Crystal structure and iron-binding properties of the R210K mutant of the N-lobe of human lactoferrin: implications for iron release from transferrins

Lactoferrin (Lf) and serum transferrin (Tf) combine high-affinity iron binding with an ability to release this iron at reduced pH. Lf, however, retains iron to significantly lower pH than Tf, giving the two proteins distinct functional roles. In this paper, we compared the iron-release profiles for human Lf, Tf, and their N-lobe half-molecules Lf(N) and Tf(N) and showed that half of the difference in iron retention at low pH ( approximately 1.3 pH units) results from interlobe interactions in Lf. To probe factors intrinsic to the N-lobes, we further examined the specific role of two basic residues that are proposed to form a pH-sensitive dilysine trigger for iron release in the N-lobe of Tf [Dewan, J. C., Mikami, B., Hirose, M., and Sacchettini, J. C. (1993) Biochemistry 32, 11963-11968] by mutating Arg 210 to Lys in the N-lobe half-molecule Lf(N). The R210K mutant was expressed, purified, and crystallized, and its crystal structure was determined and refined at 2.0-A resolution to a final R factor (R(free)) of 19.8% (25.0%). The structure showed that Lys 210 and Lys 301 in R210K do not form a dilysine interaction like that between Lys 206 and Lys 296 in human Tf. The R210K mutant retained iron to lower pH than Tf(N), consistent with the absence of the dilysine interaction but released iron at approximately 0.7 pH units higher than Lf(N). We conclude that (i) the ability of Lf to retain iron to significantly lower pH than Tf is due equally to interlobe interactions and to the absence in Lfs of an interaction analogous to the dilysine pair in Tfs, even when two lysines are present at the corresponding sequence positions, and (ii) an appropriately positioned basic residue (Arg 210 in human Lf) modulates iron release by inhibiting protonation of the N-lobe iron ligands, specifically His 253.     

The transferrin receptor expressed by gonococcal strain FA1090 is required for the experimental infection of human male volunteers

Iron, an essential nutrient for most microorganisms, is sequestered by the host to decrease the concentration of iron available to bacterial pathogens. Neisseria gonorrhoeae , the causative agent of gonorrhoea, can acquire iron by direct interaction with human iron-binding proteins, including the serum glycoprotein, transferrin. Iron internalization from host transferrin requires the expression of a bacterial receptor, which specifically recognizes the human form of transferrin. Two gonococcal transferrin-binding proteins have been implicated in transferrin receptor function, TbpA and TbpB. We constructed a gonococcal transferrin receptor mutant without the introduction of additional antibiotic resistance markers and tested its ability to cause experimental urethritis in human male volunteers. The transferrin receptor mutant was incapable of initiating urethritis, although the same inoculum size of the wild-type parent strain, FA1090, causes urethritis in >90% of inoculated volunteers. To our knowledge, this is the first experimental demonstration that a bacterial iron acquisition system is an essential virulence factor for human infection.     

Lactoferrin receptors in Gram-negative bacteria: Insights into the iron acquisition process

One component of the anti-microbial function of lactoferrin (Lf) is its ability to sequester iron from potential pathogens. To overcome this iron limitation, a number of gram-negative bacterial pathogens have developed a mechanism for acquiring iron directly from this host glycoprotein. This mechanism involves surface receptors capable of specifically binding Lf from the host, removing iron and transporting it across the outer membrane. The iron is then bound by a periplasmic iron-binding protein, FbpA, and transported into the cell via an inner membrane complex comprised of FbpB and FbpC. The receptor has been shown to consist of two proteins, LbpA and LbpB. LbpB is bilobed lipoprotein anchored to the outer membrane via fatty acyl groups attached to the N-terminal cysteine. LbpA is a homologue of siderophore receptors, which consist of an N-terminal plug and a C-terminal beta-barrel region. We propose that the receptor proteins, LbpA and LbpB, induce conformational changes in human Lf (hLf) that lower its affinity for iron that binding by FbpA can drive the transport across the outer membrane, a mechanism shared with transferrin (Tf) receptors. The interaction between the receptor proteins and Lf is quite extensive and has been previously studied by using chimeric proteins comprised of Lf & Tf. In an attempt to evaluate the role of FbpA in the transport process, a series of site-directed mutants of FbpA were prepared and used to replace the wild-type protein in the iron acquisition pathway. The mutations were made in the iron-binding and anion-binding ligands of FbpA and were designed to result in altered binding properties. Protein crystallography of the iron-bound form of the Q58L mutant protein revealed that it was in the open conformation with iron coordinated by Y195 and Y196 from the C-terminal domain but not by the other iron-liganding amino acids from the N-terminal domain, H9 and E57. Replacement of the native FbpA in Neisseria meningitidis with wild-type or mutant Haemophilus influenzae FbpAs resulted in a defect in growth on Tf or Lf, suggesting that there may be a barrier to functional expression of H. influenzae FbpAs in Neisseria meningitidis. Thus mutants of the N. meningitidis FbpA are being prepared to replace wild-type protein in order to test their ability to mediate transport from hLf.     

Studies on the mechanism of specificity of human natural killer cells for tumor cells: correlation between target cell transferrin receptor expression and competitive activity

Previous studies to determine the nature of the specificity of natural killer (NK) cells for leukemic cells indicated that functional transferrin (Tf) receptors may be one of the determinants recognized by NK cells. To further investigate these observations, the relationship between cellular Tf receptor expression and ability to compete with a control K562 cell preparation in a standard chromium release assay was studied. K562 cells were selected at different phases of growth by removing cells from tissue culture at 1, 3, and 5 days postfeeding. Under these conditions, K562 cells, respectively, displayed relatively high, medium, and low numbers of Tf receptors and corresponding competitive activity against a control K562 cell preparation. K562 cells were modified by either trypsin, heat, or sodium butyrate (differentiation inducer) pretreatment. An NK-resistant clone was also studied. There was a good correlation between Tf receptor expression and cold competitive activity of the above K562 cell preparations (r = 0.82, P less than 0.01). The different tumor target cell lines, K562, Molt-4, Raji, HL-60, and MeWo, which would be expected to express different ranges of specificity, did not show a significant correlation between Tf receptor expression and their cold competitive activities against Cr-51-labeled K562 cells. Rabbit reticulocytes which express high numbers of Tf receptors were tested for their ability to compete with K562 cells for NK cells. These cells were able to compete with K562 cells while mature rabbit red blood cells which do not express Tf receptors did not compete well. These findings support the contention that the Tf receptor may be involved in NK cell recognition of some tumor cells.     

Insulin receptors with defective tyrosine kinase inhibit normal receptor function at the level of substrate phosphorylation

Rat 1 fibroblasts have been transfected with the cDNA encoding a kinase-defective mutant human insulin receptor (A/K1018). Expression of this cDNA results in a receptor that is not only biologically inactive but also inhibits normal insulin action through the normal endogenous rat receptors in this fibroblast line (McClain, D. A., Maegawa, H., Lee, J., Dull, T. J., Ullrich, A., and Olefsky, J. M. (1987) J. Biol. Chem. 262, 14663-14671). We have investigated the mechanism of this inhibition and show that: 1) rat receptors are expressed at normal to increased levels in two cell lines which also express A/K1018 receptors at low (A/K1018-A, 5700 total receptors) or high (A/K1018-B, 2.2 x 10(5) total receptors) levels. 2) The rat receptors in the A/K1018 lines can be normally autophosphorylated under the control of insulin in vitro. 3) A/K1018 receptors do not inhibit the kinase activity of normal receptors when mixed together in vitro. 4) In intact A/K1018-B cells, the ability of insulin to stimulate autophosphorylation of the rat receptor is unimpaired; furthermore, the autophosphorylated rat receptor becomes normally activated as a tyrosine kinase. 5) The expression of receptors for insulin-like growth factor I and stimulation of hexose uptake mediated by this receptor are unaffected in cells expressing inhibitory A/K1018 receptors. 6) Expression of the A/K1018 receptor inhibits insulin-stimulated phosphorylation of two endogenous protein substrates (pp220 and pp170) by the native rat receptors. We conclude that the inhibition of insulin action seen in the A/K1018 cells is not mediated at the levels of native receptor expression or activation, nor is the effector (hexose uptake) mechanism affected by the A/K1018 receptors. The expression of this kinase-defective receptor does, however, inhibit the phosphorylation of substrate molecules by the normally activated endogenous rat receptors.     

Pharmacokinetics and brain uptake of lactoferrin in rats

Lactoferrin (Lf) is an iron-binding glycoprotein belonging to the transferrin (Tf) family. Lf was reported to cross the blood brain barrier (BBB) via receptor-mediated transcytosis in an in vitro model of the BBB. In the present study, we compared the in vivo brain uptake of Lf with that of OX26, an anti-Tf receptor antibody, and Tf. These three proteins were radiolabeled with 125I and administered to rats by i.v. injection. We found that Lf was more rapidly eliminated from the blood compared with OX26 and Tf (The half-life of Lf was approximately 8 and 6 times shorter than that of OX26 and Tf, respectively; the area under the blood concentration-time curve of Lf was approximately 15 and 17 times smaller than that of OX26 and Tf, respectively), and mainly accumulated in the liver, spleen, and kidney. Markedly high brain uptake was observed for Lf relative to Tf and OX26. Lf might be useful as a ligand for facilitating drug delivery into the brain.     

Cellular internalization of lactoferrin in intestinal epithelial cells

We studied the cellular internalization of lactoferrin (Lf) in an intestinal epithelial cell line, Caco-2, to investigate the mechanism of biological actions of ingested Lf. RT-PCR and Western blotting analyses revealed that differentiated Caco-2 cells express LfR mRNA and its protein with a 34 kD molecular weight under reducing conditions. Biotin-labeled Lf showed specific binding to the cellular membrane of differentiated Caco-2 cells with a dissociation constant (Kd) of 0.16 microM. The cellular internalization of Lf was studied in differentiated Caco-2 cells grown as monolayers on Transwell inserts, and compared to that of human transferrin (Tf). After labeling with fluorescent dye, either Lf or Tf was added to Caco-2 cells from the apical side or the basolateral one. Laser scanning confocal microscopy showed that labeled Lf was internalized only from the apical side and localized to the nuclei. On the other hand, labeled Tf was internalized from the basolateral side, not from the apical side, and localized in the cytoplasm. The internalization of labeled Lf was inhibited by excess of unlabeled Lf, but not of Tf. The internalization of labeled Lf, but not of labeled Tf, was also suppressed by heparin. This indicates that a heparin-binding site in the N-terminal region of Lf could be important for the internalization of Lf. These findings suggest that ingested Lf might be internalized by the intestinal epithelium in a manner different from Tf and might function in the nucleus.     

Comparison of the interactions of transferrin receptor and transferrin receptor 2 with transferrin and the hereditary hemochromatosis protein HFE

The transferrin receptor (TfR) interacts with two proteins important for iron metabolism, transferrin (Tf) and HFE, the protein mutated in hereditary hemochromatosis. A second receptor for Tf, TfR2, was recently identified and found to be functional for iron uptake in transfected cells (Kawabata, H., Germain, R. S., Vuong, P. T., Nakamaki, T., Said, J. W., and Koeffler, H. P. (2000) J. Biol. Chem. 275, 16618-16625). TfR2 has a pattern of expression and regulation that is distinct from TfR, and mutations in TfR2 have been recognized as the cause of a non-HFE linked form of hemochromatosis (Camaschella, C., Roetto, A., Cali, A., De Gobbi, M., Garozzo, G., Carella, M., Majorano, N., Totaro, A., and Gasparini, P. (2000) Nat. Genet. 25, 14-15). To investigate the relationship between TfR, TfR2, Tf, and HFE, we performed a series of binding experiments using soluble forms of these proteins. We find no detectable binding between TfR2 and HFE by co-immunoprecipitation or using a surface plasmon resonance-based assay. The affinity of TfR2 for iron-loaded Tf was determined to be 27 nm, 25-fold lower than the affinity of TfR for Tf. These results imply that HFE regulates Tf-mediated iron uptake only from the classical TfR and that TfR2 does not compete for HFE binding in cells expressing both forms of TfR.     

Mutation of the two carboxyl-terminal tyrosines results in an insulin receptor with normal metabolic signaling but enhanced mitogenic signaling properties

Our previous studies have shown that the deletion of the insulin receptor carboxyl terminus impairs metabolic, but augments mitogenic, signaling (McClain, D. A., Maegawa, H., Levy, J., Huecksteadt, T., Dull, T. J., Lee, J., Ullrich, A., and Olefsky, J. M. (1988) J. Biol. Chem. 263, 8904-8911; Thies, R.S., Ulrich, A., and McClain, D. A. (1989) J. Biol. Chem. 264, 12820-12825). To explore further the regulatory role of the insulin receptor carboxyl terminus, a mutant insulin receptor was constructed in which the two tyrosines (Y1316 and Y1322) on the carboxyl terminus were replaced with phenylalanines. Rat 1 fibroblasts expressing high levels of this mutant receptor (Y/F2 cells) exhibited normal insulin binding and normal insulin internalization. The absence of the two tyrosines in the carboxyl terminus did not affect the phosphotransferase activity of the beta-subunit and insulin-stimulated glucose transport. However, the Y/F2 cells showed markedly enhanced sensitivity for insulin-stimulated DNA synthesis. Dose-response curves for both insulin-stimulated thymidine uptake and 5-bromo-2-deoxyuridine incorporation in the Y/F2 cell lines were shifted to the left (4-10-fold) compared with those observed in the cells expressing similar numbers of wild type receptors. Thus, the two tyrosines of the insulin receptor carboxyl terminus do not modulate the kinase function of the insulin receptor, although they are autophosphorylated in native receptors. Moreover, these tyrosines are not necessary for stimulation of glucose transport. On the other hand, these results suggest that the two carboxyl-terminal tyrosine residues exert an inhibitory effect on mitogenic signaling in native insulin receptors.     

Functional expression of the human transferrin receptor cDNA in Chinese hamster ovary cells deficient in endogenous transferrin receptor

Transferrin (Tf) receptor-variant Chinese hamster ovary cells have been isolated by selection for resistance to two Tf-toxin conjugates. The hybrid toxins contain Tf covalently linked to ricin A chain or a genetically engineered diphtheria toxin fragment. The Tf-receptor-variant (TRV) cells do not have detectable cell-surface Tf receptor; they do not bind fluorescein-Tf or 125I-Tf. TRV cells are at least 100-fold more resistant to the Tf-diphtheria toxin conjugate than are the parent cells. The TRV cells have retained sensitivity to native diphtheria toxin, indicating that the increased resistance to the conjugate is correlated with the loss of Tf binding. The endocytosis of fluorescein-labeled alpha 2-macroglobulin is normal in TRV cells, demonstrating that the defect does not pleiotropically affect endocytosis. Since these cells lack endogenous Tf receptor activity, they are ideally suited for studies of the functional expression of normal or altered Tf receptors introduced into the cells by cDNA transfection. One advantage of this system is that Tf binding and uptake can be used to monitor the behavior of the transfected receptor. A cDNA clone of the human Tf receptor has been transfected into TRV cells. In the stably expressing transfectants, the behavior of the human receptor is very similar to that of the endogenous Chinese hamster ovary cell Tf receptor. Tf binds to cell surface receptors, and is internalized into the para-Golgi region of the cell. Iron is released from Tf, and the apo-Tf and its receptor are recycled back to the cell surface. Thus, the TRV cells can be used to study the behavior of genetically altered Tf receptors in the absence of interfering effects from endogenous receptors.     

Heme-binding of bovine lactoferrin: the potential presence of a heme-binding capacity in an ancestral transferrin gene

Lactoferrin (Lf) and transferrin (Tf) are iron-binding proteins that can bind various metal ions. This study demonstrates the heme-binding activity of bovine Lf and Tf using biotinylated hemin. When both proteins were coated on separate plate wells, each directly bound biotinylated hemin. On the other hand, when biotinylated hemin was immobilized on an avidin-coated plate, soluble native Lf bound to the immobilized biotinylated hemin whereas native Tf did not, suggesting that a conformational change triggered by coating on the plate allows the binding of denatured Tf with hemin. Incubation of Lf with hemin-agarose resulted in negligible binding of Lf with biotinylated hemin. Lf in bovine milk also bound to immobilized biotinylated hemin. These results demonstrate that bovine Lf has specific heme-binding activity, which is different from Tf, suggesting that either Tf lost heme-binding activity during its evolution or that Lf evolved heme-binding activity from its Tf ancestral gene. Additionally, Lf in bovine milk may bind heme directly, but may also bind heme indirectly by interaction with other milk iron- and/or heme-binding proteins.     

Activation of the mitogen-activated protein kinase pathway by a Gq/11-coupled muscarinic receptor is independent of receptor internalization

A number of recent studies have demonstrated an essential role for receptor endocytosis in the activation of the mitogen-activated protein (MAP) kinases, Erk-1 and Erk-2 (extracellular activated protein kinases 1 and 2), by growth factor receptors and the G-protein coupled beta2-adrenergic receptor. Because ligand-mediated receptor endocytosis and activation of the MAP kinase pathway are common phenomena among G-protein coupled receptors, it has been suggested that the essential role of endocytosis in MAP kinase activation identified for the beta2-adrenergic receptor may be universal for all G-protein coupled receptors (Daaka,Y., Luttrell, L. M., Ahn, S., Della Rocca, G. J., Ferguson, S. S. G., Caron, M. G., and Lefkowitz, R. J. (1998) J. Biol. Chem. 273, 685-688). We tested this hypothesis using the Gq/11-coupled m3-muscarinic receptor expressed in Chinese hamster ovary cells and an m3-muscarinic receptor mutant that does not undergo endocytosis. We demonstrate that inhibition of endocytosis by concanavalin A and cytochalasin D does not affect the ability of the wild type m3-muscarinic receptor to activate Erk-1/2. Furthermore, the mutant m3-muscarinic receptor that is unable to undergo endocytosis, activates the MAP kinase pathway in an identical manner to the wild type receptor. We conclude that receptor endocytosis is not universally essential for MAP kinase activation by G-protein coupled receptors. We discuss the possibility that the differential roles played by endocytosis in MAP kinase activation between various receptor subtypes may be linked to the mechanism of upstream activation of Raf-1.     

"Dilysine trigger" in transferrins probed by mutagenesis of lactoferrin: crystal structures of the R210G,R210E,and R210L mutants of human lactoferrin

The mammalian iron-binding proteins lactoferrin (Lf) and transferrin (Tf) bind iron very tightly, but reversibly. Despite homologous structures and essentially identical iron binding sites, Tf begins to release iron at pH 6.0, whereas Lf retains iron to pH approximately 3.5. This difference in iron retention gives the two proteins different biological roles. Two lysine residues, Lys 206 and Lys 296, which form a hydrogen-bonded dilysine pair in human Tf, have been shown to strongly influence iron release from the N-lobe. The equivalent residues in human Lf are Arg 210 and Lys 301, and we have here mutated Arg 210 in the N-lobe half-molecule of human lactoferrin, Lf(N), to probe its role in iron release. The Lf(N) mutants R210G, R210E, and R210L were expressed, purified, and crystallized, and their crystal structures were determined and refined at resolutions of 1.95 A (R210G), 2.2 A (R210E), and 2.0 A (R210L). The overall structures are very similar to that of wild-type Lf(N), but with small differences in domain orientations. In each of the mutants, however, Lys 301 (equivalent to Lys 296 in Tf) changes its conformation to fill the space occupied by Arg 210 Neta2 in wild-type Lf(N), interacting with the two tyrosine ligands Tyr 92 and Tyr 192. By comparison with other Lf and Tf structures, we conclude that Lys 301 (or Lys 296 in Tf) only occupies this site when residue 210 (206 in Tf) is nonpositive (neutral as in R210G and R210L or negative as in R210E). Thus, Lys 206 in the Tf dilysine pair is identified as having a depressed pK(a). Three specific sites are variably occupied by polar groups in the Lf mutants and other Lf and Tf proteins, and when coupled with iron-release data, these give new insights into the factors that most influence iron retention at low pH.     

The hereditary hemochromatosis protein, HFE, specifically regulates transferrin-mediated iron uptake in HeLa cells

HFE is the protein product of the gene mutated in the autosomal recessive disease hereditary hemochromatosis (Feder, J. N., Gnirke, A., Thomas, W., Tsuchihashi, Z., Ruddy, D. A., Basava, A., Dormishian, F., Domingo, R. J., Ellis, M. C., Fullan, A., Hinton, L. M., Jones, N. L., Kimmel, B. E., Kronmal, G. S., Lauer, P., Lee, V. K., Loeb, D. B., Mapa, F. A., McClelland, E., Meyer, N. C., Mintier, G. A., Moeller, N., Moore, T., Morikang, E., Prasss, C. E., Quintana, L., Starnes, S. M., Schatzman, R. C., Brunke, K. J., Drayna, D. T., Risch, N. J., Bacon, B. R., and Wolff, R. R. (1996) Nat. Genet. 13, 399-408). At the cell surface, HFE complexes with transferrin receptor (TfR), increasing the dissociation constant of transferrin (Tf) for its receptor 10-fold (Gross, C. N., Irrinki, A., Feder, J. N., and Enns, C. A. (1998) J. Biol. Chem. 273, 22068-22074; Feder, J. N., Penny, D. M., Irrinki, A., Lee, V. K., Lebron, J. A., Watson, N. , Tsuchihashi, Z., Sigal, E., Bjorkman, P. J., and Schatzman, R. C. (1998) Proc. Natl. Acad. Sci. U S A 95, 1472-1477). HFE does not remain at the cell surface, but traffics with TfR to Tf-positive internal compartments (Gross et al., 1998). Using a HeLa cell line in which the expression of HFE is controlled by tetracycline, we show that the expression of HFE reduces 55Fe uptake from Tf by 33% but does not affect the endocytic or exocytic rates of TfR cycling. Therefore, HFE appears to reduce cellular acquisition of iron from Tf within endocytic compartments. HFE specifically reduces iron uptake from Tf, as non-Tf-mediated iron uptake from Fe-nitrilotriacetic acid is not altered. These results explain the decreased ferritin levels seen in our HeLa cell system and demonstrate the specific control of HFE over the Tf-mediated pathway of iron uptake. These results also have implications for the understanding of cellular iron homeostasis in organs such as the liver, pancreas, heart, and spleen that are iron loaded in hereditary hemochromatotic individuals lacking functional HFE.     

The soluble form of the membrane-bound transferrin homologue, melanotransferrin, inefficiently donates iron to cells via nonspecific internalization and degradation of the protein.

Melanotransferrin (MTf) is a membrane-bound transferrin (Tf) homologue found particularly in melanoma cells. Apart from membrane-bound MTf, a soluble form of the molecule (sMTf) has been identified in vitro[Food, M.R., Rothenberger, S., Gabathuler, R., Haidl, I.D., Reid, G. & Jefferies, W.A. (1994) J. Biol. Chem.269, 3034-3040] and in vivo in Alzheimer's disease. However, nothing is known about the function of sMTf or its role in Fe uptake. In this study, sMTf labelled with 59Fe and 125I was used to examine its ability to donate 59Fe to SK-Mel-28 melanoma cells and other cell types. sMTf donated 59Fe to cells at 14% of the rate of Tf. Analysis of sMTf binding showed that unlike Tf, sMTf did not bind to a saturable Tf-binding site. Studies with Chinese hamster ovary cells with and without specific Tf receptors showed that unlike Tf, sMTf did not donate its 59Fe via these pathways. This was confirmed by experiments using lysosomotropic agents that markedly reduced 59Fe uptake from Tf, but had far less effect on 59Fe uptake from sMTf. In addition, an excess of 56Fe-labelled Tf or sMTf had no effect on 125I-labelled sMTf uptake, suggesting a nonspecific interaction of sMTf with cells. Protein-free 125I determinations demonstrated that in contrast with Tf, sMTf was markedly degraded. We suggest that unlike the binding of Tf to specific receptors, sMTf was donating Fe to cells via an inefficient mechanism involving nonspecific internalization and subsequent degradation.     

Effects of epidermal growth factor on transferrin receptor phosphorylation and surface expression in malignant epithelial cells

The transferrin (Tf) receptor is a major transmembrane protein which provides iron for normal and malignant cell growth. Epidermal growth factor (EGF) has been reported to rapidly and transiently alter the number of surface Tf receptors in normal and transformed epithelial cells. To investigate mechanisms of EGF-induced changes in surface Tf display, EGF effects on surface Tf receptors were compared in two cell lines which differ in their number of EGF receptors and growth responses to EGF. In cloned A431 cells with high receptor numbers which are growth-inhibited by EGF, EGF caused a 50% decrease in Tf receptor expression after 30 min. In contrast, EGF induced a rapid, transitory increase (within 5 min) in the number of surface Tf receptors on KB carcinoma cells which returned to basal levels by 15 min. The observed changes in Tf receptor display were due to altered receptor distribution and not changes in ligand affinity or total cellular transferrin receptor pools. Anti-EGF receptor monoclonal antibody blocked effects of EGF on transferrin receptor expression. Since the antibody is internalized and causes EGF receptor down-regulation, effects on transferrin receptor expression were independent of these events. EGF-induced alterations in Tf receptor display occurred even when cells were pretreated with colchicine, suggesting that changes in surface Tf binding were not mediated by cytoskeletal components. Na orthovanadate, which mimics some early cellular effects of EGF, duplicated EGF's effects on A431 Tf receptors, but had no effect on KB cells, suggesting these responses occur by differing mechanisms. To determine whether EGF caused changes in Tf receptor phosphorylation, 32P-labelled Tf receptors were immunoprecipitated after EGF treatment. After exposure to EGF, A431 cells showed no change in Tf phosphorylation, but KB cells showed a transient, 6-fold increase in transferrin receptor phosphorylation on serine residues. In both A431 and KB cells, phorbol ester (PMA) also increased phosphorylation on transferrin receptors, but had little effect on surface Tf receptor expression. In malignant cell lines, EGE induces rapid, variable changes in transferrin receptor expression and phosphorylation which differ from the effects of PMA. These early responses to EGF appear to differ with the cell type and correlate poorly with alterations in Tf receptor phosphorylation. These results suggest Tf receptor phosphorylation does not regulate Tf receptor display in all cells.     

Substitution of arginine for histidine at position 209 in the alpha-subunit of the human insulin receptor. A mutation that impairs receptor dimerization and transport of receptors to the cell surface.

Receptors for insulin and epidermal growth factor contain cysteine-rich domains in the extracellular portion of the molecule. His209 (insulin receptor numbering system) is 1 of 2 amino acid residues that are identically conserved in the cysteine-rich domains of insulin receptors, epidermal growth factor receptors, and other homologous receptors. Previously, we have identified a patient with leprechaunism who is homozygous for a mutation substituting Arg for His209 in the insulin receptor gene (Kadowaki, T., Kadowaki, H., Rechler, M. M., Serrano-Rios, M., Roth, J., Gorden, P., and Taylor, S. I. (1990) J. Clin. Invest. 86, 254-264). In this investigation, the Arg209 mutant receptor was expressed by transfection of mutant cDNA into NIH-3T3 cells. The mutation impairs several steps in the post-translational processing of the insulin receptor:dimerization of 190-kDa proreceptors into a disulfide linked species, proteolytic cleavage of the proreceptor into alpha- and beta-subunits, and terminal processing of the high mannose form of N-linked oligosaccharide into complex carbohydrate. In addition, the defects in post-translational processing within the endoplasmic reticulum and Golgi apparatus are associated with a marked inhibition in transport of receptors to the plasma membrane. Nevertheless, a small number (approximately 10%) of the receptors are transported to the cell surface. These receptors on the cell surface bind insulin with normal affinity and have normal tyrosine kinase activity.     

Role of lactoferrin and its receptors on biliary epithelium

Human lactoferrin is an iron-binding glycoprotein present at high concentrations in breast milk and colostrum. It is produced by many exocrine glands and widely distributed in a variety of body fluids. This protein has antimicrobial, immunomodulatory, antioxidant, and anticancer properties. Two important hLf receptors have been identified: LDL receptor related protein (LRP1), a low specificity receptor, and intelectin-1 (ITLN1), a high specificity receptor. No data are present on the role of hLf on the biliary epithelium. Our aims have been to evaluate the expression of Lf and its receptors in human and murine cholangiocytes and its effect on proliferation. Immunohistochemistry and immunofluorescence (IF) were conducted on human healthy and primary biliary cholangitis (PBC) liver samples as well as on liver samples obtained from normal and bile duct ligated (BDL) mice to evaluate the expression of Lf, LRP1 and ITLN1. Cell proliferation in vitro studies were performed on human cholangiocyte cell lines via 3-(4,5-dimetiltiazol-2-il)-2,5-diphenyltetrazolium assay as well as IF to evaluate proliferating cell nuclear antigen (PCNA) expression. Our results show that mouse and human cholangiocytes express Lf, LRP1 and ITLN1, at higher extent in cholangiocytes from BDL and PBC samples. Furthermore, the in vitro addition of bovine Lf (bLf) has a proliferative effect on human cholangiocyte cell line. The results support a proliferative role of hLf on the biliary epithelium; this pro-proliferative effect of hLf and bLf on cholangiocytes could be particularly relevant in human cholangiopathies such as PBC, characterized by cholangiocyte death and ductopenia.     

Low binding capacity and altered O-linked glycosylation of low density lipoprotein receptor in a monensin-resistant mutant of Chinese hamster ovary cells

We have studied function and structure of the low density lipoprotein (LDL) receptors in a monensin-resistant (Monr-31) mutant isolated from Chinese hamster ovary (CHO) cells. To assay the ability of the receptor to bind LDL, we employed three methods, 125I-LDL binding to the cells at 4 degrees C, 125I-LDL binding to the receptor-phospholipid complex (Schneider, W.J., Goldstein, J.L., and Brown, M.S. (1980) J. Biol. Chem. 255, 11442-11447), and ligand blotting (Daniel, T.O., Schneider, W.J., Goldstein, J.L., and Brown, M.S. (1983) J. Biol. Chem. 258, 4606-4611). The LDL receptor number was similar in both CHO and Monr-31, but the binding affinity was reduced in the mutant. The semi-quantitative immunoblotting assay with an antibody directed against the COOH-terminal 14 amino acids and the ligand-blotting assay with LDL also showed that the relative steady-state level of the receptor in Monr-31 was comparable to that in CHO, whereas the binding capacity of the receptor in Monr-31 was lower than that in CHO. The precursor and degradation forms of the LDL receptors produced in the mutant cells were similar in size to those in the parental cells, but the apparent molecular mass of the mature receptor protein in sodium dodecyl sulfate-polyacrylamide gels was reduced about 5000 daltons in the mutant. These results suggest a structural change at the NH2-terminal LDL binding domain. Tests of the effects of tunicamycin, endo-alpha-N-acetylgalactosaminidase (O-glycanase), and sialidase (neuraminidase) on the molecular size of the mature receptors indicated that the reduced size of the receptor in the mutant cells resulted from altered oligosaccharide chain(s) linked to serine/threonine residues in the binding domain. We compared the molecular sizes and binding activity of human LDL receptors in several clones derived from CHO and Monr-31 cells which were transfected with human LDL receptor cDNA. The human LDL receptors produced in the transfected clones of Monr-31 were also smaller in molecular size and lower in binding capacity than those produced in the transfected clones of CHO. These results suggest that both structural and functional alteration of the LDL receptor of Monr-31 is not caused by a mutation in the structural gene of the LDL receptor but by altered processing or maturation of the receptor. The correlation of the decrease in molecular size and reduced binding capacity of the LDL receptor is discussed.     

Evidence for hybrid rodent and human insulin receptors in transfected cells.

Chinese hamster ovary cells and NIH 3T3 cells overexpressing mutant human insulin receptors were examined for the presence of hybrid receptors composed of human and rodent insulin receptors. In the present studies, most of the endogenous rodent receptors were found to be immunoprecipitated from the transfected cells but not the parental cells with a monoclonal antibody specific for human receptor. These data indicate that in these transfected cells, most of the endogenous rodent receptors are in a hybrid complex with the overexpressed human receptor. These results together with the in vitro studies of Treadway et al. (Treadway, J.L., Morrison, B.D., Soos, M.A., Siddle, K., Olefsky, J., Ullrich, A., McClain, D.A., and Pessin, J.E. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 214-218) showing that hybrid receptors exhibit transdominant inhibition explain the prior finding indicating that overexpression of defective insulin receptors interferes with the normal signaling of endogenous receptors.