre-beta HDLs in tissue fluids, we collected prenodal peripheral lymph from 34 fasted normal men. By crossed immunoelectrophoresis, the concentration of pre-beta HDLs in lymph averaged 20% of that in plasma. On multiple regression analysis, pre-beta apoA-I concentration in lymph was directly related to pre-beta apoA-I concentration in plasma and independently to alpha apoA-I concentration in lymph. Similar results were obtained when the same apoA-I-containing particles were quantified by size exclusion chromatography. Lymph pre-beta apoA-I concentration was low in a subject with familial lecithin:cholesterol acyltransferase deficiency, despite a normal plasma pre-beta apoA-I concentration, but was normal in a subject with familial lipoprotein lipase deficiency. These results suggest that the concentration of small pre-beta HDLs in human tissue fluids is determined only in part by the transfer of pre-beta HDLs across capillary endothelium from plasma. Local production, by remodeling of spheroidal alpha HDLs in tissue fluids, may be equally important. Lipolysis of triglyceride-rich lipoproteins by lipoprotein lipase appears to have little effect.

Exploitation of major histocompatibility complex class I molecules and caveolae by simian virus 40

Simian virus 40 (SV40), a non-enveloped DNA virus, is transported from the cell surface to the nucleus where virus replication occurs. This pathway of virus uptake involves binding to surface MHC class I molecules, entry via non-coated pits, and subsequent transport to the endoplasmic reticulum (ER). At some stage in this pathway the virus must cross a membrane to reach the cytosol. In the present review, the cellular machinery which the virus has utilized to enter the cell will be examined. In particular, we will consider recent evidence for the involvement of caveolae in the infectious entry step and propose a model involving recruitment of caveolar proteins around the membrane-bound virus. We also speculate that a similar mechanism may have been exploited by bacterial pathogens. The subsequent steps by which SV40 reaches the ER remain unclear but recent evidence suggests that this pathway may be shared with several other proteins that are transported from surface caveolae to the ER.

Transendothelial transport of modified low-density lipoproteins

Human umbilical vein endothelial cell monolayers were grown as monolayers on porous filters and their transcellular transport and degradation of 125I-labelled native and modified forms of LDL, supplied to either the intimal or the luminal face, were measured. Intact native, acetylated and oxidized LDL were all transported in both directions across the cell monolayers by receptor-independent mechanisms, and all forms of LDL were transported at similar rates. However, the mass of intact LDL transported from the intimal to the luminal face of the monolayer was always fourfold more than that transported in the opposite direction under similar conditions. In addition to LDL transport, endothelial cell monolayers also degraded native and modified forms of LDL by predominantly receptor-dependent routes, in that these could be inhibited (> 70%) by the addition of a 20-fold excess of the same form of (but unlabelled) LDL. The measured amounts of lipoprotein degraded were the same whether supplied to the intimal or the luminal face. Incubation of endothelial cells with oxidized LDL led to intracellular accumulation of a pool of macromolecular apo B which was apparently resistant to lysosomal proteolysis.

Recent evidence for the involvement of catecholamines and of macrophages in atherosclerotic processes

The atherogenetic uptake of low density lipoprotein (LDL) is accelerated by the catecholamines adrenaline and noradrenaline at their pathophysical blood concentrations in rabbits and rats. A similar effect in man could account for the coronary risk factor status of cigarette smoking, hypertension, stress, etc. when associated with elevated circulating catecholamines. In human atheromatous lesions, the concentration of macrophages is greater in plaques which have fissured or ulcerated than in those which have not. This is compatible with the proposition that macrophages contribute to plaque fissure, the commonest immediate cause of coronary thrombosis.

Transport pathways of beta-VLDL by aortic endothelium of normal and hypercholesterolemic rabbits

The uptake and transport of beta-VLDL by the aortic endothelium was investigated in normal and hyperlipidemic rabbits fed a cholesterol-enriched diet for 1 week to 5 months. Weekly (in the first month) or every other week afterwards, animals were given one of the following probes: (a) [125I]-beta-VLDL injected in vivo and after 24 h the whole aorta or its intima and media were separately collected and examined by spectrometry and autoradiography; (b) [125I]-beta-VLDL coupled to the fluorescent probe 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate perfused in situ for 1-2 h and aorta examined by radioassay and fluorescence microscopy; (c) beta-VLDL-gold complex perfused in situ for 10-15 min and aortic fragments examined by electron microscopy. In addition, cryosections of aortic wall were processed for the immunocytochemical detection of apolipoprotein B and apolipoprotein E. The results showed that both in normal and hyperlipidemic rabbits, the aortic endothelium transports plasma beta-VLDL by a dual pathway: (i) endocytosis involving coated pits and vesicles, endosomes, multivesicular bodies and lysosomes, and (ii) transcytosis, the predominant process, carried out by plasmalemmal vesicles. Both processes, and especially transcytosis, are markedly increased in hyperlipidemia leading to progressive accumulation of beta-VLDL or/and its components in the subendothelial extracellular matrix. In prelesional stages of atherogenesis, beta-VLDL-gold complexes or deposits of apo B and apo E were detected in close association with extracellular liposomes. With the appearance of intimal macrophage-derived foam cells, the immunoperoxidase reaction product, revealing the presence of the two apolipoproteins, could also be seen in intracellular lipid inclusions.

Increased uptake of methylated low-density lipoprotein induced by noradrenaline in carotid arteries of anaesthetized rabbits

Atherosclerosis is accelerated in hyperlipidaemias but, apart from the concentration of low-density lipoprotein (LDL) in the blood, very little is known about other influences on the disease process. We now provide evidence that in anaesthetized rabbits the atherogenic uptake of LDL by arterial walls is accelerated by noradrenaline at its physiological concentrations in rabbit and human blood. The principle of the experiments was to compare the uptake of intravenously injected, radioactively labelled LDL, methylated to prevent removal by high-affinity receptors, in the two carotid arteries of anaesthetized rabbits after infusing low concentrations of noradrenaline into one carotid and saline as control into the other, the volume rates of infusion being about 1% of the carotid blood flows. Human LDL, which behaves sufficiently like rabbit LDL for these purposes, was prepared, methylated and radio-iodinated by standard methods. At the end of the infusions, the arteries were excised and their radioactivities determined. Noradrenaline infused for 2 h to produce local blood concentrations of nominally 1, 10, 50 and 100 nM significantly increased the LDL radioactivities of the walls of the noradrenaline-infused carotids. Concentrations of nominally 100 nM also increased the LDL radioactivities of the walls of the saline-infused carotids; this was associated with significant increases in their blood noradrenaline concentrations. These results may contribute towards an explanation for the accelerated atherosclerosis and the increased incidence of its clinical manifestations in conditions associated with elevated blood noradrenaline concentrations, including the episodic increases associated with stress and cigarette smoking as well as the more persistent increases caused by phaeochromocytoma.

Comparison of uptake and degradation of low density lipoproteins by arteries and veins of rabbits

The uptake and degradation of low density lipoproteins were compared in arteries and veins of rabbits in vivo. Rabbits were injected with low density lipoproteins labelled singly with 125I (125I-LDL) or doubly with 131I and with [125I]tyramine cellobiose (TC-LDL) which remains trapped intracellularly. After increasing times up to 24 h the largest vessels were perfused to remove blood, fixed in situ and excised. When the uptake of 125I-LDL by arteries and veins was compared on the basis of luminal surface area, there were no significant differences up to 3.5 h. When degradation rates of double labelled LDL were compared after 24 h on the same basis, the arteries contained significantly more TC label than the veins, but when the comparison was based on dry weights there were no significant differences. These results show that veins, like arteries, take up LDL from the circulating blood; and they suggest that the preferential accumulation of LDL in arteries susceptible to atherosclerosis may be accounted for, at least in part, by more degradation of LDL in arteries than in veins.

Increased uptake of monocyte-treated low density lipoproteins by aortic endothelium in vivo

A new technique was elaborated for measuring LDL uptake by rat aortic endothelial cells in vivo, using a fluorescent marker (Dil)-labelled LDL and quantifying the fluorescence in cells selectively removed from the aorta. This technique was used to study the endothelial uptake of LDL modified by activated human monocytes (LDL-A) in comparison with native LDL (LDL-N) protected from oxidation by vitamin E during the preparation. Incubation of LDL with activated monocytes increased endothelial uptake in vivo by 9.3-fold and also by 4.4-fold in cultured confluent porcine endothelium. In contrast, only a 1.5-fold increase in uptake of LDL-A was observed in sparse cultures. Cytotoxicity of monocyte-altered or native LDL did not differ as measured by the [3H]deoxyglucose-release test on cultured endothelium. Our results suggest that modification of LDL in the circulation by monocytes may make an important contribution to atherogenesis.