Retinol-binding protein and asialo-orosomucoid are taken up by different pathways in liver cells

The intracellular transport and degradation of in vivo endocytosed retinol-binding protein was compared with that of asialo-orosomucoid, a marker for receptor-mediated endocytosis through coated pits. The transport pathways were studied in rat liver cells by means of subcellular fractionation in Nycodenz and sucrose density gradients and by immunoelectron microscopy. Retinol-binding protein and asialo-orosomucoid were labeled by covalent attachment of radioiodinated tyramine cellobiose, an adduct which is incapable of crossing cellular membranes and thus provides a marker for the organelles where the protein has been taken up and degraded. The data obtained from subcellular fractionation studies, as well as from immunoelectron microscopy, showed that retinol-binding protein and asialo-orosomucoid were initially localized in different endocytic vesicles. Retinol-binding protein co-localized in density gradients with markers for potocytosis, an alternative endocytic pathway which uses internalization through caveolae instead of clathrin-coated pits. Later, retinol-binding protein and asialo-orosomucoid comigrated in the gradients and they were also observed in the same larger vesicles by immunoelectron microscopy. These data suggest that retinol-binding protein is taken up by liver cells by potocytosis and that a fraction of the retinol-binding protein is later transferred to larger vesicles located deeper in the cytoplasm where degradation takes place.

Interaction of the microtubule cytoskeleton with endocytic vesicles and cytoplasmic dynein in cultured rat hepatocytes

In a recent study (Goltz, J.S., Wolkoff, A.W., Novikoff, P.M., Stockert, R.J., and Satir, P. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 7026-7030), we found that ligand- and receptor-containing endocytic vesicles bind to endogenous microtubules in vitro after 60 min of receptor-mediated endocytosis of asialo-orosomucoid. In the presence of ATP, ligand-containing endocytic vesicles are released from microtubules, while those containing receptor are not. We hypothesized that cytoplasmic dynein may associate with ligand-containing, but not receptor-containing, domains of endocytic vesicles and might be involved in the movement of ligand-containing vesicles along microtubules during sorting of ligand from receptor. Direct evidence in support of this hypothesis has been obtained in the present study. Binding of ligand-containing vesicles to microtubules correlates highly (p < 0.001) with binding of dynein, but not kinesin, under a variety of conditions. Binding of receptor-containing vesicles to microtubules is independent of both cytoplasmic dynein and kinesin binding. Tight association of cytoplasmic dynein with a population of ligand-containing vesicles is seen directly by immunoprecipitation. These results support the view that in receptor-mediated endocytosis, ligand-containing vesicles become bound to microtubules by cytoplasmic dynein. While receptor domains of endosomes remain attached to microtubules in an ATP-independent manner, ligand-containing domains might be moved away toward pericentrosomal lysosomes by this motor molecule.

CD23 molecule acts as a galactose-binding lectin in the cell aggregation of EBV-transformed human B-cell lines

Epstein-Barr virus (EBV)-transformed human B-cell lines, L-KT9 and DH3 cells express CD23 antigen, and grow in a mixture of single and aggregated cells. The CD23 molecule has high amino acid sequence homology with C-type lectin and recently we have shown that the solubilized CD23 molecule can really interact with galactose residues on glycoproteins. In this study, therefore, we tested whether CD23 antigen on the cell surface really acts as a galactose-binding lectin in the aggregation of these cells. The EBV-transformed cells (L-KT9) were separated into an aggregated-cell-rich fraction and a single-cell-rich fraction. Aggregated cells disaggregated after removal of galactose by beta-galactosidase treatment, whereas single cells made large aggregation on sialidase treatment, and this aggregation was inhibited in the presence of asialo-fetuin. On the other hand, naturally aggregated cells become single cells with anti-CD23 monoclonal antibody (mAB) as well as the soluble form of CD23, but not with anti-CD21 mAB. In addition, L-KT9 and DH3 cells bound to asialo-fetuin-coupled Sepharose (ASF-Sepharose) and this binding was significantly inhibited by pre-treatment of cells with anti-CD23, but not with anti-CD21 or other anti-adhesion molecules. From these results, we conclude that the naturally aggregated state of EBV-transformed cells occurs mainly through the interaction of CD23 as a lectin molecule and galactose residues as its ligand.

A molecular receptor-binding contrast agent for magnetic resonance imaging of the liver

RATIONALE AND OBJECTIVES

A gadolinium complex of polydiethylenetriamine pentaacetic acid polyneogalactosyl polylysine (Gd-DTPA-gal-PL) was developed and tested as a paramagnetic contrast agent for magnetic resonance (MR) imaging of the liver. The agent was designed for receptor-mediated uptake by the asialoglycoprotein receptor (ASGP-R), which is unique to hepatocytes and exhibits high specificity for galactose-terminated glycoconjugates.

METHODS

Polylysine was alkylated with a mixed anhydride of diethylenetriamine pentaacetic acid. This product was complexed with gadolinium and N-alkylated with 3-oxopropyl-1-thio-beta-D-galactopyranoside. With this reaction sequence, we prepared a gadolinium complex consisting of 2284 galactose groups and 858 chelators per polylysine having 2136 amino groups. Hepatic enhancement was tested by MR imaging of nine rats with liver-implanted mammary adenocarcinoma before and after injection of 20 x 10(-9) mol/kg Gd-DTPA858-gal2284-PL2136. The conjugate was labeled with technetium-99m and tested (1.5 x 10(-10) mol/kg) for hepatic specificity via nuclear imaging.

RESULTS

Mean hepatic enhancement was 86% within 10 min and remained constant for 25 min. Hepatic relative intensity exceeded preinjection intensities by at least four times the standard deviation of the preinjection values (p < .01). The tumors, which are devoid of ASGP-R, did not exhibit significant enhancement (p > .1). The liver accumulated 90% of the technetium-99m-labeled conjugate.

CONCLUSION

A molecular paramagnetic ligand to the asialoglycoprotein receptor has been developed for hepatocyte-specific MR contrast enhancement.

The differences in structural specificity for recognition and binding between asialoglycoprotein receptors of liver and macrophages

The Gal/GalNAc-specific lectin on the surface of rat peritoneal macrophages (macrophage asialoglycoprotein binding protein, M-ASGP-BP), which consists of a single polypeptide chain of 42 kDa, can form a homo-oligomeric receptor exhibiting high affinity for asialoorosomucoid (ASOR) [Ozaki K., Ii M., Itoh N., Kawasaki T. (1992) J Biol Chem 267: 9229-35]. In this study, the binding affinity of M-ASGP-BP was studied by using a series of synthetic or natural glycosides as inhibitors of 125I-ASOR binding to recombinant M-ASGP-BP expressed on COS-1 cells (rM-ASGP-BP), and the results were compared with those of human hepatic lectin (HHL) on Hep G2 cells. Clustering of multiple Gal (or GalNAc) residues increased the binding affinity to M-ASGP-BP as well as to HHL. In contrast to HHL and other mammalian hepatic lectins, rM-ASGP-BP bound Gal residues tighter than GalNAc residues. A galactose-terminated triantennary N-glycoside, having one N-acetyl-lactosamine unit on the 6 branch and two N-acetyl-lactosamine units on the 3 branch of the trimannosyl core structure, showed affinity enhancement of approximately 10(5) over a monovalent ligand for HHL, while the same glycopeptide showed enhancement of about 2000-fold for rM-ASGP-BP. These results suggest that spatial arrangements of sugar combining sites and subunit organization of macrophage and hepatic lectins are different.

High-yield purification and characterization of human asialoglycoprotein receptor

The human asialoglycoprotein receptor (ASGPR) represents a major component of the hepatocellular membrane. To study its native composition, approximately 30% of receptor activity from liver specimens was recovered in highly purified ASGPR preparations. Discontinuous, denaturing SDS-gel electrophoresis based on Tris-Tricine buffer indicated the presence of a multimeric ASGPR corresponding to H1 and H2 polypeptides as confirmed by peptide-specific immunoblotting. FPLC-gel filtration of ASGPR preparations revealed a molecular mass for receptor complexes at 150 and 95 kDa, suggesting functional heterotrimers and dimers of H1/H2 subunits. Gel filtration of SDS-denatured protein indicated a single peak at 50 kDa apparently corresponding to dissociated subunits H1 and H2. beta-Mercaptoethanol treatment followed by affinity chromatography separated functionally active and inactive receptors. The H2 subunit was strikingly enriched in the inactive fraction of receptors. Both active and inactive ASGPR preparations consistently showed peaks at 150 and 95 kDa by gel filtration. Receptor activity retained in such heteromers was linked to a lower glycosylation state of ASGPR. These results suggest that native human ASGPR consists of sulfide- and non-sulfide-linked heterotrimers and -dimers from H1 and H2 subunits with a functional restriction to their glycosylation states.

Functional liver imaging with asialoglycoprotein receptors and serum hyaluronate in a patient with amyloidosis

A 47-year-old man with primary amyloidosis was admitted with abdominal pain. A new radionuclide liver imaging technique using Technetium-99m diethylenetriamine-pentaacetic acid-galactosyl human serum albumin, was performed and the serum hyaluronate concentration was measured. Although ordinary laboratory tests revealed only slight abnormalities, the uptake of the radiolabelled ligand for hepatocyte asialo-glycoprotein receptors was decreased, and marked hepatomegaly was revealed. Furthermore, the serum hyaluronate level was elevated. Histological examination of a hepatic needle biopsy specimen revealed a marked deposition of amyloid in the hepatic perisinusoidal spaces. These results indicate that this new radionuclide liver imaging technique (using Technetium-99m diethylenetriamine-pentaacetic acid-galactosyl human serum albumin) and the measurement of serum hyaluronate may be useful supplementary tools for identifying amyloid deposition in the hepatic perisinusoidal spaces in patients with amyloidosis.

Inhibition of autophagy and multiple steps in asialoglycoprotein endocytosis by inhibitors of tyrosine protein kinases (tyrphostins)

In isolated rat hepatocytes, several tyrosine protein kinase inhibitors (tyrphostins) reduced the autophagic sequestration of electroinjected [3H]raffinose by 40-75% at doses that did not significantly affect cellular ATP levels or plasma membrane integrity. Tyrphostin 46 specifically inhibited autophagy, whereas tyrphostins 1, 25 and 51 also suppressed the receptor-mediated endocytic uptake of 125I-tyramine-cellobiose-asialoorosomucoid, 125I-TC-AOM, by 20-30% and its degradation by 70-90%. Tyrphostins 1 and 51, and the microtubule inhibitor vinblastine, inhibited an early endocytic step (endosome maturation/multivesiculation?), causing accumulation of endocytosed 125I-TC-AOM in a recycling compartment that corresponded to light endosomes (1.10-1.11 g/ml) in sucrose density gradients. In the electron microscope, these endosomes could be recognized as small, peripheral endocytic vesicles and tubules accumulating endocytosed AOM-gold. The serine/threonine protein phosphatase inhibitor okadaic acid inhibited an intermediate endocytic step (detachment of multivesicular endosomes from the tubulovesicular network?), causing accumulation of 125I-TC-AOM in a recycling compartment corresponding to light endosomes (1.10-1.11 g/ml), but with a multivesicular rather than a tubulovesicular morphology. Tyrphostin 25 inhibited endocytosis at a late step (endosome-lysosome fusion?), causing accumulation of 125I-TC-AOM in a non-recycling compartment corresponding to dense, multivesicular endosomes (1.14 g/ml) that had probably detached from the light endosomal network.

Characterization of carbohydrate binding proteins in Trypanosoma cruzi

Trypanosoma cruzi is an obligatory intracellular protozoan parasite that causes Chagas' disease in humans and invades a great variety of mammalian cells. The nature of the ligand(s) and receptor components in both T. cruzi and target cells remains controversial, although it seems to involve an interaction with oligosaccharides. In an attempt to identify possible ligands on the parasite, we have searched for the presence of carbohydrate binding proteins (CBPs) in T. cruzi. By fluorescence-activated cell sorter analysis using a panel of fluoresceinated glyco- and neoglycopeptides with well characterized glycans, the presence of at least two different CBPs was identified on the surface of T. cruzi epimastigotes and trypomastigotes. The specificity of binding of the two CBPs seems to be mediated by galactose and mannose residues. The mannose- and galactose-mediated CBPs from epimastigotes and trypomastigotes were purified to homogeneity by affinity chromatography on immobilized thyroglobulin and identified as 60-70-kDa glycoproteins. Purified CBPs were able to specifically bind with high affinity to murine and human macrophages as well as other cell types susceptible to infection by T. cruzi but not to fat or neuronal cells. This binding was inhibited by the corresponding ligands. Moreover, the mannose-mediated CBP binding was completely abolished by alpha-mannosidase treatment of the cells. These results suggest a possible role for the CBPs in the recognition events between the parasite and target cells and/or in the interaction of the epimastigotes with the insect gut cells.

Functional expression of low density lipoprotein receptor-related protein is controlled by receptor-associated protein in vivo

The 39-kDa receptor-associated protein (RAP) associates with the multifunctional low density lipoprotein (LDL) receptor-related protein (LRP) and thereby prevents the binding of all known ligands, including alpha 2-macroglobulin and chylomicron remnants. RAP is predominantly localized in the endoplasmic reticulum, raising the possibility that it functions as a chaperone or escort protein in the biosynthesis or intracellular transport of LRP. Here we have used gene targeting to show that RAP promotes the expression of functional LRP in vivo. The amount of mature, processed LRP is reduced in liver and brain of RAP-deficient mice. As a result, hepatic clearance of alpha 2-macroglobulin is impaired and remnant lipoproteins accumulate in the plasma of RAP-deficient mice that also lack functional LDL receptors. These results are consistent with the hypothesis that RAP stabilizes LRP within the secretory pathway. They also suggest a further mechanism by which the activity of an endocytic receptor may be modulated in vivo.

Role of cytoskeleton and acidification of endocytic compartment in asialoglycoprotein metabolism in isolated rat hepatocyte couplets

The process of receptor-mediated endocytosis is common to a variety of species and cell types. One of the best characterized receptor-ligand systems is the hepatocyte receptor for asialoglycoproteins. We investigated the morphological features of the uptake and intracellular transport of gold-conjugated asialofetuin in isolated rat hepatocyte couplets. We assessed the effects of colchicine, lumicolchicine, cytochalasin B, and chloroquine on the uptake and intracellular transport of asialoglycoproteins. Isolated rat hepatocyte couplets were incubated with gold-conjugated asialofetuin, and transmission electron micrographs of these cells were analyzed to determine the density and distribution of gold particles in the peripheral and pericanalicular areas. Results were analyzed morphometrically. Colchicine significantly inhibited the uptake and intracellular transport of asialoglycoproteins, but did not affect membrane fusion of endocytic compartments in the peripheral area. Lumicolchicine and cytochalasin B had minimal effects on these processes. Chloroquine inhibited the uptake of asialoglycoproteins, but did not affect the intracellular transport of asialoglycoproteins. Results suggest that the microtubule is essential for intracellular movement of endocytosed asialoglycoproteins and receptor recycling, and that endocytic structures in the peripheral regions can fuse in the absence of intact microtubules. We also found that uptake and intracellular transport of asialoglycoproteins were independent of the microfilaments, and the pH gradient in endocytic compartments was important in receptor-mediated endocytosis of asialoglycoproteins.

Expression and secretion of a recombinant ricin immunotoxin from murine myeloma cells

Expression plasmids carrying a humanized N901 immunoglobulin heavy chain gene (hN901HC) fused to a gene encoding the native B chain of ricin toxin (RTB), hN901HC-RTB, or a sugar binding mutant of RTB, hN901HC-RTB delta gly, were constructed. In each case, the fused gene constructions were co-expressed in murine myeloma cells (Sp2/0) with the gene for humanized N901 immunoglobulin light chain to produce the secreted recombinant products hN901-RTB and hN901-RTB delta gly, respectively. When purified by affinity chromatography, both the hN901-RTB and hN901-RTB delta gly products were found to have an apparent molecular mass of M(r) = 210,000 and to be composed of two hN901 antibody heavy chains each fused to a full-length copy of RTB and two hN901 antibody light chains. In each of the recombinant fusions the hN901 antibody moiety retained the full binding affinity and specificity for its cognate antigen, CD56. Moreover, when mixtures of hN901-RTB and native ricin A chain were incubated in the presence of the antigen-positive target cell line SW-2, antigen-specific potentiation of ricin A chain cytotoxicity was observed. It has been demonstrated previously that lectin activity of the B chain is essential for A chain cytotoxicity, and we conclude that the fused wild-type B chain was properly folded and maintained lectin activity. These data demonstrate that feasibility of using recombinant ricin B chain in an immunotoxin and of using mammalian cell culture for its expression. The use of recombinant hN901-RTB fusion protein to evaluate the contribution of the lectin activity of ricin B chain in the penetration of cell membranes by ricin A chain is proposed.

Synthesis of cluster galactosides with high affinity for the hepatic asialoglycoprotein receptor

High-affinity ligands for the asialoglycoprotein receptor, which is uniquely localized on the parenchymal liver cell and recognizes oligoantennary galactosides, might be utilized as homing device to specifically target drugs or genes to parenchymal liver cells. In the present study, the synthesis of galactose-terminated triantennary glycosides, provided with various spacers between the beta-galactopyranosyl moieties and the branching point of the dendrite, is described. N-[Tris[[(methylthio)methoxy]methyl]methyl]-N alpha-[1-(6- methyladipy)]glycinamide (3b) was glycosylated with monogalactosyl derivatives, containing propanediol or ethylene glycol units as hydrophilic spacer moieties, to yield the corresponding cluster galactosides. To determine the affinity of the cluster galactosides for the asialoglycoprotein receptor, we have performed competition studies of [125I]ASOR binding, a specific ligand for the asialoglycoprotein receptor, to isolated parenchymal cells. The affinity for the asialoglycoprotein receptor significantly increased with increasing spacer length. N-[[[Tris-O-(beta-D-galactopyranosyl)-3,6,9-trioxaunde- canoxy]methoxy]methyl]-N-alpha-[1-(6-methyladipyl)]glycinami de (4e), a cluster galactoside provided with a 20 A spacer, possessed an at least 2000-fold higher affinity for the receptor than N-[[tris-O-(beta-D-galactopyranosyl)methyl]methyl]-N alpha-[1-(6- methyladipyl)]glycinamide (4a), a cluster galactoside lacking the spacer. It is concluded that vicinal galactosyl moieties within a cluster galactoside are more optimal recognized by the galactose binding sites of the asialoglycoprotein receptor upon proper spacing. The most potent galactoside, TG(20A), may constitute an attractive targeting device for the specific delivery of drugs and/or genes to the parenchymal liver cell.

Enzymic transfer of 6-modified D-galactosyl residues: synthesis of biantennary penta- and hepta-saccharides having two 6-deoxy-D-galactose residues at the nonreducing end and evaluation of 6-deoxy-D-galactosyl transfer to glycoprotein using bovine beta-(1-->4)-galactosyltransferase and UDP-6-deoxy-D-galactose

UDP-6-Deoxy-D-galactose and UDP-6-deoxy-6-fluoro-D-galactose were synthesized and their transfer to 2-acetamido-2-deoxy-D-glucose (N-acetyl-D-glucosamine) by beta-(1-->4)-galactosyltransferase was examined. The transfer rates of 6-deoxy-D-galactose and 6-deoxy-6-fluoro-D-galactose were 1.3 and 0.2% of that of D-galactosyl transfer, respectively. The 2-acetamido-4-O-(6-deoxy-beta-D-galactopyranosyl)-2-deoxy-D-glucopyranose (6'-deoxy-N-acetyllactosamine) and methyl 2-acetamido-4-O-(6-deoxy-6-fluoro-beta-D-galactopyranosyl)-2-deoxy-D- glucopyranoside (6'-deoxy-6'-fluoro-N-acetyllactosamine) were synthesized enzymatically in 30 and 59% yields, respectively. Further, 6-deoxy-D-galactose could be completely transferred to N-linked type biantennary oligosaccharides having two N-acetyl-D-glucosaminyl residues at the nonreducing end to give the corresponding penta- and hepta-saccharides in 55 and 57% yields, respectively. An assay of 6-deoxy-D-galactosyl transfer using asialo agalacto alpha 1-acid glycoprotein as an acceptor suggested that 6-deoxy-D-galactose was transferred to about 30% of the N-acetyl-D-glucosaminyl residues in the N-linked oligosaccharides of the glycoprotein.

Effect of hyperosmolarity on both receptor-mediated and fluid-phase endocytosis in ethanol-fed animals

We have shown previously that chronic ethanol administration impairs hepatic receptor-mediated endocytosis (RME) of asialoorosomucoid (ASOR), epidermal growth factor and insulin, whereas early uptake by fluid-phase endocytosis (FPE) of a fluorescent dye, Lucifer Yellow (LY), is not altered. Results of these studies suggested that ethanol-induced injury was primarily affecting endocytosis in coated pit areas of the plasma membrane while internalization in noncoated membrane areas was unaffected. In the present study, we investigated the effects of blocking clathrin-coated pit mediated endocytosis by hyperosmolarity on FPE of LY and on RME of ASOR. We also examined the effects of hyperosmolarity on the binding and internalization of insulin, a ligand endocytosed by both RME and FPE. Uptake of LY by noncoated regions of the membrane was not altered in control animals, whereas in hepatocytes from ethanol-fed animals uptake of LY was decreased by 35-40% in the presence of 0.12 M sucrose (P < 0.05). These hyperosmolar conditions almost completely inhibited (> 85%) the endocytosis of 125I-ASOR by RME in both ethanol and control cells. Results with insulin showed slight effects (20-30% impairment) on uptake of the ligand in the presence of sucrose. These results are consistent with previous reports that in normal cells the coated pit pathway is impaired by hyperosmolarity, whereas endocytosis in noncoated regions is unaltered. It appears, however, that both FPE and RME in hepatocytes from ethanol-fed animals are susceptible to perturbation by hyperosmolarity. These results indicate that the noncoated pit pathway may be sensitive to stressful conditions such as hyperosmolarity after ethanol treatment.

Neisseria gonorrhoeae utilizes and enhances the biosynthesis of the asialoglycoprotein receptor expressed on the surface of the hepatic HepG2 cell line

One of the lipooligosaccharide (LOS) structures of Neisseria gonorrhoeae contains a terminal Gal(beta 1-4)GlcNAc residue which is a good candidate to serve as a ligand for human asialoglycoprotein receptors (ASGP-R). These receptors have been shown to be present on macrophages, sperm cells, and hepatocytes. The human tissue culture cell line used most often to study this receptor, HepG2, was used in our investigations only as a model. We also chose N. gonorrhoeae 1291 for these studies because, unlike many other gonococcal strains, this strain expresses one main species of LOS. The LOS structure expressed by this strain has also been fully characterized. Using well-established assays for the utilization of the ASGP-R, we found that incubation of HepG2 cells with gonococci expressing the terminal Gal(beta 1-4)GlcNAc asialo-LOS carbohydrate structure competitively inhibited the ASGP-R from binding to one of its well-known ligands, asialo-alpha-acid-1-glycoprotein. The inhibition was specific to the ASGP-R, since binding of two other ligands to their specific receptors in the same model cell system was not affected. Immunoblot analysis for the ASGP-R suggested that gonococci seemed to stimulate the HepG2 cells to increase the expression of the major (46-kDa) receptor species. This observation was confirmed both by functional analysis, which showed that the concentration of total receptor molecules, as well as surface receptors, was about 60% higher after incubation with gonococci than in control cells and by Northern (RNA) blot analysis using a cDNA probe of the major human H1 subunit. Poly(A) RNA purified from control and HepG2 cells exposed to gonococci indicated the presence of increased amounts of mRNA coding for the ASGP-R after incubation with gonococci. This result supports the idea that the molecular mechanism controlling the receptor level after gonococcal exposure is under transcriptional regulation.

Translocation of different forms of transferrin from blood to bile in the rat

Five different forms of transferrin (rat apo [iron-free], rat diferric, diferric rat asialo, human diferric, and diferric human asialotransferrin type 3) were used to monitor the passage of this protein and its metal to the bile. Cumulative biliary excretion of the dose over 3 hours was determined. In addition, an excretion profile was constructed from the concentration of tracer in bile samples collected over 10-minute intervals. The profile obtained with apotransferrin was very similar to that found in an earlier study with albumin, the implication being that the apo form is transferred passively (e.g., by diffusion). Behavior of rat diferric transferrin, however, was consistent with the assumption that this form is transferred both passively and actively (i.e., in vesicles). The three other transferrins were investigated with the intent of broadening the spectrum of ligand affinities for the plasmalemma of hepatocyte. The higher this attraction was, the larger fraction of the dose appeared in bile. When transferrin was targeted to lysosomes, the bile contained several intermediate proteolytic fragments. Double-labeled (125I, 59Fe) transferrin was used to measure recovery of iron (Fe) relative to the protein (P) in bile. With rat diferric transferrin, the Fe/P ratio was 0.72. Lower values were recorded with transferrins (human or asialo) that had higher affinities for the plasmalemma and therefore were expected to be transported to a larger extent in vesicles. Of the biliary 59Fe, 85% to 92% was protein bound. The proportion of the protein-bound fraction was essentially independent of the magnitude of Fe/P ratios.

Cloning, expression, and chromosome mapping of human galectin-7

The galectins are a family of beta-galactoside-binding proteins implicated in modulating cell-cell and cell-matrix interactions. Here we report the cloning and expression of a novel member of this family (galectin-7) that correspond to IEF (isoelectric focusing) 17 (12,700 Da; pI, 7.6) in the human keratinocyte protein data base, and that is strikingly down-regulated in SV40 transformed keratinocytes (K14). The cDNA was cloned from a lambda gt11 cDNA expression library using degenerated oligodeoxyribonucleotides back-translated from an IEF 17 peptide sequence. The protein encoded by the galectin-7 clone comigrated with IEF 17 as determined by two-dimensional (two-dimensional gel electrophoresis) analysis of proteins expressed by transiently transfected COS-1 cells, and bound lactose. Alignment of the amino acid sequences with other members of the family showed that the amino acids central to the beta-galactoside interaction are conserved. Galectin-7 was partially externalized to the medium by keratinocytes although it has no typical secretion signal peptide. Immunoblotting as well as immunofluorescence analysis of human tissues with a specific galectin-7 antibody revealed a narrow distribution of the protein which was found mainly in stratified squamous epithelium. The antigen localized to basal keratinocytes, although it was also found, albeit at lower levels, in the suprabasal layers where it concentrated to areas of cell to cell contact. Both, its cellular localization as well as its striking down-regulation in K14 keratinocytes imply a role in cell-cell and/or cell-matrix interactions necessary for normal growth control. The galectin-7 gene was mapped to chromosome 19.

Cholesterol derivative of a new triantennary cluster galactoside lowers serum cholesterol levels and enhances secretion of bile acids in the rat

BACKGROUND

Previous studies have demonstrated that cholesterol-derivatized galactosides exert a hypocholesterolemic effect by inducing hepatic uptake of atherogenic lipoproteins by means of galactose-recognizing receptors in the liver. However, a prolonged infusion of high concentrations of these compounds was required for this effect, possibly because of low affinity for the galactose-recognizing asialoglycoprotein receptor on the parenchymal liver cell.

METHODS AND RESULTS

We have designed a new series of triantennary galactosides to optimize the affinity and specificity for this receptor. The affinity of a triantennary galactoside for the asialoglycoprotein receptor appeared to be dramatically enhanced by proper spacing of the three terminal galactose groups. In rats, a single injection of N-[tris-O-(3,6,9-trioxaundecanyl-beta-D-galacto- pyranosyl)methoxymethyl]methyl-N alpha-[1-(6-(5-cholesten-3 beta- yloxy)glycyl)adipyl]glycinamide [TG(20A)C], the cholesterol derivative of the most selective galactoside, causes a dose-dependent decrease of < or = 45% in the serum cholesterol concentration (P < .001). This decrease is mainly attributed to a decrease in the level of serum HDL (P = .0066) and, to a lesser extent, serum LDL (P = .036). In addition, TG(20A)C strongly enhances the bile-acid secretion in rats during the first 2 hours after administration, which indicates that TG(20A)C-induced clearance of cholesterol from the bloodstream is efficiently coupled to hepatic bile-acid secretion.

CONCLUSIONS

We conclude that TG(20A)C efficiently directs lipoproteins that contain cholesterol to the liver at a 30-fold-lower concentration than previously developed cholesterol-derived cluster galactosides. This newly developed approach to lower cholesterol levels may prove valuable for familial hypercholesterolemic patients or those with familial defective apolipoprotein B-100 who do not respond or who respond insufficiently, respectively, to conventional therapies.

Conditional immortalization of Gunn rat hepatocytes: an ex vivo model for evaluating methods for bilirubin-UDP-glucuronosyltransferase gene transfer

Viral vectors and protein carriers utilizing asialoglycoprotein receptor (ASGR)-mediated endocytosis are being developed to transfer genes for the correction of bilirubin-UDP-glucuronosyltransferase (bilirubin-UGT) deficiency. Ex vivo evaluation of these gene transfer vectors would be facilitated by a cell system that lacks bilirubin-UGT, but expresses differentiated liver functions, including ASGR. We immortalized primary Gunn rat hepatocytes by transduction with a recombinant Moloney murine leukemia virus expressing a thermolabile mutant SV40 large T antigen (tsA58). At 33 degrees C, the immortalized hepatocyte clones expressed SV40 large T antigen, synthesized DNA, and doubled in number every 2 to 3 days. At this temperature, differentiated hepatocyte markers, e.g., albumin, ASGR, and androsterone-UGT, were expressed at 5% to 10% of the levels found in primary hepatocytes maintained in culture for 24 hours. Glutathione-S-transferase Yp (GST-Yp), an oncofetal protein, was expressed in these cells at 33 degrees C, but was undetectable in primary hepatocytes. In contrast, when the cells were cultured at 39 degrees C or 37 degrees C, the large T antigen was degraded, DNA synthesis and cell growth stopped, and morphologic characteristics of differentiated hepatocytes were observed. The expression of albumin, ASGR, and androsterone-UGT, and their corresponding mRNAs, increased to 25% to 40% of the level in primary hepatocytes, whereas GST-Yp expression decreased. Functionality of ASGR was demonstrated by internalization of Texas red-labeled asialoorosomucoid, and binding and degradation of 125I-asialoorosomucoid. After liposome-mediated transfer of a plasmid containing the coding region of human bilirubin-UGT1, driven by the SV40 large T promoter, active human bilirubin-UGT1 was expressed in these cells. The immortalized cells were not tumorigenic after transplantation into severe combined immunodeficiency mice. These conditionally immortalized cells will be useful for ex vivo evaluation of bilirubin-UGT gene transfer vectors.

Carbohydrate-dependent binding of the cell-free hemagglutinin of Vibrio cholerae to glycoprotein and glycolipid

The carbohydrate-binding specificity of the cell-free hemagglutinin (HA) of Vibrio cholerae (K.K. Banerjee, A.N. Ghose, K. Datta-Roy, S.C. Pal, and A.C. Ghose, Infect. Immun.58:3698-3705, 1990) was studied by using glycoconjugates with defined sugar sequences. The HA was not inhibited by simple sugars including glucobiose, galabiose, and their N-acetylated derivatives. The hemagglutination of rabbit erythrocytes by the HA was inhibited moderately by fetuin, calf thyroglobulin, and human alpha 1-acid glycoprotein, all of which contain multiple asparagine-linked complex-type oligosaccharide units alone or in combination with serine/threonine-linked oligosaccharide units. The inhibitory potencies of the glycoproteins increased approximately 10-fold following removal of the terminal sialic acid and were completely destroyed by exhausative proteolysis. The HA agglutinated phosphatidylcholine liposomes containing GM1-ganglioside or its asialo-derivative in the presence of Ca2+ ions. The association constants of the complexes of the HA with asialofetuin, asialothyroglobulin, GM1-ganglioside, and asialo-GM1-ganglioside were determined by an enzyme-linked immunosorbent assay-based assay and found to be 1.7 x 10(7) M-1, 1.5 x 10(7) M-1, 1.8 x 10(7) M-1, and 2.4 x 10(7) M-1, respectively. Studies using chemically modified glycoproteins and plant lectins with defined sugar specificity revealed that the HA recognized the terminal beta 1-galactosyl moiety of these glycoconjugates. There was no evidence for the presence of an extended carbohydrate-binding domain in the HA molecule or a preference of the HA for a complex, branched oligosaccharide structure. Similar to the mechanisms proposed for the binding of cholera toxin and Shiga toxin to glycolipids and neoglycoproteins, the strong interaction of V. cholerae cell-free HA with glycoconjugates appeared to be a consequence of multiple weak binding to terminal beta1-galactosyl moieties of the glycoproteins or glycolipids.

Detection and quantification of soluble asialoglycoprotein receptor in human serum

We describe the first evidence that soluble asialoglycoprotein receptors (AGPR) are present in human serum and that they are quantifiable by an enzyme-linked immunosorbent assay (ELISA). An affinity chromatography gel immobilized with monoclonal antibodies (McAbs) against human liver AGPR was mixed with normal sera, and the bound fraction was analyzed both by sodium dodecyl sulfate polyacrylamide gel electrophoresis and by Western blot analysis. Immunoreactive bands corresponding to 35 to 40 kd were obtained, which were lower than those of liver AGPR (41 kd and 46 kd). Soluble AGPR in human serum was able to bind to D-galactose-immobilized beads, indicating that the soluble AGPR remained ligand-binding activity. In order to quantify soluble AGPR, we established an ELISA using a monoclonal antibody (30220 McAb)-immobilized microplate and horseradish peroxidase-labeled F(ab')2 of another monoclonal antibody (30201 McAb). Reproducibility of intra- and interassay of the ELISA were 4% to 14% and 7% to 14%, respectively. Analytical recoveries ranged from 93% to 99%. The detection limit was estimated to be 0.1 micrograms/L. By nonparametolic analysis, a median and a 90% tile of serum AGPR level obtained from 283 normal volunteers were 0.4 micrograms/L and 2.4 micrograms/L, respectively.

Role of clathrin-coated vesicles in glycoprotein transport from the cell surface to the Golgi complex

Plasma membrane glycoproteins recycle to the Golgi complex, but the route followed by these proteins is not known. To elucidate the pathway of transport, the involvement of clathrin-coated vesicles was tested. This was accomplished by comparing the traffic of wild type low density lipoprotein receptor (LDLR) and FH 683, a mutant receptor whose endocytosis from the cell surface in coated vesicles is reduced by 90-95%. Wild type LDLR traveled from the cell surface to the sialyltransferase compartment of the Golgi with a half-time of 2.5 h in K562 human leukemia cells expressing receptor from a transfected cDNA. In contrast, FH 683 LDLR recycled to the Golgi at 33% of the wild type rate, suggesting that wild type LDLR is largely transported to the Golgi by a pathway that involves clathrin-coated vesicles. Moreover, because clathrin-coated vesicles that bud from the plasma membrane are transported to endosomes, surface-to-Golgi transport probably involves an endosomal intermediate. Finally, because there was substantial transport of mutant LDLR to the Golgi even though its endocytosis in coated vesicles was greatly reduced, there may be a second pathway of surface-to-Golgi traffic. Our results suggest that wild type LDLR may move from plasma membrane to Golgi by two routes. Two-thirds of the traffic proceeds via a coated vesicle-mediated pathway while the remainder may follow a clathrin-independent pathway.

Receptor-mediated regulation of differentiation and proliferation of hepatocytes by synthetic polymer model of asialoglycoprotein

Morphology and responses of hepatocytes are investigated using an artificial asialoglycoprotein model polymer--lactose-carrying polystyrene (PVLA) as a culture substratum, especially in focusing on the effect of the surface density of the PVLA substratum. The surface density of PVLA on polystyrene dishes was determined using fluorescein-labeled PVLA as a probe under a fluorescence laser microscope. PVLA-coated surfaces were observed by scanning electron microscope and atomic force microscopies under air and water, which showed that PVLA molecules were adsorbed patchily on low density surfaces and uniformly concentrated all over the dish on high density surfaces. It is suggested from the requirement of the Ca2+ ion, inhibition of galactosyl substances, and localization of receptors that the adhesion of hepatocytes to both low and high PVLA-density surfaces is mediated by galactose-specific interactions between PVLA and asialoglycoprotein receptors. At low PVLA densities (0.07 micrograms cm-2), the hepatocytes were flat and expressed high levels of 3H-thymidine uptake and low levels of bile acid secretion. Contrastingly, at high PVLA densities (1.08 micrograms cm-2), they were round and expressed a low level of 3H-thymidine uptake and a high level of bile acid secretion. The shapes, proliferation, and differentiation of hepatocytes could be regulated by varying the densities of PVLA adsorbed to polystyrene dishes. We assume that there are two recognition mechanisms operating between PVLA and hepatocytes: (1) adhesion through highly concentrated or clustered galactose-specific interaction; and (2) responses in shape, proliferation, and differentiation by PVLA-coating densities.

An organotypical in vitro model of the liver parenchyma for uptake studies of diagnostic MR receptor agents

Testing of receptor-specific MR contrast agents targeted to the liver is hampered by a shortage of viable in vitro models with in vivo-like hepatocellular morphology. Coated pits are ultrastructural signs of an active receptor mediated endocytosis in hepatocytes. Expression of coated pits by matrix overlaid hepatocytes was studied by transmission electron microscopy. Binding of a rhodaminated asialoglycoprotein receptor agent (MION-ASF-rh) was assessed by fluorescence microscopy. Fluorescence of cells exposed to MION-ASF-rh with D(+)-galactose reduced fluorescent light emission to a level of 58% of MION-ASF-rh-induced fluorescence. After preincubation with the hepatotoxin CCl4 a dose-dependent decrease in fluorescent light emission resulted. Hepatocytes maintained a homogeneous cell surface expression, with microprojections, coated pits, and vesicles on both sinusoidal surfaces. Matrix overlaid primary hepatocytes constitute a viable, morphologically and functionally differentiated model. This model can be used to study receptor binding, uptake, and blockage of diagnostic magnetopharmaceuticals under controlled conditions.