Uptake and degradation of human very-low-density lipoproteins by rat liver endothelial cells in culture

T lymphocyte activation results in an increased expression of beta-1,4-galactosyltransferase: phorbol ester induces a similar enhancement in the absence of mitosis

We previously showed that in vitro activated human T lymphocytes expressed increased amounts of beta-1,6-branched N-linked oligosaccharides (Lemaire S etal. (1994) J Biol Chem269: 8069-74), which have been proposed to participate in the regulation of the immune process. In the present paper, we compared the activity and expression of beta-1,4-galactosyltransferase (GalT), one of the glycosyltransferases involved in the biosynthesis of these beta-1,6-branched N-linked oligosaccharides, before and after in vitro activation of T lymphocytes after a 40h treatment with a mixture of phorbol 12-myristate 13-acetate and Phaseolus vulgaris lectin. After treatment, the enzymatic activity of the GalT was significantly increased and immunoblot experiments performed with a monoclonal antibody to human GalT showed an increased intensity of the GalT band at 49 kDa, attributable to an enhancement of GalT mRNA level, as shown by Northern blots. However, treatment of the same T-lymphocytes by phorbol ester alone, which is unable to induce mitosis, resulted in a comparable increase of the expression of GalT. Moreover, these phorbol ester-treated T lymphocytes, analysed by flow cytometry exhibited a two-fold increase in the expression of GalT. Finally, confocal fluorescence microscopy performed on all T lymphocytes (treated or not) showed that the flow cytometric signal of GalT originates from intracellular, Golgi-associated antigen only since no surface GalT was detected.

Transcytosis of albumin in endothelial cells is brefeldin A--independent

To determine whether in endothelial cells (EC) the pathways of endocytosis and transcytosis of macromolecules interconnect, the effect of Brefeldin A (BFA) on these processes was tested. To this purpose EC were grown to confluence on plastic culture dishes or on cell culture chamber inserts placed into corresponding wells, so as to obtain a dual chamber system. The cells maintained the typical characteristics of EC and had an electrical resistance in the range of 30-60 Ohm.cm2. Transendothelial transport of albumin conjugated to the fluorochrom Texas Red (Alb-TR) and of horseradish peroxidase (HRP) added to the upper compartment, in the absence or presence of BFA (0-25 micrograms/ml), was evaluated in aliquots collected from the lower compartment. At different time intervals, quantitative data were obtained by fluorimetry and spectrophotometry. In other experiments transcytosis of Alb-TR was examined in the presence of 100 microM forskolin (an inhibitor of BFA effect). The endocytosis of Alb-TR and HRP was evaluated by incubating EC with the probes, and the internalized tracers determined in the cell lysate using the methods described above. The results showed that BFA has no significant effect on transcytosis of albumin and HRP. In contradistinction, BFA (5 micrograms/ml) reduced markedly endocytosis of HRP (by 47%). Forskolin has no effect on transcytosis. The data indicate that the BFA-induced perturbance in the endocytic route does not affect the transcytotic pathway of albumin, and suggest that in EC, transcytosis of macromolecules may represent a shortcut for rapid and direct transport of some plasma molecules across the cell.

Labelling of polysaccharides using [11C]cyanogen bromide. In vivo and in vitro evaluation of 11C-hyaluronan uptake kinetics

A method for the 11C-labelling of polysaccharides in high specific radioactivity is described. Dextran and hyaluronan were treated with [11C]cyanogen bromide in aqueous solution at pH 11.5 to give 30-47% radiochemical yields with higher than 98% radiochemical purity in synthesis times of 24-26 min counted from the end of bombardment. Specific radioactivities at the end of synthesis ranged from 0.12 to 3.1 Ci/mumol. The biodistribution kinetics of [11C]hyaluronan injected intravenously was studied in rats by means of positron emission tomography, showing a rapid and displaceable uptake in liver. Uptake and displacement of [11C]hyaluronan was also demonstrated in cultured rat liver endothelial cells.

Labelling of high molecular weight hyaluronan with 125I-tyrosine: studies in vitro and in vivo in the rat

Studies on the metabolism of the polysaccharide hyaluronan has previously been hampered by the lack of radioactive hyaluronan of high molecular weight (MW) and high specific activity. In the present study 125I-tyrosine (T)-labelled hyaluronan was produced after CNBr-activation of the polysaccharide. A specific activity of approximately 0.1 MBq microgram-1 was achieved using 100 micrograms of 0.5 x 10(6) Da hyaluronan labelled for 2 h with 18 MBq 125I. The 125I-T-hyaluronan kept a high MW-profile upon gel filtration chromatography and was found to be cleared from the circulation with the kinetics and organ distribution reported for biosynthetically labelled hyaluronan of high MW. The 125I-labelled polysaccharide is also taken up by liver endothelial cells both in vivo and in vitro, indicating that the labelling does not interfere with the binding to specific cell-surface receptors found on these cells. The intracellular degradation is slower than that earlier reported for biosynthetically labelled hyaluronan and seems to be halted at the level of low MW oligo- or mono-saccharides that eventually leave the organism via the urine. Scintigraphic images of rats after intravenous injection of 125I-T-hyaluronan showed rapid uptake in the liver and a redistribution of radioactivity from liver to urine with time. Our results indicate that the 125I-T-hyaluronan is suitable for studies of hyaluronan-metabolism in a number of ways. The gamma emitters 125I and 131I are easy to monitor and can be used also for in vivo 3D-imaging using single photon emission computer tomography.

Cultured rat hepatic sinusoidal endothelial cells express intercellular adhesion molecule-1 (ICAM-1) by tumor necrosis factor-alpha or interleukin-1 alpha stimulation

This study investigated the expression of intercellular adhesion molecule-1, a leukocyte adhesion molecule, on cultured rat hepatic sinusoidal endothelial cells during stimulation with tumor necrosis factor-alpha or interleukin-1 alpha. Using immunoelectron microscopy and the immunogold technique against intercellular adhesion molecule-1, gold particles were shown to increase significantly on the surface of sinusoidal epithelial cells treated with tumor necrosis factor-alpha (100 U/ml) or interleukin-1 alpha (10 U/ml) for 8 h compared with unstimulated cells. In addition, semi-quantitative analysis of intercellular adhesion molecule-1 on the sinusoidal endothelial cells was performed by cytofluorometer. Even without stimulation, intercellular adhesion molecule-1 was weakly expressed. However, 8 h after tumor necrosis factor-alpha or interleukin-1 alpha treatment, the expression of intercellular adhesion molecule-1 on cells was increased in a dose-dependent manner. Kinetic analysis showed that the expression of intercellular adhesion molecule-1 on sinusoidal endothelial cells treated with these cytokines increased gradually from the beginning of stimulation to 24 h. These findings suggest that hepatic sinusoidal endothelial cells may mediate the direct interaction between leukocytes and sinusoidal endothelial cells by expressing leukocyte adhesion molecules such as intercellular adhesion molecule-1.

Determination of the osmotic active drug concentration in the cytoplasm of anthracycline-resistant and -sensitive K562 cells

A fluorescence method was used to follow the interaction of 4'-o-tetrahydropyranyladriamycin (THP-ADR) with drug-resistant and -sensitive K562 cells. The amounts of drug bound to the nuclei at the steady state, Cn and at the equilibrium state, CN, once the membrane has been solubilized with Triton X-100, have been determined as a function of the pH outside the cells (pHe): Cn increased and CN decreased as pHe increased. At a given pH value outside the cells, CN is the same for both sensitive and resistant cells, whereas Cn is lower in resistant cells as compared to sensitive cells. Using the observation that the essential binding characteristics of THP-ADR in nuclei are the same for both types of cell, the osmotic active drug concentration, Ci, in the cytoplasm of the cells was determined at different values of pHe. Using fluorescent dye, the cytoplasmic pH was determined and found equal to 7.2 +/- 0.1 in both types of cell. In sensitive cells, the equilibrium transmembrane concentrations verified the relation [DH+]i/[DH+]e = [H +]i/[H+]e where [DH +]i and [DH +]e stand for the concentration of protonated form of the drug inside and outside the cells, respectively. This indicates that the uptake of the drug occurs through free permeation of the neutral form of the drug in response to delta pH gradient. Such a relation is not verified in the case of resistant cells.

Hyaluronan-binding proteins on cultured J 774 macrophages

Cultivated macrophages of murine cell-line J 774 were found to bind high-molecular-weight (molecular weight average approx. 5.10(6) [3H]hyaluronan (HA) by a saturable mechanism at 4 degrees C. Half-maximal binding was observed at 7-8 microgram/ml (1.4-1.6 nM) and the maximal binding was reached at 30-40 microgram/ml. Scatchard plot analysis revealed that approx. 20,000 molecules could bind to each cell with a Kd of 1.5 nM. The binding could be effectively inhibited by unlabeled HA. Also chondroitin sulphate inhibited the binding, but only to about 50%. At 37 degrees C the J 774 cells took up and degraded the polysaccharide effectively. Affinity chromatography on HA coupled to agarose of solubilized surface-iodinated J 774 cells, revealed that a protein of approx. 60 kDa, when analyzed by sodium dodecylsulfate polyacrylamide gel electrophoresis and autoradiography, could be specifically eluted with HA-oligosaccharides. Our results suggest that J 774 macrophages can bind HA by a mechanism compatible with receptor-binding, and carry a 60 kDa HA-binding protein on their surface. This receptor-binding may mediate uptake and degradation of the polysaccharide and influence the levels and turnover of HA in interstitial fluid as well as the release of HA into the bloodstream.

Scavenger functions of the liver endothelial cell

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Sinusoidal endothelial cells of the liver: fine structure and function in relation to age

Liver endothelial cells form a continuous lining of the liver capillaries, or sinusoids, separating parenchymal cells and fat-storing cells from sinusoidal blood. Liver sinusoidal endothelial cells differ in fine structure from endothelial cells lining larger blood vessels and from other capillary endothelia in that they lack a distinct basement membrane and also contain open pores, or fenestrae, in the thin cytoplasmic projections which constitute the sinusoidal wall. This distinctive morphology supports the protective role played by liver endothelium, the cells forming a general barrier against pathogenic agents and serving as a selective sieve for substances passing from the blood to parenchymal and fat-storing cells, and vice versa. Sinusoidal endothelial cells, furthermore, significantly participate in the metabolic and clearance functions of the liver. They have been shown to be involved in the endocytosis and metabolism of a wide range of macromolecules, including glycoproteins, lipoproteins, extracellular matrix components, and inert colloids, establishing endothelial cells as a vital link in the complex network of cellular interactions and cooperation in the liver. Fine structural studies in combination with the development of cell isolation and culture techniques from both experimental animal and human liver have greatly contributed to the elucidation of these endothelial cell functions. Morphological and biochemical investigations have both revealed little changes with age except for an accumulation of iron ferritin and a decrease in the activities of glucose-6-phosphatase, Mg-ATPase, and in glucagon-stimulated adenylcyclase. Future studies are likely to disclose more fully the role of sinusoidal endothelial cells in the regulation of liver hemodynamics, in liver metabolism and blood clearance, in the maintenance of hepatic structure, in the pathogenesis of various liver diseases, and in the aging process in the liver.

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.

Apolipoprotein-E-binding proteins of rat liver endothelial cells

In an attempt to characterise the apolipoprotein-E-binding proteins of rat liver endothelial cells, we prepared membranes from monolayer cultures of liver endothelial cells as an enriched source of membrane receptors. The membranes could specifically bind iodinated very-low-density lipoproteins (VLDL) and the binding could be inhibited effectively by unlabelled VLDL and high-density lipoproteins, but only moderately by low-density lipoproteins. To identify the binding proteins, we performed immunoprecipitation studies of solubilised iodinated liver endothelial cells and cell membranes, respectively, using purified apolipoprotein E and monospecific polyclonal IgG directed towards this apolipoprotein. The antibodies together with the bound apolipoprotein E and iodinated liver endothelial cell proteins were harvested with staphylococcal protein A-Sepharose. The immunoprecipitates were subjected to sodium dodecyl sulphate-polyacrylamide gel electrophoresis, and after autoradiography of the dried gel, the Mr of the liver endothelial cell proteins bound to apolipoprotein E could be determined. Two protein bands with molecular masses of 55-60 and 110, and a weak band of 170 kDa could be detected from intact cells. These proteins were specifically precipitated only in the presence of divalent cations, and might represent cell-surface receptors for apolipoprotein-E-containing lipoproteins. Additional bands were seen when cell membranes were used, the most prominent ones having molecular masses of 32 and 35 kDa. These proteins could be of intracellular origin, or they may be degradation products of the other apolipoprotein-E-binding proteins.

Identification of a fibronectin receptor specific for rat liver endothelial cells

Antibodies raised against the fibronectin receptor of rat hepatocytes recognized one protein (Mr 120 and 135 kDa for unreduced and reduced samples, respectively) in immunoblotting of solubilized rat liver endothelial cells (LEC). The antibodies specifically precipitated a 200-kDa protein together with the 135-kDa component from 125I-labeled LEC. Spreading of LEC on fibronectin, but not on laminin or collagen, was inhibited by monovalent Fab fragments of the antibodies, implicating that the 135/200-kDa complex is a specific fibronectin receptor. The results indicate that LEC, hepatocytes, and fibroblasts of rat carry different fibronectin receptors, suggesting that the interaction of fibronectin with these cells may have different functional roles.

The catabolic fate of hyaluronic acid

Part of the hyaluronic acid (HA) synthesized in peripheral tissues enters the blood circulation through the lymph. It is rapidly taken up by the endothelial cells in the liver (half-life in blood is 2.5-5.5 minutes) and degraded. Pure primary cultures of liver endothelial cells were obtained by a newly developed technique and used to follow the metabolism of the polysaccharide on the cell surface. At 37 degrees C the HA is effectively endocytosed and degraded to acetate and lactate. A radioassay specific for HA and sensitive in the nanogram range has been developed to follow the concentration of HA in serum. The normal level in man is 10 to 100 micrograms/l. Elevated serum levels of HA are seen in liver cirrhosis, rheumatoid arthritis and scleroderma indicating that both an impaired catabolism in the liver and an increased synthesis in the peripheral tissues can modify the HA level.