Transendothelial transport of modified low-density lipoproteins

Palladium-catalyzed three-component reaction for the synthesis of 3,3-disubstituted allylic alcohols with regio- and stereoselectivity

A novel three-component assembly of allenic alcohols, aryl iodides and 1,3-dicarbonyl compounds into 3,3-disubstituted allylic alcohols was promoted in the presence of a palladium source.

A multiscale modelling approach to understand atherosclerosis formation: A patient-specific case study in the aortic bifurcation

Atherogenesis, the formation of plaques in the wall of blood vessels, starts as a result of lipid accumulation (low-density lipoprotein cholesterol) in the vessel wall. Such accumulation is related to the site of endothelial mechanotransduction, the endothelial response to mechanical stimuli and haemodynamics, which determines biochemical processes regulating the vessel wall permeability. This interaction between biomechanical and biochemical phenomena is complex, spanning different biological scales and is patient-specific, requiring tools able to capture such mathematical and biological complexity in a unified framework. Mathematical models offer an elegant and efficient way of doing this, by taking into account multifactorial and multiscale processes and mechanisms, in order to capture the fundamentals of plaque formation in individual patients. In this study, a mathematical model to understand plaque and calcification locations is presented: this model provides a strong interpretability and physical meaning through a multiscale, complex index or metric (the penetration site of low-density lipoprotein cholesterol, expressed as volumetric flux). Computed tomography scans of the aortic bifurcation and iliac arteries are analysed and compared with the results of the multifactorial model. The results indicate that the model shows potential to predict the majority of the plaque locations, also not predicting regions where plaques are absent. The promising results from this case study provide a proof of concept that can be applied to a larger patient population.

Regulation of blood vascular permeability in the skin

Regulation of blood vessel permeability is essential for the homeostasis of peripheral tissues. This regulation controls the trafficking of plasma contents, including water, vitamins, ions, hormones, cytokines, amyloids, lipoproteins, carrier proteins, and immunoglobulins. The properties of blood vessels vary among tissues based on their structural differences: continuous, fenestrated, or sinusoidal. These three types of blood vessels have different charge and size barrier properties. The anionic luminal glycocalyx layer on endothelial cells establishes the "charge barrier" that repels the attachment of negatively charged blood cells and plasma molecules. In contrast, the "size barrier" of blood vessels largely relies on the interendothelial junctions (IEJs) between endothelial cells, which define the paracellular permeability. As in most peripheral tissues, blood capillaries in the skin are composed of continuous and/or fenestrated blood vessels that have relatively tighter IEJs compared to those in the internal organs. Small vesicles in the capillary endothelium were discovered in the 1950s, and studies have since confirmed that blood endothelial cells transport the plasma contents by endocytosis and subsequent transcytosis and exocytosis-this process is called transcellular permeability. The permeability of blood vessels is highly variable as a result of intrinsic and extrinsic factors. It is significantly elevated upon tissue inflammations as a result of disabled IEJs and increased paracellular permeability due to inflammatory mediators. An increase in transcellular permeability during inflammation has also been postulated. Here, we provide an overview of the general properties of vascular permeability based on our recent observations of murine skin inflammation models, and we discuss its physiological significance in peripheral homeostasis.

Human microvascular endothelial cells displaying reduced angiogenesis and increased uptake of lipids during in vitro culture

Human microvascular ECs from the neonatal foreskin of two donors purchased from one distributor were used in an angiogenesis assay under the same culture conditions. Different angiogenic potency was apparent in these two batches (ECang and ECnon-ang). During the cultivation period of three weeks, ECang ran through all stages of angiogenesis starting from proliferation to migration up to the formation of three-dimensional capillary-like structures. Despite of expression of endothelial markers, ECnon-ang showed excessive intracellular storage of lipids in form of multilamellar bodies and decreased angiogenic potency in contrast to its counterpart, ECang. Results indicate that lipid metabolism differs in ECang versus ECnon-ang. This study points up that these differences are based on the different donors and presents a novel and valuable model for the study of mechanisms of atherosclerosis in endothelial cells in vitro.

Remodelling of membrane rafts expression in lung cells as an early sign of mechanotransduction-signalling in pulmonary edema

Membrane rafts (MRs) are clusters of lipids, organized in a "quasicrystalline" liquid-order phase, organized on the cell surface and whose pattern of molecules and physicochemical properties are distinct from those of the surrounding plasma membrane. MRs may be considered an efficient and fairly rapid cell-activated mechanism to express or mask surface receptors aimed at triggering specific response pathways. This paper reports observations concerning the role of MRs in the control of lung extravascular water that ought to be kept at minimum to assure gas diffusion, supporting the hypothesis that MRs expression is a potential mechanism of sensing minor changes in the volume of extravascular water. We present the evidence that MRs expression specifically relates to signal-transduction processes evoked by mechanical stimuli arising in the interstitial lung compartment when a small increase in extravascular volume occurs. We further hypothesize that a differential expression of MRs might also reflect the damage to precise components of the extracellular matrix caused by the perturbation in water balance and thus can trigger a molecule-oriented specific matrix remodelling.

In vitro accumulation of complexes of endotoxin and low-density lipoproteins by macrophages and arterial wall

Binding and uptake of complexes of endotoxin and low-density lipoproteins (LPS-LDL) in the arterial wall and mononuclear phagocytes were studied under in vitro conditions. Incubation of aortic explants from Wistar rats with complexes of (125)I-LDL and S. minnesota R595 LPS or (125)I-LDL was accompanied by a 6-fold increase in binding (0 degrees C) and 2-fold increase in the uptake (37 degrees C) of LDL-LPS complexes as compared to free LDL. Binding and degradation of (125)I-LDL-LPS complexes in the culture of peritoneal macrophages were higher compared to the corresponding parameters for free (125)I-LDL. Our results suggest that the formation of LDL-LPS complexes is followed by the increased binding and accumulation of LDL in the arterial wall and macrophages. These changes probably induce the cascade of major atherogenic events in the vascular wall.

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.

In vitro study of LDL transport under pressurized (convective) conditions

It is difficult to assess the transport pathways that carry low-density lipoprotein (LDL) into the artery wall in vivo, and there has been no previous in vitro study that has examined transendothelial transport under physiologically relevant pressurized (convective) conditions. Therefore, we measured water, albumin, and LDL fluxes across bovine aortic endothelial cell (BAEC) monolayers in vitro and determined the relative contributions of vesicles, paracellular transport through "breaks" in the tight junction, and "leaky" junctions associated with dying or dividing cells. Our results show that leaky junctions are the dominant pathway for LDL transport (>90%) under convective conditions and that albumin also has a significant component of transport through leaky junctions (44%). Transcellular transport of LDL by receptor-mediated processes makes a minor contribution (<10%) to overall transport under convective conditions.

Role of Caveolae and Caveolins in Health and Disease

Although they were discovered more than 50 years ago, caveolae have remained enigmatic plasmalemmal organelles. With their characteristic “flasklike” shape and virtually ubiquitous tissue distribution, these interesting structures have been implicated in a wide range of cellular functions. Similar to clathrin-coated pits, caveolae function as macromolecular vesicular transporters, while their unique lipid composition classifies them as plasma membrane lipid rafts, structures enriched in a variety of signaling molecules. The caveolin proteins (caveolin-1, -2, and -3) serve as the structural components of caveolae, while also functioning as scaffolding proteins, capable of recruiting numerous signaling molecules to caveolae, as well as regulating their activity. That so many signaling molecules and signaling cascades are regulated by an interaction with the caveolins provides a paradigm by which numerous disease processes may be affected by ablation or mutation of these proteins. Indeed, studies in caveolin-deficient mice have implicated these structures in a host of human diseases, including diabetes, cancer, cardiovascular disease, atherosclerosis, pulmonary fibrosis, and a variety of degenerative muscular dystrophies. In this review, we provide an in depth summary regarding the mechanisms by which caveolae and caveolins participate in human disease processes.

Genetic Ablation of Caveolin-1 Confers Protection Against Atherosclerosis

Objective— The development of atherosclerosis is a process characterized by the accumulation of lipids in the form of modified lipoproteins in the subendothelial space. This initiating step is followed by the subsequent recruitment and proliferation of other cell types, including monocytes/macrophages and smooth muscle cells. Here, we evaluate the potential role of caveolae membrane domains in the pathogenesis of atherosclerosis by using apolipoprotein E-deficient (ApoE−/−) mice as a model system.

Methods and Results— Caveolin-1 (Cav-1) is a principal structural protein component of caveolae membrane domains. To directly assess the in vivo role of caveolae and Cav-1 in atherosclerosis, we interbred Cav-1−/− mice with ApoE−/− mice. Interestingly, loss of Cav-1 resulted in a dramatic >2-fold increase in non-HDL plasma cholesterol levels in the ApoE−/− background. However, despite this hypercholesterolemia, we found that loss of Cav-1 gene expression was clearly protective against the development of aortic atheromas, with up to an ≈70% reduction in atherosclerotic lesion area. Mechanistically, we demonstrated that loss of Cav-1 resulted in the dramatic downregulation of certain proatherogenic molecules, namely, CD36 and vascular cell adhesion molecule-1.

Conclusions— Taken together, our results indicate that loss of Cav-1 can counteract the detrimental effects of atherogenic lipoproteins. Thus, Cav-1 is a novel target for drug development in the pharmacologic prevention of atheroma formation. Our current data also provide the first molecular genetic evidence to support the hypothesis that caveolar transcytosis of modified lipoproteins (from the blood to the sub-endothelial space) is a critical initiating step in atherosclerosis.

Caveolin, caveolae, and endothelial cell function

Caveolae are 50- to 100-nm cell-surface plasma membrane invaginations observed in terminally differentiated cells. They are particularly abundant in endothelial cells, where they are believed to play a major role in the regulation of endothelial vesicular trafficking and signal transduction. The use of caveolin-1-deficient mice has provided many new insights into the roles of caveolae and caveolin-1 in the regulation of endothelial cell function. These novel findings suggest an important role for caveolin-1 in the pathogenesis of cancer, atherosclerosis, and vascular disease.

Evidence for the Involvement of annexin 6 in the trafficking between the endocytic compartment and lysosomes

Annexins are a family of calcium-dependent phospholipid-binding proteins, which have been implicated in a variety of biological processes including membrane trafficking. The annexin 6/lgp120 prelysosomal compartment of NRK cells was loaded with low-density lipoprotein (LDL) and then its transport from this endocytic compartment and its degradation in lysosomes were studied. NRK cells were microinjected with the mutated annexin 6 (anx6(1-175)), to assess the possible involvement of annexin 6 in the transport of LDL from the prelysosomal compartment. The results indicated that microinjection of mutated annexin 6, in NRK cells, showed the accumulation of LDL in larger endocytic structures, denoting retention of LDL in the prelysosomal compartment. To confirm the involvement of annexin 6 in the trafficking and the degradation of LDL we used CHO cells transfected with mutated annexin 6(1-175). Thus, in agreement with NRK cells the results obtained in CHO cells demonstrated a significant inhibition of LDL degradation in CHO cells expressing the mutated form of annexin 6 compared to controls overexpressing wild-type annexin 6. Therefore, we conclude that annexin 6 is involved in the trafficking events leading to LDL degradation.

Identification of a γ-Interferon–Responsive Element in the Promoter of the Human Macrophage Scavenger Receptor A Gene

Abstract —In the present study, we demonstrate γ-interferon (γ-IFN)–inducible scavenger receptor A (SR-A) mRNA expression during the early stages of THP-1 and blood monocyte differentiation. Predominant induction of SR-A type II mRNA parallels the increased accumulation of cholesteryl esters under these conditions. A potential signal transducer and activator of transcription (STAT1) binding site (γ-interferon activation site) in the SR-A promoter demonstrates γ-IFN–inducible DNA binding activity and is most likely responsible for the γ-IFN–dependent expression of an SR-A promoter–luciferase fusion construct. In contrast, γ-IFN inhibits SR-A expression in mature macrophages as well as after prolonged γ-IFN incubation of THP-1 monocytes. Taken together, these results demonstrate opposite effects of γ-IFN on SR-A expression and activity during the early versus late stages of monocyte maturation. γ-IFN–induced STAT1 activation, leading to increased SR-A expression, could therefore play an important role in the initial steps of foam cell formation and xanthomatosis.

Role of the macrophage galactose lectin in the uptake of desialylated LDL

Desialylated low density lipoprotein (LDL) is rapidly taken up and accumulated by both peripheral blood monocytes and cells isolated from human arterial intima consisting predominantly of smooth muscle cells. It is shown that thioglycollate (TG)-elicited mouse macrophages and mouse peritoneal macrophages stimulated with lipopolysaccharide (LPS) show increased expression of a membrane-bound, galactose-specific lectin that could be responsible for this uptake. In LPS-stimulated macrophages accumulation of desialylated LDL is increased ca. 2.6-fold. Accumulation of acetylated LDL in the same cells is reduced, suggesting that the galactose-specific lectin might be responsible for the uptake of desialylated LDL. Transfection of cells with the mouse macrophage Gal/GalNAc-specific lectin (MMGL) increased their capacity to take up asialofetuin (ASF) and, to a smaller extent, desialylated LDL. The uptake of desialylated LDL was small, most likely due to the high k(d) of MMGL for biantennary oligosaccharides as found on LDL, and low concentration of LDL achieved in tissue culture experiments. The data suggest that the expression of galactose-specific lectins can be elevated under inflammatory conditions, and that these receptors could contribute to foam cell formation under conditions of high desialylated LDL concentration, as might be found in arterial intima.

Annexin VI stimulates endocytosis and is involved in the trafficking of low density lipoprotein to the prelysosomal compartment

Annexins are calcium-binding proteins with a wide distribution in most polarized and nonpolarized cells that participate in a variety of membrane-membrane interactions. At the cell surface, annexin VI is thought to remodel the spectrin cytoskeleton to facilitate budding of coated pits. However, annexin VI is also found in late endocytic compartments in a number of cell types, indicating an additional important role at later stages of the endocytic pathway. Therefore overexpression of annexin VI in Chinese hamster ovary cells was used to investigate its possible role in endocytosis and intracellular trafficking of low density lipoprotein (LDL) and transferrin. While overexpression of annexin VI alone did not alter endocytosis and degradation of LDL, coexpression of annexin VI and LDL receptor resulted in an increase in LDL uptake with a concomitant increase of its degradation. Whereas annexin VI showed a wide intracellular distribution in resting Chinese hamster ovary cells, it was mainly found in the endocytic compartment and remained associated with LDL-containing vesicles even at later stages of the endocytic pathway. Thus, data presented in this study suggest that after stimulating endocytosis at the cell surface, annexin VI remains bound to endocytic vesicles to regulate entry of ligands into the prelysosomal compartment.

Tumor vascular endothelium: barrier or target in tumor directed drug delivery and immunotherapy

The therapy of solid tumors with conventional chemotherapeutics, drug delivery preparations and immunomodulatory agents directed against the tumor cells is corrupted by a major barrier presented by the tumor vasculature. Permeability of the tumor blood vessels for transport of small molecules and macromolecular drug-carrier conjugates is only sufficient in the blood vessels at the tumor-host interface. Downregulation of the expression of adhesion molecules, required for the facilitation of immune cell recruitment, by the tumor vascular endothelium results in an escape of the tumor from host defence. New therapeutic approaches for the treatment of solid tumors are aimed at the tumor vasculature, either at the endothelial cells themselves or at basement membrane or tumor stroma components. Angiogenesis can be directly blocked with angiogenesis inhibitors, while angiogenesis related factors can serve as tumor vasculature specific epitopes for drug delivery strategies. Some glycoproteins expressed by tumor endothelial cells or present in the basement membrane and tumor stroma are also potential tumor selective targets. Therapeutic modalities that are suitable for site specific delivery are agents that increase tumor accumulation of (targeted) chemo/radiotherapeutics through increasing tumor vascular permeability. The observation that for tumor growth the blood supply is a limiting factor, led to the development of strategies to inhibit angiogenesis or block the tumor blood flow. Manipulation of the expression of endothelial cell adhesion molecules by selectively delivering modulatory agents at or in the tumor vascular endothelial cells may induce (bispecific antibody mediated) host defense activity directed against the tumor cells.

Oxidation of plasma low-density lipoprotein accelerates its accumulation and degradation in the arterial wall in vivo

BACKGROUND

The aim of the present study was to investigate whether oxidized LDL (ox-LDL) in the arterial intima could be derived from LDL already oxidized in plasma.

METHODS AND RESULTS

Rabbits received an intravenous injection of 125I-labeled normal LDL (N-LDL) mixed with 131I-labeled LDL that had been mildly oxidized through exposure to Cu2+. The aortic accumulation of undegraded labeled LDL was expressed as plasma equivalents and cakulated as radioactivity in the intima/inner media (cpm/cm2) divided by the time-averaged concentration of radioactivity in plasma (cpm/nL): for the thoracic aorta, the accumulation of undegraded ox-LDL in the intima/ inner media exceeded that of undegraded N-LDL by 286% (n = 6, P < .04), 863% (n = 7, P < .02), and 364% (n = 8, P < .01) after 1, 3, and 24 hours of exposure, respectively. There was a strong positive association between the extent of oxidation and the excess accumulation of undegraded ox-LDL compared with N-LDL (thoracic aorta; 3 hours of exposure: r = .97, n = 14, P < .00001). To measure degradation of N-LDL and ox-LDL, 125I-LDL labeled with 131I-tyramine cellobiose was injected intravenously 24 hours before the aortic intima/inner media was removed: for the thoracic aorta, the accumulation of degradation products from ox-LDL (n = 6) exceeded that from N-LDL (n = 6) by 301% (P < .04).

CONCLUSIONS

The present data suggest a novel mechanism: mildly oxidized LDL may circulate in plasma for a period sufficiently long to enter, accumulate, and be degraded in the arterial intima in preference to N-LDL.