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.     

HDL endocytosis and resecretion

HDL removes excess cholesterol from peripheral tissues and delivers it to the liver and steroidogenic tissues via selective lipid uptake without catabolism of the HDL particle itself. In addition, endocytosis of HDL holo-particles has been debated for nearly 40 years. However, neither the connection between HDL endocytosis and selective lipid uptake, nor the physiological relevance of HDL uptake has been delineated clearly. This review will focus on HDL endocytosis and resecretion and its relation to cholesterol transfer. We will discuss the role of HDL endocytosis in maintaining cholesterol homeostasis in tissues and cell types involved in atherosclerosis, focusing on liver, macrophages and endothelium. We will critically summarize the current knowledge on the receptors mediating HDL endocytosis including SR-BI, F₁-ATPase and CD36 and on intracellular HDL transport routes. Dependent on the tissue, HDL is either resecreted (retro-endocytosis) or degraded after endocytosis. Finally, findings on HDL transcytosis across the endothelial barrier will be summarized. We suggest that HDL endocytosis and resecretion is a rather redundant pathway under physiologic conditions. In case of disturbed lipid metabolism, however, HDL retro-endocytosis represents an alternative pathway that enables tissues to maintain cellular cholesterol homeostasis.     

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.     

Effect of irreversibly glycated LDL in human vascular smooth muscle cells: lipid loading,oxidative and inflammatory stress

The major complication of diabetes is accelerated atherosclerosis, the progression of which entails complex interactions between the modified low‐density lipoproteins (LDL) and the cells of the arterial wall. Advanced glycation end product‐modified‐LDL (AGE‐LDL) that occurs at high rate in diabetes contributes to diabetic atherosclerosis, but the underlying mechanisms are not fully understood. The aim of this study was to assess the direct effect of AGE‐LDL on human vascular smooth muscle cells (hSMC) dysfunction. Cultured hSMC incubated (24 hrs) with human AGE‐LDL, native LDL (nLDL) or oxidized LDL (oxLDL) were subjected to: (i) quantification of the expression of the receptors for modified LDL and AGE proteins (LRP1, CD36, RAGE) and estimation of lipid loading, (ii) determination of NADPH oxidase activity and reactive oxygen species (ROS) production and (iii) evaluation of the expression of monocyte chemoattractant protein‐1 (MCP‐1). The results show that exposure of hSMC to AGE‐LDL (compared to nLDL) induced: (a) increased NADPH oxidase activity (30%) and ROS production (28%) by up‐regulation of NOX1, NOX4, p22phox and p67phox expression, (b) accumulation of intracellular cholesteryl esters, (c) enhanced gene expression of LRP1 (160%) and CD36 (35%), and protein expression of LRP1, CD36 and RAGE, (d) increased MCP‐1 gene expression (160%) and protein secretion (300%) and (e) augmented cell proliferation (30%). In conclusion, AGE‐LDL activates hSMC (increasing CD36, LRP1, RAGE), inducing a pro‐oxidant state (activation of NADPHox), lipid accumulation and a pro‐inflammatory state (expression of MCP‐1). These results may partly explain the contribution of AGE‐LDL and hSMC to the accelerated atherosclerosis in diabetes.     

Endostatin binds biglycan and LDL and interferes with LDL retention to the subendothelial matrix during atherosclerosis

Retention of lipoproteins to proteoglycans in the subendothelial matrix (SEM) is an early event in atherosclerosis. We recently reported that collagen XVIII and its proteolytically released fragment endostatin (ES) are differentially depleted in blood vessels affected by atherosclerosis. Loss of collagen XVIII/ES in atherosclerosis-prone mice enhanced plaque neovascularization and increased the vascular permeability to lipids by distinct mechanisms. Impaired endothelial barrier function increased the influx of lipoproteins across the endothelium; however, we hypothesized that enhanced retention might be a second mechanism leading to the increased lipid content in atheromas lacking collagen XVIII. We now demonstrate a novel property of ES that binds both the matrix proteoglycan biglycan and LDL and interferes with LDL retention to biglycan and to SEM. A peptide encompassing the alpha coil in the ES crystal structure mediates the major blocking effect of ES on LDL retention. ES inhibits the macrophage uptake of biglycan-associated LDL indirectly by interfering with LDL retention to biglycan, but it has no direct effect on the macrophage uptake of native or modified lipoproteins. Thus, loss of ES in advanced atheromas enhances lipoprotein retention in SEM. Our data reveal a third protective role of this vascular basement membrane component during atherosclerosis.     

Lipid retention in the arterial wall of two mouse strains with different atherosclerosis susceptibility

LDL deposition in the subendothelium of arterial walls is the initial event in the development of atherosclerosis. The deposited LDL undergoes oxidative modification by arterial wall cells to become oxidized LDL and consequently contributes to atherosclerotic formation. Using mouse strains C57BL/6J (B6) and C3H/HeJ (C3H), which differ markedly in susceptibility to atherosclerosis, we determined whether variation in subendothelial retention of apolipoprotein B (apoB)-containing lipoproteins constitutes a genetic component in atherosclerosis. Lipoprotein retention was quantitated by Western blot analysis to detect the presence of apoB in aortic walls before foam cells developed. In both dietary and apoE-deficient models, B6 mice exhibited up to a 2-fold increase of apoB in the aortic wall compared with C3H mice. This increase could not be attributed to differences in plasma lipid levels of the two strains. In vitro, endothelial cells from C3H mice took up more acetylated and oxidized LDL but not native LDL and converted more native LDL to oxidized LDL than did endothelial cells from B6 mice. C3H mice expressed more scavenger receptor A in their aortic wall than B6 mice. Thus, variation in the subendothelial retention of apoB-containing lipoproteins cannot explain the dramatic difference in atherosclerosis susceptibility between B6 and C3H mice, and endothelial cells may play a role in alleviating lipid accumulation in arterial walls.     

Role of caveolin-1 in the regulation of lipoprotein metabolism

Lipoprotein metabolism plays an important role in the development of several human diseases, including coronary artery disease and the metabolic syndrome. A good comprehension of the factors that regulate the metabolism of the various lipoproteins is therefore key to better understanding the variables associated with the development of these diseases. Among the players identified are regulators such as caveolins and caveolae. Caveolae are small plasma membrane invaginations that are observed in terminally differentiated cells. Their most important protein marker, caveolin-1, has been shown to play a key role in the regulation of several cellular signaling pathways and in the regulation of plasma lipoprotein metabolism. In the present paper, we have examined the role of caveolin-1 in lipoprotein metabolism using caveolin-1-deficient (Cav-1(-/-)) mice. Our data show that, while Cav-1(-/-) mice show increased plasma triglyceride levels, they also display reduced hepatic very low-density lipoprotein (VLDL) secretion. Additionally, we also found that a caveolin-1 deficiency is associated with an increase in high-density lipoprotein (HDL), and these HDL particles are enriched in cholesteryl ester in Cav-1(-/-) mice when compared with HDL obtained from wild-type mice. Finally, our data suggest that a caveolin-1 deficiency prevents the transcytosis of LDL across endothelial cells, and therefore, that caveolin-1 may be implicated in the regulation of plasma LDL levels. Taken together, our studies suggest that caveolin-1 plays an important role in the regulation of lipoprotein metabolism by controlling their plasma levels as well as their lipid composition. Thus caveolin-1 may also play an important role in the development of atherosclerosis.     

Fisetin, morin and myricetin attenuate CD36 expression and oxLDL uptake in U937-derived macrophages

Dietary flavonoid intake has been reported to be inversely associated with the incidence of coronary artery disease. To clarify the possible role of flavonoids in the prevention of atherosclerosis, we investigated the effects of some of these compounds, including fisetin, morin and myricetin, on the susceptibility of low-density lipoprotein (LDL) to oxidative modification and on oxLDL uptake in macrophages. The results demonstrated that fisetin had stronger inhibitory activity than the other two on inhibiting Cu(2+)-mediated LDL oxidation measured by thiobarbituric acid-reactive substances assay (TBARS), conjugated diene formation and electrophoretic mobility. The class B scavenger receptor, CD36, to which oxLDL binds, is present in atherosclerotic lesions. Treatment of U937-derived macrophages with myricetin (20 microM) significantly inhibited CD36 cell surface protein and mRNA expression (p<0.01). Fisetin, morin and myricetin (20 microM) also reduced the feed-forward induction of CD36 mRNA and surface protein expression by PPARgamma. The inhibition of CD36 by flavonols was mediated by interference with PPARgamma activation thus counteracting the deleterious autoamplification loop of CD36 expression stimulated by PPARgamma ligand. All three flavonols (10 and 20 microM) markedly decreased the uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanide perchlorate (DiI)-labeled oxLDL uptake in U937-derived macrophages dose-dependently. Current evidences indicate that fisetin, morin and myricetin not only prevent LDL from oxidation but also block oxLDL uptake by macrophages at least in part through reducing CD36 gene expression on macrophages. In conclusion, flavonols may play a role in ameliorating atherosclerosis.     

Impaired Phagocytosis in Caveolin-1 Deficient Macrophages

Caveolae are plasma membrane invaginations that function as important regulators of numerous cellular processes, including signal transduction, cholesterol trafficking, and endocytosis. Caveolin-1 (Cav-1) constitutes the main structural protein of caveolae membrane. Here, we report an in vivo increase in the number of apoptotic cells in the thymus and spleen of Cav-1 deficient mice, following whole-body &gamma;-irradiation. We demonstrate that this increase in apoptotic cells is not due to increased apoptosis in lymphocytes per se, which normally do not express Cav-1, but rather to the decreased phagocytic clearance of apoptotic cells by macrophages, which do express Cav-1. Utilizing in vitro phagocytosis assays of both apoptotic thymocytes and Escherichia coli K-12 BioParticles, we demonstrate that the loss of Cav-1 decreases the phagocytic ability of thioglycollate-elicited peritoneal macrophages. We suggest that impaired macrophage phagocytosis in Cav-1 knockout mice could have implications for altered innate immunity against pathogens, the regulation of inflammatory responses, and the development of autoimmune disease.     

alpha-Tocopherol disturbs macrophage LXRalpha regulation of ABCA1/G1 and cholesterol handling

Based on the oxidation hypothesis high doses of α-tocopherol have been advocated to prevent atherosclerosis, but clinical trials failed to demonstrate a benefit. As specific oxylipids activate PPARγ and LXRα, master regulators of lipid metabolism and cholesterol exporters, we hypothesized, that high dose α-tocopherol might interfere with reverse cholesterol transport out of the vessel wall. Human THP-1 cells, a foam cell model, were preincubated with α-tocopherol or carrier before exposure to oxidized LDL, delipidated HDL or control buffer. Specific mRNAs were quantified by real-time RT-PCR, LXRα activation by a reporter gene assay and cellular cholesterol homeostasis by oxLDL and dHDL facilitated uptake and efflux assays. α-Tocopherol significantly reduced baseline expression and stimulation by oxLDL of LXRα activity, CD36, ABCA1, and ABCG1. α-Tocopherol also reversed the suppression of CD36 and ABCA1 by dHDL. Thus α-Tocopherol compromises cellular lipid scavenging and channelling of cholesterol into reverse transport out of the vessel wall.     

TGF-beta1 downregulates CD36 and scavenger receptor A but upregulates LOX-1 in human macrophages

Transforming growth factor-beta1 (TGF-beta1), a key cytokine for control of cell growth, extracellular matrix formation, and inflammation control, is secreted by many cells present in the arteriosclerotic plaque. Lipid accumulation in the vessel wall is regarded as an early step in atherogenesis and depends on uptake of modified low-density lipoprotein (LDL) by macrophages through scavenger receptors and their transformation into foam cells. Prominent members of the scavenger receptor family are the class A type I and II receptors (ScR-A), the class B receptor CD36, and the recently detected lectin-like oxidized LDL receptor-1 (LOX-1), which, unlike the native LDL receptor (LDL-R), are not feedback controlled. CD36 is responsible for >50% of modified LDL uptake into human monocyte-derived macrophages. We therefore studied whether TGF-beta1 influences expression and function of ScR-A, CD36, and LOX-1 in monocytes using RT-PCR and flow cytometry. Total uptake of oxidized LDL by monocytoid cells, reflecting the combined function of all scavenger receptors, was significantly reduced by TGF-beta1. At initially low picomolar concentrations, TGF-beta1 decreased CD36 mRNA and protein surface expression and ScR-A mRNA levels in the human monocytic cell line THP-1 and in freshly isolated and cultivated human monocytes, whereas LOX-1 mRNA was increased. Expression of LDL-R and beta-actin was not affected by TGF-beta1. In conclusion, depression of scavenger receptor function in monocytes by TGF-beta1 in low concentrations reduces foam cell formation. Together with matrix control by TGF-beta1, this may be important for atherogenesis and plaque stabilization.     

Feedback regulation of coronary artery disease susceptibility gene ADTRP and LDL receptors LDLR/CD36/LOX-1 in endothelia cell functions involved in atherosclerosis

A high level of low-density lipoprotein cholesterol (LDL) is one of the most important risk factors for coronary artery disease (CAD), the leading cause of death worldwide. However, a low concentration of LDL may be protective. Genome-wide association studies revealed that variation in ADTRP gene increased the risk of CAD. In this study, we found that a low concentration of oxidized-LDL induced the expression of ADTRP. Further analyses showed that knockdown of the expression of LDL receptor genes LDLR, CD36, or LOX-1 significantly downregulated ADTRP expression, whereas overexpression of LDLR/CD36/LOX-1 markedly increased ADTRP expression through the NF-κB pathway. Like ADTRP, LDLR, CD36 and LOX-1 were all involved in endothelial cell (EC) functions relevant to the initiation of atherosclerosis. Downregulation of LDLR/CD36/LOX-1 promoted monocyte adhesion to ECs and transendothelial migration of monocytes by increasing expression of ICAM-1, VCAM-1, E-selectin and P-selectin, decreased EC proliferation and migration, and increased EC apoptosis, thereby promoting the initiation of atherosclerosis. Opposite effects were observed with the overexpression of ADTRP and LDLR/CD36/LOX-1 in ECs. Interestingly, through the NF-κB and AKT pathways, overexpression of ADTRP significantly upregulated the expression of LDLR, CD36, and LOX-1, and knockdown of ADTRP expression significantly downregulated the expression of LDLR, CD36, and LOX-1. These data suggest that ADTRP and LDL receptors LDLR/CD36/LOX-1 positively regulate each other, and form a positive regulatory loop that regulates endothelial cell functions, thereby providing a potential protective mechanism against atherosclerosis. Our findings provide a new molecular mechanism by which deregulation of ADTRP and LDLR/CD36/LOX-1 promote the development of atherosclerosis and CAD.     

Endothelial nitric oxide synthase and calcium production in arterial geometries: an integrated fluid mechanics/cell model

It is well known that atherosclerosis occurs at very specific locations throughout the human vasculature, such as arterial bifurcations and bends, all of which are subjected to low wall shear stress. A key player in the pathology of atherosclerosis is the endothelium, controlling the passage of material to and from the artery wall. Endothelial dysfunction refers to the condition where the normal regulation of processes by the endothelium is diminished. In this paper, the blood flow and transport of the low diffusion coefficient species adenosine triphosphate (ATP) are investigated in a variety of arterial geometries: a bifurcation with varying inner angle, and an artery bend. A mathematical model of endothelial calcium and endothelial nitric oxide synthase cellular dynamics is used to investigate spatial variations in the physiology of the endothelium. This model allows assessment of regions of the artery wall deficient in nitric oxide (NO). The models here aim to determine whether 3D flow fields are important in determining ATP concentration and endothelial function. For ATP transport, the effects of a coronary and carotid wave form on mass transport is investigated for low Womersley number. For the carotid, the Womersley number is then increased to determine whether this is an important factor. The results show that regions of low wall shear stress correspond with regions of impaired endothetial nitric oxide synthase signaling, therefore reduced availability of NO. However, experimental work is required to determine if this level is significant. The results also suggest that bifurcation angle is an important factor and acute angle bifurcations are more susceptible to disease than large angle bifurcations. It has been evidenced that complex 3D flow fields play an important role in determining signaling within endothelial cells. Furthermore, the distribution of ATP in blood is highly dependent on secondary flow features. The models here use ATP concentration simulated under steady conditions. This has been evidenced to reproduce essential features of time-averaged ATP concentration over a cardiac cycle for small Womersley numbers. However, when the Womersley number is increased, some differences are observed. Transient variations are overall insignificant, suggesting that spatial variation is more important than temporal. It has been determined that acute angle bifurcations are potentially more susceptible to atherogenesis and steady-state ATP transport reproduces essential features of time-averaged pulsatile transport for small Womersley number. Larger Womersley numbers appear to be an important factor in time-dependent mass transfer.     

Filipin-sensitive caveolae-mediated transport in endothelium: reduced transcytosis, scavenger endocytosis, and capillary permeability of select macromolecules

Caveolae or noncoated plasmalemmal vesicles found in a variety of cells have been implicated in a number of important cellular functions including endocytosis, transcytosis, and potocytosis. Their function in transport across endothelium has been especially controversial, at least in part because there has not been any way to selectively inhibit this putative pathway. We now show that the ability of sterol binding agents such as filipin to disassemble endothelial noncoated but not coated plasmalemmal vesicles selectively inhibits caveolae-mediated intracellular and transcellular transport of select macromolecules in endothelium. Filipin significantly reduces the transcellular transport of insulin and albumin across cultured endothelial cell monolayers. Rat lung microvascular permeability to albumin in situ is significantly decreased after filipin perfusion. Conversely, paracellular transport of the small solute inulin is not inhibited in vitro or in situ. In addition, we show that caveolae mediate the scavenger endocytosis of conformationally modified albumins for delivery to endosomes and lysosomes for degradation. This intracellular transport is inhibited by filipin both in vitro and in situ. Other sterol binding agents including nystatin and digitonin also inhibit this degradative process. Conversely, the endocytosis and degradation of activated alpha 2- macroglobulin, a known ligand of the clathrin-dependent pathway, is not affected. Interestingly, filipin appears to inhibit insulin uptake by endothelium for transcytosis, a caveolae-mediated process, but not endocytosis for degradation, apparently mediated by the clathrin-coated pathway. Such selective inhibition of caveolae not only provides critical evidence for the role of caveolae in the intracellular and transcellular transport of select macromolecules in endothelium but also may be useful for distinguishing transport mediated by coated versus noncoated vesicles.     

Ceruloplasmin and oxidized LDL colocalize in atherosclerotic lesions of hamster

Epidemiological studies show that the risk for cardiovascular diseases increases with increasing levels of free-copper in plasma. It is known that intact ceruloplasmin (CP), the major protein transporter of copper in human plasma, oxidizes low density lipoproteins (LDL) in vitro. Our aim was to study the interaction between LDL and CP in vitro and in vivo, in an animal model of diet-induced atherosclerosis. In order to visualize the pathway of LDL into the arterial wall, human native LDL was labeled with fluorescent DiI and injected into male, Golden Syrian hyperlipemic hamsters. In vitro results demonstrated that slightly degraded CP has a significant oxidation potential against LDL at neutral pH. In vivo, after 24 hours circulation, LDL-DiI was taken up by the enlarged intima and fatty streaks of the arterial wall. Immunohistochemical localization of oxidized LDL and CP revealed their presence in the same areas of the arteries that take up LDL-DiI. Co-localization of LDL and CP in the enlarged intima of pro-atherosclerotic areas might explain the possible copper-induced oxidation process that might occur after native LDL is taken-up from the blood, transcytosed through the endothelium and accumulated in focalized deposits.     

Caveolin-1-mediated negative signaling plays a critical role in the induction of regulatory dendritic cells by DNA and protein coimmunization

Induction of Ag-specific regulatory T cells (iTregs) by vaccination is a promising strategy for treating autoimmune diseases. We previously demonstrated that DNA and protein covaccination converted naive T cells to Ag-specific iTregs by inducing CD11c(+)CD40(low)IL-10(+) regulatory dendritic cells (DCregs). However, it is unclear how coimmunization induces the DCregs. In this paper, we report that the event is initiated by coentry of sequence-matched DNA and protein immunogens into the same DC via caveolae-mediated endocytosis, which leads to inhibition of phosphorylation of caveolin-1 (Cav-1), the main component of caveolae, and upregulation of Tollip. This triggers downstream signaling that upregulates suppressor of cytokine signaling 1 and downregulates NF-κB and STAT-1α. Silencing either Cav-1 or Tollip blocks the negative signaling, leading to upregulated expression of CD40, downregulated production of IL-10, and loss of iTreg-inducing function. We further show that DCregs can be induced in culture from primary DCs and JAWS II DC lines by feeding them sequence-matched DNA and protein immunogens. The in vitro-generated DCregs are effective in ameliorating autoimmune and inflammatory diseases in several mouse models. Our study thus suggests that DNA and protein coimmunization induces DCregs through Cav-1- and Tollip-mediated negative signaling. It also describes a novel method for generating therapeutic DCregs in vitro.     

Computational study of LDL mass transport in the artery wall

A two-dimensional (2D) numerical simulation of convective–diffusive transport of LDL in the artery wall, coupled with the wall shear stress gradient (WSSG)-dependent LDL consumption of smooth muscle cells (SMCs) is presented. SMCs are modeled as an array of solid cylindrical pillars embedded in a continuous porous media which represents the interstitial proteoglycan and collagen fiber matrix. The internal elastic lamina (IEL), which separates the artery media from the intima, is modeled as an impermeable barrier to both water and LDL except for the fenestral pores that are assumed to be uniformly distributed over the IEL. The predictions demonstrate a range of interesting features of LDL transport and uptake in the media. For cells immediately below the fenestral pores, LDL uptake of SMCs is highly dependent on WSSG. Moreover, the rate of LDL consumption by SMCs is also affected by the diameter of the fenestral pore. This will be helpful in understanding the involvement of transmural transport processes in the initiation and development of atherosclerosis.     

Vav protein guanine nucleotide exchange factor regulates CD36 protein-mediated macrophage foam cell formation via calcium and dynamin-dependent processes

Atherosclerosis, a chronic inflammatory disease, results in part from the accumulation of modified lipoproteins in the arterial wall and formation of lipid-laden macrophages, known as "foam cells." Recently, we reported that CD36, a scavenger receptor, contributes to activation of Vav-family guanine nucleotide exchange factors by oxidatively modified LDL in macrophages. We also discovered that CD36-dependent uptake of oxidized LDL (oxLDL) in vitro and foam cell formation in vitro and in vivo was significantly reduced in macrophages deficient of Vav proteins. The goal of the present study was to identify the mechanisms by which Vav proteins regulate CD36-dependent foam cell formation. We now show that a Vav-dynamin signaling axis plays a critical role in generating calcium signals in mouse macrophages exposed to CD36-specific oxidized phospholipid ligands. Chelation of intracellular Ca(2+) or inhibition of phospholipase C-γ (PLC-γ) inhibited Vav activation (85 and 70%, respectively, compared with vehicle control) and reduced foam cell formation (approximately 75%). Knockdown of expression by siRNA or inhibition of GTPase activity of dynamin 2, a Vav-interacting protein involved in endocytic vesicle fission, significantly blocked oxLDL uptake and inhibited foam cell formation. Immunofluorescence microscopy studies showed that Vav1 and dynamin 2 colocalized with internalized oxLDL in macrophages and that activation and mobilization of dynamin 2 by oxLDL was impaired in vav null cells. These studies identified previously unknown components of the CD36 signaling pathway, demonstrating that Vav proteins regulate oxLDL uptake and foam cell formation via calcium- and dynamin 2-dependent processes and thus represent novel therapeutic targets for atherosclerosis.