Thyroglobulin binding and TSH regulation of the RHL-1 subunit of the asialoglycoprotein receptor in rat thyroid

The ability of asialo-thyroglobulin to bind the thyroid RHL-1 subunit of the asialoglycoprotein receptor has been investigated. Ligand blot assays show that the recombinant carbohydrate recognition domain of the thyroid RHL-1 subunit specifically interacts with rat desialated thyroglobulin. Moreover, RT-PCR and Western blot assays show that TSH deprivation decreases RHL-1 expression in PC C13 thyroid differentiated cells whereas insulin deprivation does not have any effect. The simultaneous absence of both TSH and insulin dramatically decreases the level of RHL-1 expression.

Nucleotide exchange in genomic DNA of rat hepatocytes using RNA/DNA oligonucleotides. Targeted delivery of liposomes and polyethyleneimine to the asialoglycoprotein receptor

Chimeric RNA/DNA oligonucleotides have been shown to promote single nucleotide exchange in genomic DNA. A chimeric molecule was designed to introduce an A to C nucleotide conversion at the Ser365 position of the rat factor IX gene. The oligonucleotides were encapsulated in positive, neutral, and negatively charged liposomes containing galactocerebroside or complexed with lactosylated polyethyleneimine. The formulations were evaluated for stability and efficiency in targeting hepatocytes via the asialoglycoprotein receptor. Physical characterization and electron microscopy revealed that the oligonucleotides were efficiently encapsulated within the liposomes, with the positive and negative formulations remaining stable for at least 1 month. Transfection efficiencies in isolated rat hepatocytes approached 100% with each of the formulations. However, the negative liposomes and 25-kDa lactosylated polyethyleneimine provided the most intense nuclear fluorescence with the fluorescein-labeled oligonucleotides. The lactosylated polyethyleneimine and the three different liposomal formulations resulted in A to C conversion efficiencies of 19-24%. In addition, lactosylated polyethyleneimine was also highly effective in transfecting plasmid DNA into isolated hepatocytes. The results suggest that both the liposomal and polyethyleneimine formulations are simple to prepare and stable and give reliable, reproducible results. They provide efficient delivery systems to hepatocytes for the introduction or repair of genetic mutations by the chimeric RNA/DNA oligonucleotides.

A novel immunization method to induce cytotoxic T-lymphocyte responses (CTL) against plasmid-encoded herpes simplex virus type-1 glycoprotein D

DNA molecules complexed with an asialoglycoprotein-polycation conjugate, consisting of asialoorosomucoid (ASOR) coupled to poly-L-lysine, can enter hepatocytes which bear receptors for ASOR. We used this receptor-mediated DNA delivery system to deliver plasmid DNA encoding glycoprotein D (gD) of herpes simplex virus type 1 to ASOR-positive cells. Maximum expression of gD protein was seen at 3 days after injection of this preparation in approximately 13% of cells from BALB/c mice [hepatocytes from mice injected intravenously (i.v.) or peritoneal exudate cells from mice injected intraperitoneally (i.p.)]. In comparison with mice injected with either the plasmid vector alone or the gD-containing plasmid uncomplexed to ASOR, mice immunized with gD-containing plasmid complexed with ASOR-poly-L-lysine induced marked antigen-specific CTL responses. BALB/c mice immunized with gD-DNA developed a T-cell-mediated CTL response against target cells expressing gD and MHC class II glycoproteins, but not against cells expressing only gD and MHC class I molecules. In C3H mice, gD-DNA induced a T-cell-mediated CTL response against target cells expressing gD and class I MHC molecules. Serum anti-gD antibody in low titers were produced in both strains of mice. DNA complexed with ASOR-poly-L-lysine induced CTL responses in mice.

Design and synthesis of novel amphiphilic dendritic galactosides for selective targeting of liposomes to the hepatic asialoglycoprotein receptor

A series of glycolipids have been prepared which contain a cluster galactoside moiety with high affinity for the hepatic asialoglycoprotein receptor and a bile acid ester moiety which mediates stable incorporation into liposomes. Loading of liposomes with these glycolipids at a ratio of 5% (w/w) resulted in efficient recognition and uptake of the liposomes by the liver. Preinjection with asialofetuin almost completely inhibited the uptake, establishing that the liposomes were selectively recognized and processed by the asialoglycoprotein receptor on liver parenchymal cells. In contrast, a glycolipid content of 50% (w/w) led to a liver uptake that could not be inhibited by preinjection with asialofetuin, indicating that the liposomes were now processed by the Gal/Fuc-recognizing receptor on liver macrophages. The results presented in this study are important for future targeting of water-soluble and amphiphilic drugs, enveloped in these glycolipid-laden liposomes, to parenchymal liver cells.

Expression of human alpha-l-fucosyltransferase gene homologs in monkey kidney COS cells and modification of potential fucosyltransferase acceptor substrates by an endogenous glycosidase

Previous investigations on the monkey kidney COS cell line demonstrated the weak expression of fucosylated cell surface antigens and presence of endogenous fucosyltransferase activities in cell extracts. RT-PCR analyses have now revealed expression of five homologs of human fucosyltransferase genes, FUT1, FUT4, FUT5, FUT7, and FUT8, in COS cell mRNA. The enzyme in COS cell extracts acting on unsialylated Type 2 structures is closely similar in its properties to the alpha1,3-fucosyltransferase encoded by human FUT4 gene and does not resemble the product of the FUT5 gene. Although FUT1 is expressed in the COS cell mRNA, it has not been possible to demonstrate alpha1,2-fucosyltransferase activity in cell extracts but the presence of Le(y) and blood-group A antigenic determinants on the cell surface imply the formation of H-precursor structures at some stage. The most strongly expressed fucosyltransferase in the COS cells is the alpha1,6-enzyme transferring fucose to the innermost N -acetylglucosamine unit in N -glycan chains; this enzyme is similar in its properties to the product of the human FUT8 gene. The enzymes resembling the human FUT4 and FUT8 gene products both had pH optima of 7.0 and were resistant to 10 mM NEM. The incorporation of fucose into asialo-fetuin was optimal at 5.5 and was inhibited by 10 mM NEM. This result initially suggested the presence of a third fucosyltransferase expressed in the COS cells but we have now shown that triantennary N- glycans with terminal nonreducing galactose units, similar to those present in asialo-fetuin, are modified by a weak endogenous beta-galactosidase in the COS cell extracts and thereby rendered suitable substrates for the alpha1,6-fucosyltransferase.

Comparative affinity of synthetic multi-antennary galactosyl derivatives for the Gal/GalNAc receptor of rat hepatocytes and peritoneal macrophages

The binding affinity of synthetic bi- and triantennary galactose ligands (Kichler, A. and Schuber, F. (1995) Glycoconj. Chem., 12, 275 281) has been determined for the Gal/GalNAc receptors of rat hepatocytes and macrophages. The highest affinities were observed with the triantennary structures, in agreement with the clustering effect known to occur with more complex oligosaccharide structures. However, these ligands present very similar affinities for the receptors of both cell types and thus lack the necessary selectivity for specific hepatocyte targeting.

Perioperative changes in hepatic function as assessed by asialoglycoprotein receptor indices by technetium 99m galactosyl human serum albumin

BACKGROUND/AIMS

Perioperative changes in intrinsic hepatocyte function and functional volume were investigated.

METHODOLOGY

52 cases of liver cancer, including hepatocellular carcinoma, cholangiocellular carcinoma and metastatic carcinoma were studied. There were diagnoses of liver cirrhosis, liver fibrosis and normal liver in 20, 23 and 9 cases, respectively. Hepatic resection of subsegment, one segment, two segments and three segments were performed in 11, 6, 23 and 4 cases, respectively, while 8 cases were diagnosed as inoperable. Assessments were performed before and 4 weeks after hepatectomy with asialoglycoprotein (ASGP) receptor indices, i.e., technetium-99m-galactosyl human serum albumin (TcGSA) uptake by the liver, TcGSA retention in the blood and functional volume as measured by single emission computed tomography (SPECT) with TcGSA as a probe.

RESULTS

The hyperbolic relationship between galactosyl human serum albumin (GSA) uptake by the liver and GSA retention in the blood, both of which were independent of functional volume, shifted to the right after hepatectomy. The number of conventional liver function tests correlating to the GSA uptake increased after hepatectomy. By contrast, none of the pre-operative and post-operative tests were correlated with functional volume. Post-operative decrease in intrinsic hepatocyte function can be minimized by selection of mode of hepatic resection.

CONCLUSIONS

These results indicate that GSA uptake by the liver can reveal information regarding intrinsic liver function which deteriorates and becomes a decisive factor after hepatectomy.

Recovery of hepatic asialoglycoprotein receptors after major hepatic resection

Although morphological restoration of the hepatic mass after partial hepatectomy has been well studied, fewer reports have appeared on the change of functional hepatic capacity during liver regeneration. Asialoglycoprotein receptor (ASGP-R) is a hepatic cell surface receptor specific for galactose-terminated glycoprotein. Kinetic modeling of 99mTc-labeled diethylenetriamine pentaacetic acid-galactosyl-human serum albumin (TcGSA) time-activity data yields estimates of ASGP-R concentration [R]o and amount R0, which are directly related to functional liver mass. We have investigated the changes in ASGP-R status as well as liver volume in regenerating human liver after major hepatic resection.

METHODS

Twenty-two patients (18 noncirrhotic, 4 cirrhotic) had a TcGSA study before and 3 wk after major hepatic resection, with a mean hepatic parenchymal resection rate of 36.0%.

RESULTS

[R]0 was significantly decreased from 0.683+/-0.024 micromol/L to 0.565+/-0.032 micromol/L (P < 0.001) after resection. The decrease in [R]0 was more prominent in cirrhotic patients. Recovery of ASGP-R was observed as a significantly increased R0 3 wk after the operation. Subsequent (long-term) restoration of ASGP-R appeared to be slower when compared with the volume restoration.

CONCLUSION

ASGP-R concentration of the liver significantly decreased after major hepatic resection. Subsequent recovery of ASGP-R amount was shown by TcGSA study. By estimating hepatic functional reserve expressed by ASGP-R amount and concentration, one may detect a delayed or impaired liver regeneration with higher sensitivity.

Characterization of a UDP-Gal:Galbeta1-3GalNAc alpha1, 4-galactosyltransferase activity in a Mamestra brassicae cell line

The binding of Bandeiraea simplicifolia lectin-I isolectin B4 on the endogenous glycoproteins of different insect cell lines led us to characterize for the first time a UDP-Gal:Galbeta1-3GalNAc alpha1, 4-galactosyltransferase in a Mamestra brassicae cell line (Mb). The study of the acceptor specificity indicated that the Mb alpha-galactosyltransferase prefers Galbeta1-3-R as acceptor, and among such glycans, the relative substrate activity Vmax/Km was equal to 20 microliters.mg-1.h-1 for Galbetal-3GlcNAcbeta1-O-octyl and to 330 microliters.mg-1.h-1 for Galbeta1-3GalNAcalpha-1-O-benzyl, showing clearly that Galbeta1-3GalNAc disaccharide was the more suitable acceptor substrate for Mb alpha-galactosyltransferase activity. Nuclear magnetic resonance and mass spectrometry data allowed us to establish that the Mb alpha-galactosyltransferase synthesizes one unique product, Galalpha1-4Galbeta1-3GalNAcalpha1-O-benzyl. The Galbeta1-3GalNAc disaccharide is usually present on O-glycosylation sites of numerous asialoglycoproteins and at the nonreducing end of some glycolipids. We observed that Mb alpha1,4-galactosyltransferase catalyzed the transfer of galactose onto both natural acceptors. Finally, we demonstrated that the trisaccharide Galalpha1-4Galbeta1-3GalNAcalpha1-O-benzyl was able to inhibit anti-PK monoclonal antibody-mediated hemagglutination of human blood group PK1 and PK2 erythrocytes.

Role of insulin and nutritional factors in intestinal glycoprotein fucosylation during postnatal development

This study deals with the role of insulin in the regulation of the intestinal glycoprotein fucosylation process during postnatal development in the rat. Circulating insulin level was found to increase at weaning time in parallel with alpha-1, 2-fucosyltransferase activity and with the appearance of alpha-1, 2-fucoproteins in brush-border membranes. Insulin treatment of young suckling rats induced a precocious increase in fucosyltransferase activity and in the biosynthesis of its substrate (GDP-fucose), but the sensitivity to insulin disappeared after weaning. The insulin level was lower in 22-day-old rats that received prolonged nursing (on a high-fat diet) compared with age-matched normally weaned rats (on a high-carbohydrate diet), whereas the appearance of alpha-1, 2-fucoproteins and the increase in activity of alpha-1, 2-fucosyltransferase were delayed, as was the decrease in the degradation of GDP-fucose. In 22-day-old animals that received prolonged nursing and insulin treatment, the alpha-1, 2-fucosyltransferase activity reached a level close to that observed in age-matched weaned rats, and several alpha-1,2-fucoproteins appeared in brush-border membranes with a molecular mass similar to that found in weaned rats. These results suggest that changes in insulin levels at weaning time (as caused, in the present case, by dietary modifications) may be responsible for the regulation of the glycoprotein fucosylation process, essentially by increasing fucosyltransferase activity.

Identification and analysis of a CA(2+)-dependent lactoferrin receptor in rat liver. Lactoferrin binds to the asialoglycoprotein receptor in a galactose-independent manner

We identified a 45 kDa Ca(2+)-dependent Lf binding protein on rat hepatocytes. Dithiobis(sulfosuccimidylproprionate) (DTSSP)-crosslinked 125I-Lf to a 45 kDa adduct in a Ca(2+)-dependent manner on intact cells. The 125I-labeled crosslinked complexes were absent when either surface-bound 125I-Lf was stripped prior to crosslinking or an excess of unlabeled Lf was included in the DTSSP reaction. Triton X-100 extracts of hepatocyte membrane ghosts were chromatographed on Lf-agarose, and a 45 kDa polypeptide (p45) was eluted by EGTA. Anti-p45 sera blocked vigorously 125I-Lf endocytosis to intact rat hepatocytes, confirming that p45 functions as the Ca(2+)-dependent Lf receptor on hepatocytes. Two tryptic fragments of p45 showed 100% identity with internal sequences (Leu121-->Lys126 and Phe198-->Lys220) of the major subunit (RHL-1) of the rat asialoglycoprotein receptor. Antisera against p45 and RHL-1 crossreacted equally well with each protein, and asialoorosomucoid blocked the binding of 125I-Lf to hepatocytes. We did not detect the minor subunits (RHL-2/3) of the rat asialoglycoprotein receptor in p45 preparations from Triton X-100-extracts of hepatocytes, and 125I-Lf bound to immobilized RHL-1 but not to RHL-2/3. Exoglycosidases were used to remove terminally-exposed NeuNAc and alpha- and beta-Gal from bovine Lf glycans, and lectin blotting confirmed that glycosidase-treated Lfs lacked detectable terminal Gal. Unexpectedly, deglycosylated Lf exhibited no loss in its ability to compete with unmodified Lf for binding to isolated hepatocytes. Moreover, beta-lactose but not sucrose competed vigorously for 125I-Lf endocytosis by hepatocytes, indicating that Lf binds at or near the carbohydrate-recognition domain of RHL-1. We conclude that RHL-1 is the Ca(2+)-dependent Lf receptor on hepatocytes and that it binds Lf in a Gal-independent manner.

Effect of galactose on binding and endocytosis of asialoglycoprotein in cultured rat hepatocytes

BACKGROUND

99mTc-diethylenetriaminepentaacetic acid-galactosyl-human serum albumin (99mTc-GSA) has been applied clinically in scintigraphy to estimate functioning liver mass, but it is not so sensitive in differentiating mild liver injury from normal liver. 99mTc-GSA is thought to bind to the asialoglycoprotein receptor (ASGP-R) and is then internalized and degraded in the hepatocytes. The aim of this study is to know whether D-galactose inhibits GSA binding or internalization to hepatocyes because ASGP-R recognizes galactose residues of ligands.

METHODS

Isolated rat hepatocytes were obtained by collagenase perfusion, pre-cultured for 2 h after plating, and then cultured for 16 to 18 h until use. The effect of galactose on GSA binding and internalization into cells was investigated by using cultured hepatocytes.

RESULTS

Galactose non-competitively inhibited GSA binding to cultured hepatocytes, but its Ki value was quite high (505 +/- 38 mM). Galactose significantly inhibited GSA internalization into hepatocytes at 27 mM.

CONCLUSION

It was clarified that D-galactose inhibited GSA internalization rather than binding at a low concentration. Further in vivo studies in rats are needed to know whether an administration of galactose prior to performing 99mTc-GSA scintigraphy can brake it possible to estimate the functioning mass in mild liver injury.

Ligand-induced endocytosis of the asialoglycoprotein receptor: evidence for heterogeneity in subunit oligomerization

The hepatic asialoglycoprotein receptor, a noncovalent hetero-oligomer of two subunits, is a constitutively cycling endocytic receptor. However, the ligand asialoorosomucoid caused downregulation of up to 40% of surface binding sites and a twofold increase in internalization rate. This was not the result of receptor crosslinking, since monovalent ligands had the same effect. Ligand binding thus appears to transmit a signal to the cytosolic portion of the receptor not unlike in signaling receptors. The two subunits were endocytosed at different average rates lower than that of ligand, indicating heterogeneity in oligomer formation and potentially in ligand specificity.

Endocytosed ricin and asialoorosomucoid follow different intracellular pathways in hepatocytes

Earlier studies have suggested that fluid phase endocytosis in rat hepatocytes takes place via a clathrin-independent mechanism [1,2]. This observation suggests that a relatively large amount of plasma membrane outside coated pits may be involved in hepatic endocytosis. Ricin, which binds to galactose residues on glycoproteins and glycolipids, has, in this report, been used as a general marker for the plasma membrane of hepatocytes. The endocytosis of ricin was compared with that of asialoorosomucoid (AOM) which is taken up exclusively via clathrin-coated pits. Hypertonic medium has been shown to inhibit uptake via coated pits more effectively than clathrin-independent uptake [3-5]. It was found, in this study, that the addition of 100 mM sucrose to the incubation medium inhibited the uptake of 125I-tyramine-cellobiose-asialoorosomucoid (125I-TC-AOM) more extensively than that of 125I-tyramine-cellobiose-ricin (125I-TC-ricin), compatible with the notion that the two probes are internalised via different mechanisms. Subcellular fractionation experiments indicated that 125I-TC-ricin entered a denser endocytic organelle than that receiving 125I-TC-AOM. To determine whether the separation of the two probes was due to a different transport kinetics (i.e. that 125I-TC-ricin is transported more rapidly to a later, denser compartment than 125I-TC-AOM) the cells were incubated at 18 degreesC to allow a slower internalisation/transport of the labelled probes. The results obtained showed, again, that the early endosomes containing 125I-TC-ricin were significantly denser than those containing 125I-TC-AOM. We also employed the horseradish peroxidase (HRP)-diaminobenzidine (DAB) density shift technique of Courtoy et al. [6] to determine whether 125I-TC-ricin and 125I-TC-AOM were in separate endosomes early after their uptake. The results showed that early endosomes containing 125I-TC-AOM were density shifted whereas those containing 125I-TC-ricin were unaffected by the density shift procedure. The use of probes labelled with 125I-TC allowed us to identify compartments involved in the degradation of 125I-TC-AOM and 125I-TC-ricin, by measuring acid soluble radioactivities in the gradient fractions. It was found that 125I-TC-ricin was degraded mainly in endosomes, whereas 125I-TC-AOM, as expected, was degraded mainly in lysosomes.

Modulation of biochemical activity of hepatocytes in culture by matrix substratum

The influence of extra cellular matrix on the biochemical activity of hepatocytes was studied by maintaining rat hepatocytes in primary culture in a serum free medium on different matrix protein substrata or biomatrices prepared from liver, aorta or mammary gland. There was significant difference in the individual protein synthesis and distribution by cells maintained on different substrata. Comparison of the kinetics of synthesis and secretion of albumin by cells maintained on different tissue biomatrix showed that those maintained on hepatic biomatrix synthesized more albumin and retained more of albumin synthetic capacity, when compared to those maintained on aortic and mammary gland biomatrix. Similarly, hepatocytes maintained on hepatic biomatrix synthesized significantly more apo B, the major apo protein of VLDL, than those maintained on heterologous tissue matrix. Induction of tyrosine aminotransferase by dexamethasone and the uptake of [14C]-amino isobutyric acid were found to be maximum in cells maintained on liver biomatrix than the heterologous biomatrix. But cells maintained on hepatic biomatrix incorporated less amounts of radioactivity into total cytoskeletal proteins as well as the individual proteins such as actin and the cytokeratins C8 and C18 while that by cells maintained on aortic biomatrix was significantly high. Quantitative analysis of the relative incorporation of radioactivity into individual cytoskeletal proteins and albumin in pulse labelling studies with cells maintained in culture on different matrix for different lengths of time revealed a reciprocal relationship between these two activities. These results indicate that the substrata with which the cells are in contact influence on a selective basis, the biochemical activity of hepatocytes in primary culture.

Circulating cellular fibronectin may be a natural ligand for the hepatic asialoglycoprotein receptor: possible pathway for fibronectin deposition and turnover in the rat liver

It has been postulated that the in vivo removal of many plasma glycoproteins after desialylation is mediated by their interaction with a specific endocytic receptor on hepatocytes called the asialoglycoprotein receptor (ASGP-R), which is known to have a high affinity for specific carbohydrate residues, such as galactose. However, this mechanism has never been proven in vivo, nor has a naturally occurring ligand for the ASGP-R been identified. We investigated the influence of the terminal galactose residues on plasma fibronectin (pFn) on its liver deposition and turnover in adult rats, using neuraminidase to remove sialic acid residues to expose galactose residues. We also tested the hypothesis that the normal presence of a large amount of terminal galactose residues in cellular Fn (cFn) may allow cFn to serve as a natural ligand readily able to interact with the ASGP-R. In contrast to the slow clearance of normal pFn from the blood, cFn and desialylated pFn (aFn) displayed a rapid plasma clearance (P < .001) with greater than 50% of both the 125I-cFn or 125I-aFn depositing in the liver within 15 minutes. The enhanced plasma removal and liver deposition of both 125I-cFn and 125I-aFn was competitively inhibited (P < .01) by prior intravenous infusion of excess asialofetuin, which can selectively bind to the ASGP-R. The enzymatic addition of terminal sialic acid residues onto cFn to "mask" or "cap" the normally exposed galactose residues delayed the rapid plasma removal of cFn. Accelerated degradation of 125I-aFn and 125I-cFn as compared with 125I-pFn was demonstrated in vitro by both primary cultures of normal rat hepatocytes or incubated (37 degrees C) tissue slices of livers harvested from normal rats after in vivo preloading with tracer 125I-Fn forms. Thus, the ASGP-R appears to directly participate in the rapid in vivo removal of cFn from the blood, while native pFn may be removed by an alternative pathway unless it can become desialylated in vivo. These findings suggest that cFn may be a naturally occurring ligand that does not require desialylation before removal by the ASGP-R on hepatocytes.

Binding of synthetic sulfated ligands by human splenic galectin 1, a beta-galactoside-binding lectin

The carbohydrate-binding site of galectin 1, a vertebrate beta-galactoside-binding lectin, has a pronounced specificity for the betaGal(1-->3)- and betaGal(1-->4)GlcNAc sequences. The binding inhibition study reported herein was carried out to determine whether sulfation of saccharides would influence their binding by galectin 1. The presence of 6'-OSO3- on LacNAc greatly reduces the inhibitory potency relative to LacNAc. 3'-OSO3-LacNAc, 3'-OSO3-Galbeta(1-->3)GlcNAc(beta)1-OBzl and 3-OSO3-Galbeta1-OMe are more potent inhibitors than the non-sulfated parent compounds. Surprisingly, 2'-OSO3-LacNAc showed over 40 fold less inhibitory potency relative to LacNAc. Ovarian carcinoma A121 cells were shown to synthesize sulfated macromolecules that bind to galectin 1. Modulation in vivo of saccharide sulfation may lead to modulation of galectin 1 interaction with glycoconjugates; hence, sulfation could play a role in modulating lectin functions.

Novel galactose-binding proteins in Annelida. Characterization of 29-kDa tandem repeat-type lectins from the earthworm Lumbricus terrestris

Novel type lectins were found in the phylum Annelida, i.e. in the earthworm, tubifex, leech, and lugworm. The lectins (29-31 kDa) were extracted from the worms without the use of detergent and purified by affinity chromatography on asialofetuin-agarose. On the basis of the partial primary structures of the earthworm Lumbricus terrestris 29-kDa lectin (EW29), degenerate primers were synthesized for use in the reverse transcriptase-polymerase chain reaction. An amplified 155-base pair fragment was used to screen a cDNA library. Four types of full-length clones were obtained, all of which encoded 260 amino acids, but which were found to differ at 29 nucleotide positions. Since three of them resulted in non-silent substitutions, EW29 mRNA was considered to be a mixture of at least three distinct polynucleotides encoding the following proteins: Ala44-Gln197-Ile213 (clone 5), Gly44-Gln197-Val213 (clone 7), and Ala44-His197-Ile213 (clones 8 and 9; different at the nucleotide level, but encoding an identical polypeptide). Genomic polymerase chain reaction using DNA from a single worm revealed that the single worm already had four sets of cDNAs. The EW29 protein showed two features. First, the lectin was composed of two homologous domains (14,500 Da) showing 27% identity with each other. When each of the domains was separately expressed in Escherichia coli, the C-terminal domain was found to bind to asialofetuin-agarose as strongly as the whole protein, whereas the N-terminal domain did not bind and only retardation was observed. EW29 was found to exist as a monomer under non-denaturing conditions. It had significant hemagglutinating activity, which was inhibited by a wide range of galactose-containing saccharides. Second, EW29 contained multiple short conserved motifs, "Gly-X-X-X-Gln-X-Trp." Similar motifs have been found in many carbohydrate-recognizing proteins from an extensive variety of organisms, e.g. plant lectin ricin B-chain and Clostridium botulinum 33-kDa hemagglutinin. Therefore, these carbohydrate-recognition proteins appear to form a protein superfamily.

Iron loading of isolated rat hepatocytes inhibits asialoglycoprotein receptor dynamics and induces formation of rat hepatic lectin-1 [correction of leptin-1] (RHL-1) oligomers

The major subunit [rat hepatic lectin-1 (RHL-1)] of the asialoglycoprotein (ASGP) receptor mediates endocytosis of the iron-binding protein lactoferrin (Lf) by isolated rat hepatocytes, yet iron loading of cultured adult rat hepatocytes increases the binding and endocytosis of Lf while greatly inhibiting the uptake of desialylated ligand. In the present study, we determined whether the iron-induced Lf-binding site is RHL-1 and examined the nature of the iron-induced block in ASGP receptor endocytic function. Isolated rat hepatocytes increased their non-haem iron content from 70 to 470 p.p. b. following incubation with ferric ammonium citrate (<=100 microgram/ml). These conditions blocked internalization of 125I-asialo-orosomucoid (ASOR) by approximately 90% but increased 125I-Lf endocytosis by 40%. ASOR and anti-RHL-1 sera blocked the binding and endocytosis of 125I-Lf on control cells but not on iron-loaded cells, indicating that the iron-induced Lf-binding site on hepatocytes is not RHL-1. Iron-loading of hepatocytes in the presence or absence of excess ASOR did not significantly alter the number of active ASGP receptors on the cell surface. In contrast, iron-loading decreased the number of active intracellular receptors by 40% and blocked the uptake of 125I-ASOR prebound to the cells by approximately 80%. Under these conditions, we found an iron-dependent evolution of 88 and 140 kDa RHL-1-containing, beta-mercaptoethanol-sensitive multimers that constituted up to 34 and 23%, respectively, of total immunodetectable RHL-1. We propose that iron-induced formation of cystinyl-linked RHL-1-containing multimers inhibits ASGP receptor movement between cell surface and interior and disrupts acylation of intracellular receptors.

Asialoglycoprotein (ASGP) receptors induced by apoptosis. Their immunohistochemical localization in neonatal rat hepatocytes

Immunohistochemical detection of asialoglycoprotein (ASGP) receptors has been performed on neonatal rat hepatocytes in primary cultures, incubated with TGF beta, TNF alpha and retinoic acid. These substances account for the increase in the rate of apoptosis of the cell cultures and for the increase of the expression of ASGP receptors. Both modifications are higher at 24 hours of incubation.

Mutation of the sialyltransferase S-sialylmotif alters the kinetics of the donor and acceptor substrates

Protein sequence analysis of the cloned sialyltransferase gene family has revealed the presence of two conserved protein motifs in the middle of the lumenal catalytic domain, termed L-sialylmotif and S-sialylmotif. In our previous study (Datta, A. K., and Paulson, J. C. (1995) J. Biol. Chem. 270, 1497-1500) the larger L-sialylmotif of ST6Gal I was analyzed by site-directed mutagenesis, which provided evidence that it participates in the binding of the CMP-NeuAc, a common donor substrate for all the sialyltransferases. However, none of the mutants tested in this motif had any significant effect on their binding affinities toward the acceptor substrate asialo alpha1-acid glycoprotein. In this study, we have investigated the role of the S-sialylmotif of the same enzyme ST6Gal I. In total, nine mutants have been constructed by changing the conserved amino acids of this motif to mostly alanine by site-directed mutagenesis. Kinetic analysis for the mutants which retained sialyltransferase activity showed that the mutations in the S-sialylmotif caused a change of Km values for both the donor and the acceptor substrates. Our results indicated that this motif participates in the binding of both the substrates. A sequence homology search also supported this finding, which showed that the downstream amino acid sequence of the S-sialylmotif is conserved for each subgroup of this enzyme family, indicating its association with the acceptor substrate.

Mesangial cells possess an asialoglycoprotein receptor with affinity for human immunoglobulin A

Asialoglycoprotein receptor (ASGP-R), a hepatic lectin involved in the clearance of galactose-terminal glycoproteins, is also present in extrahepatic tissues, but its expression in renal cells is not well established. This study examines the presence of ASGP-R in cultured mesangial cells (MC), key cells involved in the removal of macromolecules deposited in the glomerulus. The binding of asialo-orosomucoid (ASOR) to rat MC was saturable and galactose-specific. In addition, MC internalized and degraded ASOR in a Ca(2+)-dependent manner. Parallel studies were performed in a homologous system (human MC), obtaining similar binding curve and competition with unlabeled ASOR and carbohydrates. The purified receptor from rat MC consisted of two proteins (41 and 55 kD) with similar size to the hepatic receptor. Both subunits were detected by mRNA expression analysis (ratio 2:1). Because the hepatic receptor presents avidity for the carbohydrates of IgA1, a protein deposited in the glomerulus of patients with IgA nephropathy, the interaction of IgA1 with the mesangial ASGP-R was explored. As for the interaction with ASOR, catabolism of IgA1 by rat and human MC was Ca(2+)-dependent and was reduced with galactose. In addition, the interaction of ASOR with rat MC was partially inhibited by incubation with IgA1 and its desialylated form, but not by IgA2, as demonstrated in binding experiments and in receptor purification. It is concluded that MC possess ASGP-R specific for galactose residues of several glycoproteins, including IgA1. These data could be important for a better understanding of the pathogenesis of IgA nephropathy.

Detection of the asialoglycoprotein receptor on cell lines of extrahepatic origin

The asialoglycoprotein receptor (ASGPR) is the first lectin discovered in mammals. Despite its significant biological role in binding and internalization of desialyated glycoproteins, at least in the human, little information is available regarding its tissue distribution outside of the liver. In the present study, antibodies were raised against the H1 major subunit of the human ASGPR using synthetic peptide antigens, and their binding specificity confirmed by enzyme linked immunosorbent assay. Cell surface analysis by fluorescence activated flow cytometry on various human tissue cell lines confirmed the liver parenchymal cells as the major expression site of ASGPR. Nonetheless, ASGPR was also detectable on some extrahepatic cells such as the Jurkat T-cell line. The determination of extrahepatic expression of ASGPR will have consequences in analyzing the biological role of this receptor complex as well as having implications in designing ASGPR mediated drug- or gene-delivery strategies.

Synthesis of neoglycoconjugates containing deaminated neuraminic acid (KDN) using rat liver alpha2,6-sialyltransferase

2-Keto-3-deoxy-D- glycero -D- galacto -nononic acid (KDN) was introduced into asialotransferrin and N -acetyllactosamine (LacNAc) from CMP-KDN by using rat liver Galbeta1-->4GlcNAc alpha2, 6-sialyltransferase to form KDN-transferrin and KDN-LacNAc. These structures contain terminal KDNalpha2-->6Gal-residues, a glycotope that has not yet been described in natural glycoconjugates. KDN was transferred to all four Gal residues in asialotransferrin by this enzyme. The incorporation efficiency of KDN from CMP-KDN into asialotransferrin was about half that of Neu5Ac from CMP-Neu5Ac, based on the V max/ K m values for these donor substrates, 0.0527 min-1and 0.119 min-1, respectively. The KDNalpha2-->6Gal linkage was resistant to exosialidase treatment, in contrast to the sensitivity of the Neu5Acalpha2-->6Gal linkage. Interestingly, Sambucus sieboldiana agglutinin (SSA) was shown to prefer KDN-transferrin to the corresponding Neu5Ac-transferrin, as estimated by slot-blot analysis. The use of an alpha2,6-sialyltransferase to synthesize neoglycoproteins containing KDN has not been previously reported. Their facile synthesis using CMP-KDN and sialyltransferases with different specificities offers new possibilities to study the function of neo-KDN-glycoconjugates, and to explore their use in glycotechnology.

Increased liver uptake of liposomes and improved targeting efficacy by labeling with asialofetuin in rodents

To improve liposome-directed therapy of liver disease and gene delivery, it would be beneficial to selectively target hepatocytes. For this purpose, conventional liposomes (CL) were labeled with asialofetuin (AF), an asialoglycoprotein. The biodistribution of AF-labeled liposomes (AF-L) in mice and their incorporation into rat hepatocytes, and their potential use in acute liver injury, were investigated. AF-L displayed a quicker plasma clearance than CL, and 25.4%, 2.7%, and 1.2% of the injected dose remained in the plasma versus 47.0%, 26.1%, and 9.5% of CL, respectively at 2, 4, and 20 hours after the injection. Total liver uptake of AF-L (73%+/-3.9%) was markedly higher (P < .005) than CL (16.5%+/-1.8%) 4 hours after the injection. Liposomal radioactivity (cpm/mg) was greatly enhanced in the liver (11-fold) during the first 4 hours after the administration of 14C-AF-L, and was much higher than in 14C-CL-injected mice (1.5-fold). In vitro incubation of isolated rat hepatocytes with 14C-AF-L or intravenous injection of 14C-AF-L in rats resulted in higher hepatocyte-bound radioactivity compared with 14C-CL (P < .01-.005). AF-L-associated 1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) fluorescent signals were not only located in Kupffer cells, but also in hepatocytes, in which bile canaliculus networks were imaged. Intravenous administration of vitamin E (VE)-associated CL (VE-CL, 1 mg/mouse) significantly lowered alanine transaminase (ALT) levels in CCl4-treated mice (196+/-79 vs. 2,107+/-235 U/mL; P < .01). The ALT level in CCl4 + VE-AF-L group was decreased to 38+/-16 units/mL, which was significantly lower than the CC14 + VE-CL group (P < .05). In conclusion, labeling liposomes with AF led to a shortened liposome plasma half-life and greatly enhanced uptake of AF-L liposome by the liver. The enhanced uptake resulted from an increased incorporation of hepatocytes with AF-L liposomes. VE-associated AF liposomes further improved the protective effect of VE liposomes on CC14-induced acute liver injury in mice. Preferential hepatocyte incorporation of AF-L liposomes suggests a useful hepatocyte-targeting approach for drug delivery and gene transfection.