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#25849867   2015/04/08 Save this To Up

Oxidation by neutrophils-derived HOCl increases immunogenicity of proteins by converting them into ligands of several endocytic receptors involved in antigen uptake by dendritic cells and macrophages.

The initiation of adaptive immune responses to protein antigens has to be preceded by their uptake by antigen presenting cells and intracellular proteolytic processing. Paradoxically, endocytic receptors involved in antigen uptake do not bind the majority of proteins, which may be the main reason why purified proteins stimulate at most weak immune responses. A shared feature of different types of adjuvants, capable of boosting immunogenicity of protein vaccines, is their ability to induce acute inflammation, characterized by early influx of activated neutrophils. Neutrophils are also rapidly recruited to sites of tissue injury or infection. These cells are the source of potent oxidants, including hypochlorous acid (HOCl), causing oxidation of proteins present in inflammatory foci. We demonstrate that oxidation of proteins by endogenous, neutrophils-derived HOCl increases their immunogenicity. Upon oxidation, different, randomly chosen simple proteins (yeast alcohol dehydrogenase, human and bovine serum albumin) and glycoproteins (human apo-transferrin, ovalbumin) gain the ability to bind with high affinity to several endocytic receptors on antigen presenting cells, which seems to be the major mechanism of their increased immunogenicity. The mannose receptor (CD206), scavenger receptors A (CD204) and CD36 were responsible for the uptake and presentation of HOCl-modified proteins by murine dendritic cells and macrophages. Other scavenger receptors, SREC-I and LOX-1, as well as RAGE were also able to bind HOCl-modified proteins, but they did not contribute significantly to these ligands uptake by dendritic cells because they were either not expressed or exhibited preference for more heavily oxidised proteins. Our results indicate that oxidation by neutrophils-derived HOCl may be a physiological mechanism of conferring immunogenicity on proteins which in their native forms do not bind to endocytic receptors. This mechanism might enable the immune system to detect infections caused by pathogens not recognized by pattern recognition receptors.

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#23235478   2013/03/04 Save this To Up

Heterologous downregulation of vasopressin type 2 receptor is induced by transferrin.

Vasopressin (VP) binds to the vasopressin type 2 receptor (V2R) to trigger physiological effects including body fluid homeostasis and blood pressure regulation. Signaling is terminated by receptor downregulation involving clathrin-mediated endocytosis and V2R degradation. We report here that both native and epitope-tagged V2R are internalized from the plasma membrane of LLC-PK1 kidney epithelial cells in the presence of another ligand, transferrin (Tf). The presence of iron-saturated Tf (holo-Tf; 4 h) reduced V2R binding sites at the cell surface by up to 33% while iron-free (apo-Tf) had no effect. However, no change in green fluorescent protein-tagged V2R distribution was observed in the presence of bovine serum albumin, atrial natriuretic peptide, or ANG II. Conversely, holo-Tf did not induce the internalization of another G protein-coupled receptor, the parathyroid hormone receptor. In contrast to the effect of VP, Tf did not increase intracellular cAMP or modify aquaporin-2 distribution in these cells, although addition of VP and Tf together augmented VP-induced V2R internalization. Tf receptor coimmunoprecipitated with V2R, suggesting that they interact closely, which may explain the additive effect of VP and Tf on V2R endocytosis. Furthermore, Tf-induced V2R internalization was abolished in cells expressing a dominant negative dynamin (K44A) mutant, indicating the involvement of clathrin-coated pits. We conclude that Tf can induce heterologous downregulation of the V2R and this might desensitize VP target cells without activating downstream V2R signaling events. It also provides new insights into urine-concentrating defects observed in rat models of hemochromatosis.

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#15307055   2004/08/12 Save this To Up

A vanadium-51 NMR study of the binding of vanadate and peroxovanadate to proteins.

51V quadrupolar central transition NMR spectra of buffered (pH 7.6-8.0) solutions of bovine apo-transferrin (Tf) and bovine prostatic acid phosphatase (Pp) treated with vanadate show normal features (chemical shifts between -515 and -542 ppm) corresponding to the complexation of VO2+ to the Tf binding site and the Pp active centre, respectively. Addition of H2O2 leads to the temporary formation of complexed VO(O2)+ (delta approximately -595). Vanadate-dependent bromoperoxidase from the alga Ascophyllum nodosum exhibits an unusually high shielding both for the native (delta = -931) and the peroxo form (delta = -1135) of the enzyme. A resonance at -471 ppm is traced back to an inactive form with oxovanadium(V) in a trigonal-bipyramidal array.

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#8257710   1994/01/19 Save this To Up

Endocytosis and degradation of bovine apo- and holo-lactoferrin by isolated rat hepatocytes are mediated by recycling calcium-dependent binding sites.

We characterized endocytosis of iron-saturated (holo) and iron-depleted (apo) 125I-labeled bovine lactoferrin (Lf) by isolated rat hepatocytes. Hepatocytes ingested both Lf forms--determined by EGTA/dextran sulfate removal of surface-bound Lf--at maximal endocytic rates of 1.85 and 1.52 fmol cell-1 min-1 for 125I-apo-Lf and 125I-holo-Lf, respectively. First-order endocytic rate constants (37 degrees C) for 125I-apo-Lf and 125I-holo-Lf were 0.276 and 0.292 min-1, respectively. Regardless of Lf's iron content, hyperosmotic media (approximately 500 mmol/kg) inhibited Lf uptake by approximately 90%, indicating endocytosis of both Lf forms was primarily clathrin-dependent. Endocytosis of both Lf forms was not altered significantly in the presence of excess iron chelator desferrioxamine or rat holo-transferrin, or by cycloheximide treatment. Fluorescein isothiocyanate- and cyclohexanedione-modified Lf competed fully with native Lf for binding and endocytosis, indicating that, unlike human Lf, modification of lysine or arginine residues does not block the interaction of bovine Lf with cells. After binding Lf at 4 degrees C, cells at 37 degrees C internalized approximately 90% of Lf bound to Ca(2+)-dependent sites but not Lf bound to Ca(2+)-independent sites. Following uptake, hepatocytes released acid-soluble (degraded) products of 125I-Lf biphasically at 37 degrees C, an initial rapid phase within the first 20 min--more pronounced with 125I-holo-Lf--followed by a sustained linear release of 298 and 355 molecule equiv cell-1 min-1 for 125I-apo-Lf and 125I-holo-Lf, respectively. At 4 degrees C, both digitonin-permeabilized and intact cells bound approximately 1.1 x 10(6) 125I-Lf molecules to Ca(2+)-dependent sites per cell, indicating that hepatocytes do not contain a sizeable intracellular pool of these sites. Moreover, cells retained > 70% of Ca(2+)-dependent sites on the surface during sustained Lf endocytosis. Thus, these Lf binding sites recycle during endocytosis at an estimated 4-5 min/circuit.

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#2847791   1989/01/04 Save this To Up

A receptor for formaldehyde-treated serum albumin on human placental brush-border membrane.

Formaldehyde-treated serum albumin (f-Alb) is known to be taken up and degraded by sinusoidal liver cells via receptor-mediated endocytosis. We report that 125I-labeled f-Alb (125I-f-Alb) binding to human placental brush-border membranes also occurs. This binding reached equilibrium within 40 min at 37 degrees C. Kinetic studies demonstrated the presence of saturable binding with an apparent Kd of 2.1 micrograms of f-Alb/ml and a maximal binding of 2.3 micrograms/mg of membrane protein at pH 7.5. Maximal binding was observed at between pH 7.5 and 8.0. 125I-f-Alb binding to the membranes was little inhibited by a 1000-fold molar excess of ovalbumin, human apo-transferrin and native bovine serum albumin. No binding was observed with membranes which had been pretreated with proteinase or trypsin. This f-Alb receptor was extremely heat-stable, since the binding was not abolished even by pretreatment of the membranes at 78 degrees C for 30 min. EDTA, Ca2+ and Mg/4 had no effect on 125I-f-Alb binding, so the binding was independent of divalent cations. These data suggest that a receptor specific for f-Alb exists on human placental brush-border membranes of syncytial trophoblasts.

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