Lipoprotein cholesterol and atherosclerosis

Progressive accumulation of cholesterol in the arterial wall causes atherosclerosis, the pathologic process underlying most heart attacks and strokes. Low density lipoprotein (LDL), the major carrier of blood cholesterol, has been implicated in the buildup of cholesterol in atherosclerotic plaques. Endothelial cells that line arteries function to transport LDL into the vessel wall. Models for the mechanism of cholesterol accumulation in atherosclerotic plaques emphasize increased LDL uptake into the vessel wall or increased retention of LDL that has entered the vessel wall. This article reviews the pathways of cholesterol entry and removal, the metabolism, and the physical changes of cholesterol in the vessel wall. How these processes are believed to contribute to cholesterol buildup in atherosclerotic plaques is discussed.     

Theoretical study on flow-dependent concentration polarization of low density lipoproteins at the luminal surface of a straight artery

It is suspected that physical and fluid mechanical factors play important roles in the localization of atherosclerotic lesions and intimal hyperplasia in man by affecting the transport of cholesterol in flowing blood to arterial walls. Hence, we have studied theoretically the effects of various physical and fluid mechanical factors such as wall shear rate, diffusivity of low density lipoproteins (LDL), and filtration velocity of water at the vessel wall on surface concentration of LDL at an arterial wall by means of a computer simulation of convective and diffusive transport of LDL in flowing blood to the wall of a straight artery under conditions of a steady flow. It was found that under normal physiologic conditions prevailing in the human arterial system, due to the presence of a filtration flow of water at the vessel wall, flow-dependent concentration polarization (accumulation or depletion) of LDL occurs at a blood/endothelium boundary. The surface concentration of LDL at an arterial wall takes higher values than that in the bulk flow in that vessel, and it is affected by three major factors, that is, wall shear rate, gamma w, filtration velocity of water at the vessel wall, Vw, and the distance from the entrance of the artery, L. It increases with increasing Vw and L, and decreasing gamma w hence the flow rate. Thus, under certain circumstances, the surface concentration of LDL could rise locally to a value which is several times higher than that in the bulk flow, or drop locally to a value even lower than a critical concentration for the maintenance of normal functions and survival of cells forming the vessel wall. These results suggest the possibility that all the vascular phenomena such as the localization of atherosclerotic lesions and intimal hyperplasia, formation of cerebral aneurysms, and adaptive changes of lumen diameter and wall structure of arteries and veins to certain changes in hemodynamic conditions in the circulation are governed by this flow-dependent concentration polarization of LDL which carry cholesterol.     

Concentration polarization of atherogenic lipids in the arterial system

Nomenclature c, Normalized LDL concentration (C*/C0); C0, incoming (bulk) LDL concentration (gr/cm3); Cw, LDL concentration on the luminal surface (gr/cm3); ,wC time average value of LDL concentration on the luminal surface (gr/cm3); D, diffusion coef-ficient of LDL (cm2/s); Q, blood flow rate (mL/s); 0R, average internal radius of the artery (cm); Re, Reynolds number (002/Run); Sc, Schmidt number (/Dn); t, normalized time (00*/tuR); u, normalized axial velocity (0*/uu); 0u, time a…     

Concentration polarization of atherogenic lipids in the arterial system

The transport of atherogenic lipids (LDL) in a straight segment of an artery with a semi-permeable wall was simulated numerically. The numerical analysis predicted that a mass transport phenomenon called ’concentration polarization’ of LDL might occur in the arterial system. Under normal physiological flow conditions, the luminal surface LDL concentration was 5%–14% greater than the bulk concentration in a straight segment of an artery. The luminal surface LDL concentration at the arterial wall was flow-dependent, varying linearly with the filtration rate across the arterial wall and inversely with wall shear rate. At low wall shear rate, the luminal surface LDL concentration was very sensitive to changes in flow conditions, decreasing sharply as wall shear rate increased. In order to verify the numerical analysis, the luminal surface concentration of bovine serum albumin (as a tracer macromolecule) in the canine carotid artery was measured in vitro by directly taking liquid samples from the luminal surface of the artery. The experimental result was in very good agreement with the numerical analysis. The authors believe that the mass transport phenomenon of ‘concentration polarization’ may indeed exist in the human circulation and play an important role in the localization of atherosclerosis.     

Effects of transmural pressure and wall shear stress on LDL accumulation in the arterial wall: a numerical study using a multilayered model

The accumulation of low-density lipoprotein (LDL) is recognized as one of the main contributors in atherogenesis. Mathematical models have been constructed to simulate mass transport in large arteries and the consequent lipid accumulation in the arterial wall. The objective of this study was to investigate the influences of wall shear stress and transmural pressure on LDL accumulation in the arterial wall by a multilayered, coupled lumen-wall model. The model employs the Navier-Stokes equations and Darcy's Law for fluid dynamics, convection-diffusion-reaction equations for mass balance, and Kedem-Katchalsky equations for interfacial coupling. To determine physiologically realistic model parameters, an optimization approach that searches optimal parameters based on experimental data was developed. Two sets of model parameters corresponding to different transmural pressures were found by the optimization approach using experimental data in the literature. Furthermore, a shear-dependent hydraulic conductivity relation reported previously was adopted. The integrated multilayered model was applied to an axisymmetric stenosis simulating an idealized, mildly stenosed coronary artery. The results show that low wall shear stress leads to focal LDL accumulation by weakening the convective clearance effect of transmural flow, whereas high transmural pressure, associated with hypertension, leads to global elevation of LDL concentration in the arterial wall by facilitating the passage of LDL through wall layers.     

Visualization of the binding, endocytosis, and transcytosis of low- density lipoprotein in the arterial endothelium in situ

We investigated the interaction and transport of low-density lipoprotein (LDL) through the arterial endothelium in rat aorta and coronary artery, by perfusing in situ native, untagged human, and rat LDL. The latter was rendered electron-opaque after it interacted with the endothelial cell and was subsequently fixed within tissue. We achieved LDL electron-opacity by an improved fixation procedure using 3,3'-diaminobenzidine, and mordanting with tannic acid. The unequivocal identification of LDL was implemented by reacting immunocytochemically the perfused LDL with anti LDL-horseradish peroxidase conjugate. Results indicate that LDL is taken up and internalized through two parallel compartmented routes. (a) A relatively small amount of LDL is taken up by endocytosis via: (i) a receptor-mediated process (adsorptive endocytosis) that involved coated pits/vesicles, and endosomes, and, probably, (ii) a receptor-independent process (fluid endocytosis) carried out by a fraction of plasmalemmal vesicles. Both mechanisms bringing LDL to lysosomes supply cholesterol to the endothelial cell itself. (b) Most circulating LDL is transported across the endothelial cell by transcytosis via plasmalemmal vesicles which deliver LDL to the other cells of the vessel wall. Endocytosis is not enhanced by increasing LDL concentration, but the receptor-mediated internalization decreases at low temperature. Transcytosis is less modified by low temperature but is remarkably augmented at high concentration of LDL. While the endocytosis of homologous (rat) LDL is markedly more pronounced than that of heterologous (human) LDL, both types of LDL are similarly transported by transcytosis. These results indicate that the arterial endothelium possesses a dual mechanism for handling circulating LDL: by a high affinity process, endocytosis secures the endothelial cells' need for cholesterol; by a low-affinity nonsaturable uptake process, transcytosis supplies cholesterol to the other cells of the vascular wall, and can monitor an excessive accumulation of plasma LDL. Since in most of our experiments we used LDL concentrations above those found in normal rats, we presume that at low LDL concentrations saturable high-affinity uptake would be enhanced in relation to nonsaturable pathways.     

Effects of hypoxia on cholesterol metabolism in human monocyte-derived macrophages

We assessed the metabolism of low density lipoprotein (LDL) of human monocyte-derived macrophages under hypoxia. The specific binding and association of 125I-labeled LDL (125I-LDL) were not changed under hypoxia compared to normoxia. However, the degradation of 125I-LDL under hypoxia decreased to 60%. The rate of cholesterol esterification under hypoxia was 2-fold greater on incubation with LDL or 25-hydroxycholesterol. The cellular cholesteryl ester content was also greater under hypoxia on incubation with LDL. Secretion of apolipoprotein E into the medium was not altered under hypoxia, suggesting that apolipoprotein E independent cholesterol efflux may be reduced under hypoxia. Thus, hypoxia affects the intracellular metabolism of LDL, stimulates cholesterol esterification, and enhances cholesteryl ester accumulation in macrophages. Hypoxia is one of the important factors modifying the cellular lipid metabolism in arterial wall.     

Effect of the endothelial glycocalyx layer on arterial LDL transport under normal and high pressure

To quantitatively investigate the role of the endothelial glycocalyx layer (EGL) in protecting the artery from excessive infiltration of atherogenic lipids such as low density lipoproteins (LDLs), a multilayer model with the EGL of an arterial segment was developed to numerically simulate the flow and the transport of LDLs under normal and high pressure. The transport parameters of the layers of the model were obtained from the hydrodynamic theory, the stochastic theory, and from the literature. The results showed that the increase in the thickness of the EGL could lead to a sharp drop in LDL accumulation in the intima. A partial damage to the EGL could compromise its barrier function, hence leading to enhanced infiltration/accumulation of LDLs within the wall of the arterial model. Without the EGL, hypertension could lead to a significantly enhanced LDL transport into the wall of the model. However, the intact EGL could protect the arterial wall from hypertension so that the LDL concentration in the intima layer was almost the same as that under normal pressure conditions. The results also showed that LDL concentration within the arterial wall increased with Φ (the fraction of leaky junctions) on the intima layer. The increase in LDL concentration with Φ was much more dramatic for the model without the EGL. For instance, without the EGL, a Φ of 0.0005 could lead LDL concentration within the arterial wall to be even higher than that predicted for the EGL intact model with a Φ of 0.002. In conclusion, an intact EGL with a sufficient thickness may act as a barrier to LDL infiltration into the arterial wall and has the potential to suppress the hypertension-driven hike of LDL infiltration/accumulation in the arterial wall.     

Regulation of low density lipoprotein receptor activity by cultured human arterial smooth muscle cells.

The ability of cultured human arterial smooth muscle cells to regulate low density lipoprotein (LDL) receptor activity was tested. In contrast to human skin fibroblasts incubated with lipoprotein deficient medium under identical conditions, smooth muscle cells showed significantly reduced enhancement of 125I-labeled LDL and 125I-labeled VLDL (very low density lipoprotein) binding. Smooth muscle cells also failed to suppress LDL receptor activity during incubation with either LDL or cholesterol added to the medium, while fibroblasts shoed an active regulatory response. Thus, in comparison with the brisk LDL receptor regulation characteristic of skin fibroblasts, arterial smooth muscle cells have and attenuated capacity to regulate their LDL receptor activity. These results may be relevant to the propensity of these cells to accumulate LDL and cholesterol and form "foam cells" in the arterial wall in vivo, a process associated with atherogenesis.     

胆固醇逆转运的新途径：经肠道直接排出体外？

过多的胆固醇沉积在动脉壁对机体极为有害,可以引起动脉粥样硬化甚至心血管疾病.而胆固醇逆转运(reverse cholesterol transport,RCT)可以逆转此过程.传统的RCT是指胆固醇由外周组织转运回肝脏进行再循环或以胆汁酸的形式随粪便排出体外的过程,此过程受多种因子调控.近几年研究发现,胆固醇还可由血经过肠道直接分泌（transintestinal cholesterol efflux, TICE）通路随粪便排出体外,此过程对外界刺激更敏感.RCT已经成为防治动脉粥样硬化研究的新靶点,TICE有可能成为更有效的RCT调控通路.     

Human venous and arterial glycosaminoglycans have similar affinity for plasma low-density lipoproteins

We compared the glycosaminoglycan content of human venous and arterial walls. The most abundant glycosaminoglycan in human veins is dermatan sulfate whereas chondroitin 4/6-sulfate is preponderant in arteries. The concentrations of chondroitin 4/6-sulfate and heparan sulfate are approximately 4.8- and approximately 2.5-fold higher in arteries than in veins whereas dermatan sulfate contents are similar in the two types of blood vessels. Normal and varicose saphenous veins do not differ in their glycosaminoglycan contents. It is known that certain glycosaminoglycan species from the arterial wall, mainly high-molecular-weight fractions of dermatan sulfate+chondroitin 4/6-sulfate have greater affinity for plasma LDL. These types of glycosaminoglycans can be identified on a LDL-affinity column. We now demonstrated that a similar population of glycosaminoglycan also occurs in veins, although with a lower concentration than in the arteries due to less chondroitin 4/6-sulfate with affinity for LDL. The concentrations of dermatan sulfate species, which interact with LDL, are similar in arteries and veins. The presence of these glycosaminoglycans with affinity to plasma LDL in veins raises interesting questions concerning the role of these molecules in the pathogenesis of atherosclerosis. Possibly, the presence of these glycosaminoglycans in the vessel wall are not sufficient to cause retention of LDL and consequently endothelial dysfunction, but may require additional intrinsic factors and/or the hydrodynamic of the blood under the arterial pressure.     

17beta-estradiol reduces tumor necrosis factor-alpha-mediated LDL accumulation in the artery wall

Estrogens have direct effects on the vascular wall that may prevent the development of atherosclerosis. In particular, estrogens, such as 17beta-estradiol (estradiol), are known to have potent antioxidant activity. Tumor necrosis factor-alpha (TNF) is found in human atheroma and produces oxygen-derived free radicals. These oxygen-derived free radicals may modify low density lipoproteins (LDL) and increase LDL binding in the artery wall. We asked: 1) does TNF increase LDL accumulation in the artery wall and 2) can the TNF-mediated increase in LDL accumulation be prevented by the antioxidant activity of estradiol? Carotid arteries from ovariectomized 3-month-old rats were removed and perfused with fluorescently labeled LDL and arterial LDL flux was measured using quantitative fluorescence microscopy. In six arteries, addition of TNF (10 ng/ml) to the perfusate resulted in a 2.3-fold increase in the rate of LDL accumulation (1.50 +/- 0.37 ng/min per cm2 vs. 3.38 +/- 0.48 ng/min per cm2; P < 0.01). Estradiol (65 pg/ml) and alpha-tocopherol (6 mg/L) both attenuated TNF-mediated LDL accumulation (P < 0.05), indicating that TNF may exert its effects on LDL accumulation through cellular production of oxygen-derived free radicals. These results support an antioxidant role for estradiol in the protection against LDL accumulation in the artery wall and subsequent progression of atherosclerosis.     

Mechanobiology of low-density lipoprotein transport within an arterial wall—Impact of hyperthermia and coupling effects

The effects of hyperthermia, coupling attributes and property variations on Low-density lipoprotein (LDL) transport within a multi-layered wall while accounting for the fluid structure interaction (FSI) is analyzed in this work. To understand the potential impact of the hyperthermia process, thermo-induced attributes are incorporated, accounting for the plasma flow, mass transfer, as well as the elastic wall structure. The coupling effect of osmotic pressure, Soret and Dufour diffusion is discussed and their influence on LDL transport is examined, demonstrating that only the Soret effect needs to be accounted for. The effect of thermal expansion on changing the behavior of flow, mass transport, and elastic structure is illustrated and analyzed while incorporating the variations in the effective LDL diffusivity and consumption rate, as well as other dominating parameters. It is shown that hyperthermia results in an enhancement in LDL transport by increasing the concentration levels within the arterial wall.     

Rapid intracellular transport of LDL-derived cholesterol to the plasma membrane in cultured fibroblasts

The kinetics of low density lipoprotein (LDL) cholesterol transport to the plasma membrane of Chinese hamster ovary (CHO) cells was studied. LDL was reconstituted with [3H]cholesteryl linoleate and added to CHO cells in a pulse-chase experiment. The internalization and lysosomal cleavage of reconstituted LDL (rLDL) [3H]cholesteryl linoleate to free [3H]cholesterol occurred with a half-time of 37 min after a 30-min lag. The rate of transport of released [3H]cholesterol to the plasma membrane was measured by brief (20-30 sec) cholesterol oxidase treatment of intact, adherent cells: the half-time of transport was 42 min. The similarity in the rate of free cholesterol release from rLDL and transport of this cholesterol to the plasma membrane suggests very rapid transport of rLDL cholesterol from the lysosome to the plasma membrane. Cells were shown to be intact throughout the cholesterol oxidase treatment by the absence of cell-derived lactate dehydrogenase (LDH) activity or K+ in the assay buffer.     

Simulation of Low Density Lipoprotein (LDL) permeation into multilayer coronary arterial wall: Interactive effects of wall shear stress and fluid-structure interaction in hypertension

Due to increased atherosclerosis-caused mortality, identification of its genesis and development is of great importance. Although, key factors of the origin of the disease is still unknown, it is widely believed that cholesterol particle penetration and accumulation in arterial wall is mainly responsible for further wall thickening and decreased rate of blood flow during a gradual progression. To date, various effective components are recognized whose simultaneous consideration would lead to a more accurate approximation of Low Density Lipoprotein (LDL) distribution within the wall. In this research, a multilayer Fluid-Structure Interaction (FSI) model is studied to simulate the penetration of LDL into the arterial wall. Distention impact on wall properties is taken into account by considering FSI and Wall Shear Stress (WSS) dependent endothelium properties. The results show intensified permeation of LDL whilst the FSI approach is applied. In addition, luminal distension prompted by FSI reduces WSS along lumen/wall interface, especially in hypertension. This effect leads to a lowered endothelial resistance against LDL permeation, comparing to the case in which WSS effect is overlooked. The results are in an acceptable consistency with the clinical researches on WSS effect on atherosclerosis development.     

Inhibition of thymidine transport by low density lipoprotein and its lipid components

We previously showed that low density lipoprotein (LDL) induces an early decrease in the thymidine-transport rate and subsequently inhibits cell proliferation. We now have demonstrated that a dose-dependent decrease in the rate of thymidine transport was found in all the cell lines examined irrespective of their sensitivity to the inhibitory effect of LDL on cell growth. Thus, inhibition of both transport and cell growth by LDL are not necessarily coupled. In contrast, 3-O-methyl-D-glucose transport was not affected by an addition of LDL. The specific inhibition of transport was not suppressed by an addition of chloroquine. Thus, degradation of LDL within the lysosome was not required for LDL to inhibit thymidine transport. A mixture of lipids extracted from LDL was as inhibitory as LDL in the various types of cells examined. Calf serum also prevented the inhibitory effect of both LDL and its lipid constituents equally. Phosphatidylcholine produced no decrease in thymidine transport, but cholesterol inhibited it. We concluded that LDL-associated lipids, particularly cholesterol, play an essential role in the LDL-induced decrease of transport.     

Cholesterol delivered to macrophages by oxidized low density lipoprotein is sequestered in lysosomes and fails to efflux normally

Oxidized low density lipoprotein (LDL) has been found to exhibit numerous potentially atherogenic properties, including transformation of macrophages to foam cells. It is believed that high density lipoprotein (HDL) protects against atherosclerosis by removing excess cholesterol from cells of the artery wall, thereby retarding lipid accumulation by macrophages. In the present study, the relative rates of HDL-mediated cholesterol efflux were measured in murine resident peritoneal macrophages that had been loaded with acetylated LDL or oxidized LDL. Total cholesterol content of macrophages incubated for 24 h with either oxidized LDL or acetylated LDL was increased by 3-fold. However, there was no release of cholesterol to HDL from cells loaded with oxidized LDL under conditions in which cells loaded with acetylated LDL released about one-third of their total cholesterol to HDL. Even mild degrees of oxidation were associated with impairment of cholesterol efflux. Macrophages incubated with vortex-aggregated LDL also displayed impaired cholesterol efflux, but aggregation could not account for the entire effect of oxidized LDL. Resistance of apolipoprotein B (apoB) in oxidized LDL to lysosomal hydrolases and inactivation of hydrolases by aldehydes in oxidized LDL were also implicated. The subcellular distribution of cholesterol in oxidized LDL-loaded cells and acetylated LDL-loaded cells was investigated by density gradient fractionation, and this indicated that cholesterol derived from oxidized LDL accumulates within lysosomes. Thus impairment of cholesterol efflux in oxidized LDL-loaded macrophages appears to be due to lysosomal accumulation of oxidized LDL rather than to impaired transport of cholesterol from a cytosolic compartment to the plasma membrane.     

Computational analysis of coupled blood-wall arterial LDL transport

The transport of macromolecules, such as low density lipoproteins (LDLs), across the artery wall and their accumulation in the wall is a key step in atherogenesis. Our objective was to model fluid flow within both the lumen and wall of a constricted, axisymmetric tube simulating a stenosed artery, and to then use this flow pattern to study LDL mass transport from the blood to the artery wall. Coupled analysis of lumenal blood flow and transmural fluid flow was achieved through the solution of Brinkman's model, which is an extension of the Navier-Stokes equations for porous media. This coupled approach offers advantages over traditional analyses of this problem, which have used possibly unrealistic boundary conditions at the blood-wall interface; instead, we prescribe a more natural pressure boundary condition at the adventitial vasa vasorum, and allow variations in wall permeability due to the occurrence of plaque. Numerical complications due to the convection dominated mass transport process (low LDL diffusivity) are handled by the streamline upwind/Petrov-Galerkin (SUPG) finite element method. This new fluid-plus-porous-wall method was implemented for conditions typical of LDL transport in a stenosed artery with a 75 percent area reduction (Peclet number=2 x 10(8)). The results show an elevated LDL concentration at the downstream side of the stenosis. For the higher Darcian wall permeability thought to occur in regions containing atheromatous lesions, this leads to an increased transendothelial LDL flux downstream of the stenosis. Increased transmural filtration in such regions, when coupled with a concentration-dependent endothelial permeability to LDL, could be an important contributor to LDL infiltration into the arterial wall. Experimental work is needed to confirm these results.     

Effects of oxidatively modified LDL on cholesterol esterification in cultured macrophages

Oxidative modification of low density lipoproteins (LDL) has been shown to cause accelerated degradation of LDL via the scavenger receptor pathway in cultured macrophages, and it has been proposed that this process might lead to cholesterol accumulation in macrophages in the arterial wall in vivo. However, oxidation of LDL is accompanied by a substantial reduction in LDL total cholesterol content and hence the amount of cholesterol delivered by oxidatively modified LDL may be less than that delivered by scavenger receptor ligands such as acetyl LDL which results in massive cholesterol accumulation in cultured macrophages. The present studies were done to determine whether the decrease in total cholesterol content during LDL oxidation was due to oxidation of cholesterol and cholesteryl ester, and to determine whether the resulting oxidized sterols could affect cholesterol esterification in cultured macrophages. It was found that when LDL prelabeled with [3H]cholesteryl linoleate was oxidized, there was a decrease in cholesterol mass but no change in radioactivity. The radioactive substances derived from cholesteryl linoleate appeared more polar than the parent compound when analyzed by reverse-phase liquid chromatography, but were not identical with free cholesterol. Thin-layer chromatography of oxidized LDL lipids confirmed the loss of esterified cholesterol, and revealed multiple new bands, some of which matched reference oxysterols including 7-ketocholesterol, 5,6-epoxycholesterol, and 7-hydroxycholesterol. In addition to oxysterols, oxidized cholesteryl esters were also present. Quantitation by gas chromatography indicated that 7-ketocholesterol was the major oxysterol present.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vitro inhibition of cholesterol uptake in human and animal arteries by 7-ketocholesterol

Human and pig coronary arteries and rabbit aortas were perfused with pulsatile pressure in a modified Lindbergh apparatus with blood plasma obtained from the same species. Uptake of cholesterol by the arterial wall was measured using [³H]-cholesterol as tracer. Percent distribution of synthesized lipid fractions was determined by thin-layer chromatography and liquid scintillation counting. Inhibition of cholesterol uptake by the arterial wall was studied by the addition of 7-ketocholesterol (concentrations of from 0.05 to 1 μmoles/ml in the perfusate). The addition of 7-ketocholesterol to the perfusate reduced cholesterol uptake by the vessel by an average of 90%. At concentrations of from 0.1 to 1 μmoles/ml of perfusate, 7-ketocholesterol inhibition remained unchanged. Inhibition was reduced at concentrations of ketocholesterol of 0.05 μmoles/ml. Inhibition was present in all species, and was not due to oxidation of cholesterol to 7-ketocholesterol in the perfusate. The results suggest inhibition of cholesterol uptake in the arterial wall by a competitive process.