Iodogen-catalyzed iodination of transferrin

Hypothalamic control of mitogen-induced proliferative responses and luteinizing hormone-releasing hormone levels in thymus and peripheral blood of rat fetuses

The role of endogenous luteinizing hormone-releasing hormone (LHRH) in the development of concanavalin A (ConA)-induced proliferative responses was studied in rat fetuses. Preliminary treatment of fetuses in utero with either the LHRH receptor antagonist or anti-LHRH antibodies resulted in the suppression of ConA-induced proliferative responses of thymocytes. LHRH and LHRH-immunopositive cells, morphologically similar to thymocytes, were detected in intact fetal thymus. A significant content of LHRH was also found in the peripheral blood of fetuses. The LHRH content in thymus and plasma was similar in males and females. Surgical ablation of the hypothalamus resulted in 2-fold decreases in thymus and plasma levels of LHRH in 21-day-old fetuses compared to sham-operated fetuses. It was concluded that LHRH regulates mitogen-induced proliferative responses of thymocytes during prenatal ontogenesis in the rat. The main source of plasma LHRH at that period is the hypothalamus. Moreover, LHRH is synthesized in the fetal thymus. Thus, LHRH is suggested to have not only a central effect but also to be involved in autocrine or paracrine regulation of proliferative immune responses.

Interaction of transferrin with rat alveolar macrophages

Binding of rat transferrin to isolated alveolar macrophages was investigated in the 0.125 nM to 2 microM range. Computer analysis of the data revealed two classes of binding sites, a small number (< 1000 exposed/cell) having high affinity (dissociation constant (Kd), 3.4 nM) and a large number (approximately 4 x 10(6)/cell) having low affinity (Kd 48 microM). Measurements with a monoclonal antibody to the rat transferrin (rTf) receptor yielded values in the same range as the high-affinity sites derived from studies of ligand binding. Binding to the low-affinity sites at pH 5.8 was nearly one order of magnitude stronger than that at pH 7.3. Bovine lactoferrin (12 microM), cationized bovine serum albumin (14 microM), L-arginine (50 mM), and L-lysine (50 mM) did not compete against rTf binding to the low-affinity sites. Removal of an average of 2.6 x 10(8) sialyl residues from each cell did not affect binding. Heparan sulphate proteoglycan purified from alveolar macrophages bound strongly to immobilized rTf, thus raising the possibility that the low-affinity interaction of transferrin with these cells may be mediated, at least in part, by this glycosaminoglycan.

Interaction of transferrin and its iron-binding fragments with heparin

The interaction of heparin with transferrin (Tf; bovine and rat) and the isolated iron-binding lobes of bovine Tf were investigated. Affinity chromatography of rat Tf on heparin-agarose showed that interaction depended on both the iron content of Tf and the pH of the medium. Both the iron-free and iron-saturated forms of Tf were strongly bound by the column at pH 5.6, but only the iron-free form revealed significant affinity at pH 7.4. Desialylation of Tf moderately promoted interaction, treatment with cyclohexanedione moderately reduced interaction, and succinylation abolished it altogether. In the presence of heparin, iron release from the N-terminal lobe of native bovine Tf was accelerated and from the C-terminal lobe it was slightly reduced. The heparin effect remained qualitatively the same on each lobe after their separation by tryptic digestion and DEAE-cellulose chromatography. The affinity of native bovine Tf for heparin was very close to that of its isolated N-terminal lobe, thus suggesting that it is this portion of the molecule that binds to the glycosaminoglycan. It is concluded that the consequences for iron-binding strength of the two transferrin lobes are diagonally opposite when Tf is bound to heparin as opposed to its natural cell-surface receptor.

Reduced hepatic iron uptake from rat aglycotransferrin

Rat aglycotransferrin (rAgTf) was produced from the disialosyl diantennary fraction of rat transferrin (rTf) by treatment with peptide: N-glycosidase F. Following removal of the enzyme by gel filtration and isolation of the deglycosylated protein by lectin chromatography, rAgTf was compared to rTf both in vitro and in vivo. No significant differences were found between the two proteins with respect to affinity for iron and kinetics of Fe release from the N-lobe and C-lobe. The fluorescence emission spectrum of apo-rTf was red-shifted by approximately 3 nm relative to diferric rTf; however, no spectral difference was detected between rTf and rAgTf when the analogous forms (apo or diferric) were compared. Plasma clearance of radioactive iron administered to rats as either rTf or rAgTf was comparable. Reticulocytes took up iron from rAgTf slightly faster than from rTf. In contrast, Fe acquisition by the liver from rAgTf was significantly reduced relative to rTf. This finding contrasts sharply with earlier observations with asialotransferrin (rAsTf) and provides a basis for discounting charge loss as the mechanism of enhanced hepatic Fe uptake from rAsTf. It is suggested that the glycan complement of rTf, while unimportant for interaction of the protein with specific receptors, probably plays a role in the interaction with low-affinity hepatic binding sites.

Protein kinase C controls Fc gamma receptor-mediated endocytosis in human neutrophils

The aim of this study is to clarify which signaling mechanism operates in Fc gamma receptor-mediated endocytosis in human neutrophils. Endocytosis of immune complexes was inhibited by antibodies directed to cell membrane phospholipase C (PLC) and A2 (PLA2) (maximal inhibition obtained was 57% and 28%, respectively), being almost abolished by these antibodies if used in combination (up to 91% inhibition). The protein kinase C (PKC) activator, phorbol 12,13-dibutyrate, reversed this inhibitory effect. Four different PKC inhibitors (H-7, palmitoylcarnitine, sphingosine, and tamoxifen) produced a dose-dependent inhibition of endocytosis, up to over 80% in each case. H-8 (1-10 microM) which inhibits cyclic nucleotide protein kinases but not PKC had no effect upon endocytosis. It is concluded that Fc gamma receptor-induced activation of PLC and PLA2 triggers endocytosis by activation of PKC.

Identification of multiple ral gene products in human platelets that account for some but not all of the platelet Gn-proteins

Polyclonal antibodies raised against specific recombinant low molecular mass GTP-binding proteins were tested for their ability to recognize partially purified human platelet membrane Gn-proteins (i.e. proteins that bind [alpha-32P]GTP on nitrocellulose blots of SDS/polyacrylamide gels). An antiserum against simian ralA protein recognized a 27 kDa human platelet protein with the same apparent molecular mass as the major platelet Gn-protein (Gn27). In further analysis by two-dimensional polyacrylamide gel electrophoresis, the isoelectric focusing step permitted resolution of 12 major Gn-protein forms, seven of 27 kDa (Gn27a-g), one of 26 kDa (Gn26) and four of 24 kDa (Gn24a-d). The ralA antibody reacted strongly with the five most basic Gn27 species (a-e), weakly with Gn26 and not at all with Gn27f, Gn27g or Gn24a-d. We conclude that ral gene products account for some but probably not for all of the platelet Gn-proteins.

Rat aglycotransferrin and human monoglycotransferrin: production and metabolic properties

Rat transferrin (rTf), containing one complex glycan, and human transferrin (hTf), containing two complex glycans, were treated with peptide:N-glycosidase F (PNGase) under nondenaturing conditions. Apo rTf with a nonfucosylated standard biantennary glycan, but not its diferric counterpart, yielded satisfactory amounts (approximately 55% in 7 h) of aglyco Tf (AgTf). The presence of a chitobiose core fucose reduced yields to approximately 30%, whereas an additional NeuAc on the GlcNAc in the Man(alpha 1-3) branch had no adverse effect. Triton X-100 impaired deglycosylation. The main product (approximately 65%) obtained from apo hTf was monoglyco Tf (MgTf). Analysis of the cyanogen bromide fragments of MgTf revealed that PNGase did not discriminate between the two glycosylation sites of hTf. A negligible portion (2-4%) of AgTf, that was also obtained during the reaction, probably resulted from PNGase action on denatured hTf molecules. Modified Tfs were separated by affinity chromatography, radiolabeled, mixed with another preparation of interest, and injected in rats. Total-body radiation measurements showed that the half-life of rat AgTf was 19-20% shorter than that of rTf but 9% longer than that of asialo Tf. This suggests that close to 76% of the change in the degradative rate observed after desialylating rTf is referable to charge loss rather than the exposure of Gal residues. Human MgTf was catabolized by rats 8-9% faster than the parent protein, while human AgTf behaved in vivo like a denatured protein. It is concluded that sialyl carboxyls are a codeterminant of the normal lifetime of transferrins.

Monoclonal antibodies to the amino- and carboxyl-terminal domains of ovotransferrin

Monoclonal antibodies to the iron transport protein ovotransferrin were produced by immunizing mice simultaneously with ovotransferrin and with the proteolytically derived amino- and carboxyl-terminal half-molecule domains of ovotransferrin. Two isolated hybridoma clones (designated alpha OT + N1 and alpha OT + N2) produced antibodies (IgG1) to determinants located on holo-ovotransferrin and the amino-terminal domain; two hybridoma clones (designated alpha OT + C1 and alpha OT + C2) produced antibodies (IgG1) to determinants on holo-ovotransferrin and the carboxyl-terminal domain. One hybridoma clone (designated alpha OT-N1) produced an antibody (IgG1) that bound only the amino-terminal domain and did not bind holo-ovotransferrin. Both alpha OT + N1, and alpha OT-N1 bound to antigen less tightly after removal of iron; antibodies alpha OT + N2, alpha OT + Cl, and alpha OT + C2 were unaffected by removal of iron from holo-ovotransferrin or the isolated domains. Intact disulfide bonds in the antigens were required for binding by the antibodies. These antibodies should prove useful as probes for discrete regions of the ovotransferrin molecule, in particular, those regions involved in binding to the transferrin receptor.

Interaction of rat asialotransferrin with adult rat hepatocytes: its relevance for iron uptake and protein degradation

Comparative studies with rat transferrin (rTf) and asialotransferrin (asialo-rTf) were performed on suspended adult rat hepatocytes with the aim of elucidating the mechanism of enhanced hepatic Fe acquisition from asialo-rTf observed previously in vivo. At low ligand concentrations (0.05-20 micrograms/ml), the cells bound more asialo-rTf than rTf. However, the excess binding was abolished by incubation either in the presence of 1.55 mg/ml of diferric rTf or of 1 mg/ml of asialomucin. Following either treatment, asialo-rTf and rTf were bound to comparable extents. These findings indicate that both transferrin receptors and the hepatic galactose recognition system (lectin) are essential for preferential binding of asialo-rTf by hepatocytes. The possibility is considered that the lectin facilitates capture of asialo-rTf by the same binding sites that are normally available for rTf rather than that it functions as an alternative pathway. In agreement with this view, asialo-rTf could not be channeled into the lectin-mediated degradative pathway by blocking Tf receptors with human Tf. Enhanced Fe uptake from asialo-rTf was fully prevented by asialomucin and partially prevented by human Tf. The incomplete efficacy of human Tf in this regard supports reports in the literature about Fe uptake by the liver in a manner that is independent of Tf receptors. Rhesus asialo-Tf was deployed to show that no recognition mechanism exists for heterologous asialo-Tf in rat hepatocytes. The importance of using undenatured labeled proteins for studies with cells is demonstrated.

Quantification of rat hepatocyte transferrin receptors with poly- and monoclonal antibodies and protein A

The content and distribution of transferrin receptors (Tf-R) in suspended adult rat hepatocytes were studied using 125I-protein A in combination with either a monoclonal (MRC OX-26) or a polyclonal antibody to Tf-R. Internal receptors were made accessible by permeabilization with digitonin. The number of Tf-R detected depended on the batch of collagenase used for liver perfusion. By using the monoclonal reagent in conjunction with the less damaging of two batches of the enzyme, 129,000 receptors were found per cell, with 47,000 (37%) of these on the surface. The polyclonal reagent yielded Tf-R numbers which were consistently higher than those obtained with MRC OX-26. This difference is interpreted as being due to the binding of several (on the average 5-6) molecules of polyclonal IgG per molecule of Tf-R. Remarkably, transferrin binding by Tf-R was not affected by this cluster of associated IgG and the overlayer of protein A. Parallel studies with 131I-transferrin in a simplified binding assay system yielded surface Tf-R estimates which, in most cases, were close to the values obtained with MRC OX-26. After prolonged exposure to collagenase, the ligand-binding capacity of Tf-R was more affected than its immunoreactivity. In preliminary studies, monensin (10 microM) produced a 32%-50% shift of Tf-R from the surface to the inside, whereas short-term incubation with epidermal growth factor (0.17 mM) brought about no clear-cut Tf-R redistribution.

Domain-specific monoclonal antibodies to ovotransferrin indicate conservation of determinants involved in avian transferrin receptor recognition

1. Three of five monoclonal antibodies produced to chicken ovotransferrin bound quail ovotransferrin but none of the antibodies bound human, bovine or equine serum transferrin. 2. Equilibrium binding experiments indicate that both quail and chicken ovotransferrin bind to transferrin receptors on chick reticulocytes although the quail protein binds to 40% fewer sites with an affinity which is three times lower than chicken ovotransferrin. 3. The antibodies that recognize quail ovotransferrin block binding of both radiolabelled chicken and quail ovotransferrin to chick reticulocytes. 4. Quail NH2-terminal half-molecule domain appears to be unable to form a functional hybrid holo-ovotransferrin with chicken C-terminal half-molecule domain.

Differential effect of iodination of ovotransferrin and its two half-molecule domains on binding to transferrin receptors on chick embryo red blood cells

Iodination of the C-terminal half-molecule domain of ovotransferrin (OTF) causes a significant reduction in binding to transferrin receptors on chick reticulocytes when compared to the binding observed with holo-OTF or the N-terminal half-molecule domain. (In such studies binding of iodinated half-molecule is measured in the presence of equimolar unlabelled complementary half-molecule). In particular iodination of the C-terminal half-molecule domain by the solid-phase reagent Iodogen resulted in half the binding found when ICl was used. The iodinated N-terminal half-molecule domain labelled by either Iodogen or ICl showed consistently higher binding than was observed with the C-terminal half-molecule or Fe2OTF. Although the molecular basis for the reduced binding of these proteins relative to the N-terminal half-molecule has not been definitively established, the implication is that there is a Tyr in the C-terminal domain which is involved in receptor recognition and binding. Addition of one or more bulky iodine atoms to the Tyr interferes with the interaction. Tryptic peptide maps of unlabelled holo-OTF and half-molecule domains and of the half-molecule domains labelled by both ICl and Iodogen are presented. The maps indicate limited access of the tyrosine residues to iodination especially in the C-terminal half-molecule domain. Equilibrium binding experiments have been carried out to compare the Kd (the apparent dissociation constant for the interaction between OTF and the transferrin receptors on chick-embryo red blood cells) with the Bmax, (binding at infinite free-ligand concentration) for Fe2OTF labelled using ICl, Iodogen, Enzymobeads and Chloramine-T. The effect of labelling Fe2OTF by Bolton-Hunter reagent has also been assessed. These studies show that ICl appears to be the reagent of choice for labelling Fe2OTF and its half-molecule domains.

Metabolic stability of the fucose in rat transferrin

The metabolic behaviour of the chitobiose core fucose that is a natural constituent of a large proportion of rat transferrin molecules was studied in rats comparatively to that of the polypeptide portion of the glycoprotein by using appropriate labels ([3H]fucose and 125I) and affinity chromatographic techniques (lentil-Sepharose). No evidence was obtained to suggest that this residue is cleaved from the glycan in significant amounts before removal of the entire glycoprotein for catabolism. Similarly, [14C]fucose linked to GlcNAc residues in the antennae of human asialotransferrin was being eliminated in pigeons at the same rate as the polypeptide itself. It is concluded that in spite of transferrin's exposure to the cellular milieu, the fate of its fucose is distinctly different from that of the same in plasma membrane glycoproteins.

Anti-phospholipase C antibodies inhibit the lectin-induced proliferation of human lymphocytes

A novel approach was used to assess the role of phosphoinositide hydrolysis in the mitogenic action of phytohemagglutinin (PHA) or concanavalin A (ConA). The treatment of human peripheral blood leukocytes (PBL) with monospecific antibodies against phospholipase C (PLC) produced a dose-dependent inhibition (up to 100%) of PHA (10 micrograms/ml) or ConA (25 micrograms/ml) proliferative effects. Thus, the activation of membrane-bound PLC is a sine-qua-non condition for lectin-induced proliferation of T lymphocytes. The key-role of PLC versus protein kinase C (PKC) is stressed by the fact that the inhibition of PKC with Hidaka's compound H-7 (40 microM) produced only a partial blockade (about 25%) of lectin mitogenic effect.

Induction of the 47 kDa platelet substrate of protein kinase C during differentiation of HL-60 cells

Immunoblot analysis showed that the 47 kDa platelet substrate of protein kinase C (P47) was expressed at low levels in undifferentiated HL-60 leukaemia cells. Treatment of these cells with dimethyl sulphoxide, 1 alpha,25-dihydroxycholecalciferol or retinoic acid caused progressive increases in P47 content. Retinoic acid (1 microM) elicited the largest response, a 4-fold increase in P47 protein after 7 days that was accompanied by an increase in translatable P47 mRNA. The induction of P47 by retinoic acid preceded cessation of cell proliferation and development of the capacity to reduce Nitro Blue Tetrazolium, indicating that its expression is an early event in the myeloid differentiation of HL-60 cells.

Iodogen-catalysed iodination of human calcitonin and Tyr(0)-katacalcin and purification of their mono- and di-iodinated derivatives by chromatofocussing

Synthetic calcitonin (CT) and Tyr(0)-katacalcin (tKT) were radioiodinated with Iodogen to high specific activity and with high yields. The products of the iodination procedure were chromatographed on Sephadex G-25 to remove unreacted iodide and then separated by chromatofocussing on PBE 94 (pH 9.6-6.0 for CT and pH 7.4-4.0 for tKT). Clear separation between uniodinated peptides and their mono- and di-iodinated derivatives was achieved with specific activities of 1900 and 3800 Ci/mmol for the respective mono- and di-iodinated peptides. Yields were up to 36 and 24% of mono- and di-iodinated CT and 41 and 29% for mono- and di-iodinated tKT. Our results show that Iodogen provides an effective and gentle way to iodinate peptides with high efficiency. Chromatofocussing is a simple, inexpensive and instrumentally undemanding method that can be performed without specialized chromatographic equipment and that should be applicable to a variety of different tracer preparations.

Diacytosis of human asialotransferrin type 3 in the rat liver is due to the sequential engagement of two receptors

The possible role of transferrin receptors in the diacytosis of human asialotransferrin type 3 (HAsTf-3) by the rat liver was studied in vivo. A trace dose of the ligand was allowed to compete for hepatic binding sites against diferric transferrin, the concentration of which was varied between 5 400- and 18 000-fold. Binding of HAsTf-3 was insensitive to the presence of 2Fe-transferrin in this range, and the liver bound the ligand equally efficiently, regardless of whether it was presented in the holo or apo form. In contrast, pretreating the animals with desialylated bovine submaxillary mucin (2 mg/100 g, 2 min before the dose) prevented the asialotransferrin-liver interaction. These findings indicate that endocytosis of HAsTf-3 is mediated by the Gal/GalNAc-specific lectin and not by transferrin receptors. Although 2Fe-transferrin did not affect binding, it did reduce the half-life of the ligand in the liver, thus suggesting that transferrin receptors play an important role in the exocytic leg of the diacytic cycle. Based on our present and earlier data, a model is proposed in which the engagement of lectin and transferrin receptor in the diacytic cycle is envisaged sequentially so that HAsTf-3 switches receptors at an acidified subcellular site.

The effects of cytotropic compounds on the resialylation of human asialotransferrin type 3 in the rat

Effects of chloroquine, colchicine, leupeptin, taxol and vinblastine on the resialylation and degradation of human [125I]asialotransferrin type 3 were studied in rats. An improved experimental technique was applied that permitted the quantification of resialylated ligand produced by individual animals over 3 h by using deconvolution. All three microtubule inhibitors increased the proportion of the dose undergoing resialylation by 35-39%. In addition, colchicine, and, especially, vinblastine enhanced the overall recovery of the dose as protein-bound 125I. The dose recovery was also augmented by leupeptin without any concomitant change in resialylation. Chloroquine suppressed resialylation and this effect could only be partially lifted by the administration of colchicine. The blood of colchicine-treated rats possessed no resialylating activity toward the ligand even when supplemented with additional alkaloid in vitro. The observations support the view that the respective fractions of the ligand destined for resialylation and degradation can, to a certain extent, be varied independently of each other. The effects of short-term starvation (20 h) and refeeding (4 h) on these processes are also presented.

Two populations of prelysosomal structures transporting asialoglycoproteins in rat liver

Analyses by differential centrifugation of liver homogenates from rats that had received 131I-labeled asialoorosomucoid showed that, 1 min after injection, most of the intracellular ligand was associated with a particle that did not sediment at 2.5 X 10(5) g-min. However, by 10 min, undigested ligand became associated with a particle that did sediment at this speed. On analytical ultracentrifugation in sucrose gradients, both kinds of particles exhibited low densities (1.11-1.13 g X ml-1). In contrast to asialoorosomucoid, 125I-labeled asialotransferrin type 3, under noncatabolic conditions, remained largely confined to the nonsedimenting particle regardless of the duration of the study. Induction of catabolism of asialotransferrin was accompanied by the appearance of the ligand in the sedimentable particle. The nonsedimentable particle was separated by immunoadsorption from other subcellular particles contained in the low-density subcellular fraction. The adsorbant , prepared by immobilizing purified antibodies to the Gal/GalN-specific lectin from rat liver on coated polyacrylamide beads, removed 75-80% of the asialoorosomucoid and transferrin binding capacities present, together with a similar portion of the radioligands tested (asialoorosomucoid, asialotransferrin type 3, and human diferric transferrin). Significantly, the sialytransferase activity remained unadsorbed. From these findings, the nonsedimentable particle appears to be involved in the transport of ligands destined to such diverse fates as exocytosis or lysosomal degradation. The sedimentable particle, on the other hand, seems to represent a link between the first particle and the lysosome.

Transferrin glycans: a possible link between alcoholism and hepatic siderosis

The hepatic uptake of 59Fe from diferric rat and rabbit asialotransferrins and from human transferrin lacking two sialyl residues was investigated in rats in experiments lasting for 1 hr. The 59Fe attached to either of these preparations disappeared from the plasma more rapidly than the 59Fe introduced with the unmodified respective parent proteins. Most of the 59Fe activity that had disappeared from the circulation could be recovered with the liver. Studies with double-labeled (125I, 59Fe) preparations showed that the enhanced 59Fe clearance was not associated with increased catabolism of the modified transferrins. Prolonged, heavy alcohol consumption, as shown by others, results in the appearance of sialic acid-deficient transferrin (two residues missing) in human serum. We suggest that the increased capacity of transferrin deficient in sialic acid to selectively deposit iron in the hepatocyte may be of significance for the development of the hepatic siderosis observed in alcoholism.