A novel human serum lectin with collagen- and fibrinogen-like domains that functions as an opsonin

Collectins are C-type animal lectins with both collagenous and carbohydrate recognition domains and are involved in the first line host defense against pathogens. We report here a novel Ca(2+)-dependent and GlcNAc-binding lectin consisting of subunits of 35 kDa (P35) with a collagen-like sequence. When P35 is isolated from human serum, it forms a homopolymer by means of intermolecular disulfide bonding, as is the case with collectins. P35 cDNA was cloned from a human liver cDNA library, and the deduced amino acid sequence of 313 residues revealed that the mature form of P35 consists mainly of collagen- and fibrinogen-like domains. The latter contained two potential Ca(2+)-binding sites that may be involved in carbohydrate binding. The overall sequence of P35 was highly homologous to porcine ficolins alpha and beta. Northern blots of various human tissues showed that the major product of the 1.3-kilobase-long P35 transcript is expressed in liver. P35 enhanced phagocytosis of Salmonella typhimurium by neutrophils, suggesting an opsonic effect via the collagen region. P35 was found to bind to GlcNAc-conjugated bovine serum albumin, a neoglycoprotein, as well as to neoglycolipids containing complex-type oligosaccharides derived from glycoproteins, suggesting that P35 recognizes GlcNAc residues such as those found in microbial glycoconjugates and complex-type oligosaccharides. Therefore, P35 represents a new type of GlcNAc-binding lectin with structural and functional similarities to collectins involved in innate immunity.

Decreased binding of asialoglycoproteins to hepatocytes from ethanol-fed rats. Consequence of both impaired synthesis and inactivation of the asialoglycoprotein receptor

Chronic ethanol administration alters the process of receptor-mediated endocytosis in isolated rat hepatocytes. Using the asialoglycoprotein receptor (ASGP-R) as a model, we have previously shown decreased binding of asialoglycoproteins to this receptor after as early as 1 week of ethanol administration. In the present study, we further analyzed the mechanism(s) responsible for this impairment by determining the ligand and antibody binding characteristics of the ASGP-R in rats fed ethanol over a 5-week time course. The results presented here demonstrate that ethanol treatment for 4 days significantly impaired total ligand binding without affecting antibody binding. Ethanol administration for a longer period of 1-2 weeks resulted in intermediate impairments in both ligand and antibody binding. After 5 weeks of ethanol exposure, ligand and antibody binding were equally lowered. In contrast to total cellular receptor binding, surface binding of both ligand and antibody were decreased over the entire time course of ethanol administration. Our data indicate that the ASGP-R is initially inactivated during the time course of ethanol exposure and that a redistribution of surface receptors to intracellular compartments occurs. Northern blot analysis showed that there was a significant decrease in receptor mRNA content in the 5-week chronically fed animals but not in the animals fed for 1 week. In addition, after 5 weeks of ethanol feeding, biosynthetic labeling of the ASGP-R was decreased in the ethanol cells, indicating impaired synthesis of the ASGP-R. In summary, an early inactivation of the ASGP-R occurs during ethanol exposure followed by an actual decrease in protein and mRNA content for the receptor.

Blocking intracellular degradation of the erythropoietin and asialoglycoprotein receptors by calpain inhibitors does not result in the same increase in the levels of their membrane and secreted forms

The erythropoietin receptor (EPO-R), a type 1 membrane glycoprotein, is degraded mainly in the lysosomes or endosomes, whereas the asialoglycoprotein receptor (ASGP-R) H2a subunit, a type 2 membrane glycoprotein, is degraded exclusively in the endoplasmic reticulum. The present study describes compounds that inhibit the intracellular degradation of these receptors in an efficient manner. However, the levels of cell-surface expression and secretion of their soluble exoplasmic domains were not enhanced to the same extent. The calpain inhibitors N-acetyl-leucyl-leucyl-norleucinal (ALLN) and N-acetyl-leucyl-leucyl-methional (ALLM) inhibited EPO-R degradation profoundly. After 3 h of chase using Ba/F3 cells and NIH 3T3 fibroblasts expressing the EPO-R, virtually all of the receptor molecules were degraded, whereas 80% of the pulse-labelled receptor remained intact in the presence of the inhibitor. EPO-R cell-surface expression was elevated 1.5-fold after 1 h of incubation with ALLN. In the absence of protein synthesis, ALLN caused the accumulation of non-degraded EPO-R molecules in endosomes and lysosomes, as determined by double immunofluorescence labelling of NIH 3T3 cells expressing EPO-Rs. In Ba/F3 cells expressing a soluble EPO-R, ALLN treatment increased secretion of the soluble exoplasmic domain of the EPO-R 2-5-fold. Similarly, in NIH 3T3 cells singly transfected with the ASGP-R H2a subunit cDNA, ALLN inhibited degradation of the ASGP-R H2a subunit precursor, as well as the degradation of the 35 kDa proteolytic fragment corresponding to the receptor ectodomain, by 3-6-fold. However, accumulation of secreted proteolytic fragment in the medium was augmented in the presence of ALLN by only 1.75-fold. In cells expressing the G78R mutant of the ASGP-R H2a subunit, which is not cleaved to the 35 kDa fragment [Yuk and Lodish (1993) J. Cell Biol. 123, 1735-1749], degradation of the precursor was inhibited. Overall, our data suggest the involvement of cysteine proteinases located in the endoplasmic reticulum, as well as in post-Golgi compartments, in degradation of the EPO-R and the ASGP-R H2a subunit. The much lower effect of the inhibitory compounds on cell-surface and secreted forms of the EPO-R and ASGP-R H2a subunit illustrates the complexity and the tight regulation of the cellular localization and stability of membrane proteins.

Effects of fusogenic and DNA-binding amphiphilic compounds on the receptor-mediated gene transfer into hepatic cells by asialofetuin-labeled liposomes

Effects of fusogenic and DNA-binding amphiphilic compounds on the receptor-mediated gene transfer using asialofetuin-labeled liposomes (AF-liposomes) were examined with HepG2 cells and rat hepatocytes in primary culture. AF-liposomes were sufficiently taken up by both types of cells through the asialoglycoprotein receptor-mediated endocytosis. In HepG2 cells, bacterial beta-galactosidase (beta-Gal) gene expression was observed by transfection using AF-liposomes encapsulating plasmid pCMV beta DNA (AF-liposome-pCMV beta). By addition of dioleoylphosphatidylethanolamine (DOPE) to the liposomal lipid composition (AF-liposome(DOPE)-pCMV beta), the transfection efficiency was clearly increased. The effects of DOPE were more conspicuous in the presence of chloroquine in the medium throughout the transfection. When pCMV beta complexed with gramicidin S (pCMV beta (GrS)) was encapsulated (AF-liposome(DOPE)-pCMV beta (GrS) and was transfected to HepG2 cells, an significantly high beta-Gal activity in the cells was observed as compared with that in the cells transfected with AF-liposome(DOPE)-pCMV beta. No effects of GrS were found in the transfection using AF-non-labeled control liposomes. In primary culture of rat hepatocytes, no beta-Gal gene expression was observed even though AF-liposome(DOPE)-pCMV beta was introduced into the cells prepared from adult rats. However, following the transfection with AF-liposome(DOPE)-pCMV beta, the beta-Gal activity was expressed in the cells from immature rats cultured in the medium supplemented with epidermal growth factor and insulin, and the transfection efficiency was 2-fold higher than that transfected with pCMV beta encapsulated in AF-non-labeled control liposomes. By the complex formation of pCMV beta with GrS, the transfection efficiency of AF-liposome(DOPE)-pCMV beta (GrS) increased according to the increase of GrS in the complex. It was shown that AF-liposome(DOPE)-pCMV beta (GrS) did efficiently introduce and express beta-Gal gene in both HepG2 cells and primary hepatocytes in the receptor mediated manner.

The human asialoglycoprotein receptor is palmitoylated and fatty deacylation causes inactivation of state 2 receptors

We report here for the first time that ASGP-Rs expressed in the human hepatoma cell lines HuH-7 and HepG2 are fatty acylated. Cells were metabolically labeled with [3H]palmitate and active ASGP-Rs, which contain two subunits (HHL1 and HHL2), were purified by affinity chromatography and subjected to nonreducing SDS-PAGE and fluorography. [3H]Palmitate was covalently incorporated into both HHL1 and HHL2. When gel slices containing HHL1/HHL2 were treated at neutral pH with 1 M hydroxylamine, but not 1 M Tris, > 95% of the radioactivity was removed, indicating that the attachment of palmitate to ASGP-Rs is to cysteines. Furthermore, the same mild hydroxylamine treatment caused partial ASGP-R inactivation; 50-70% of receptors corresponding to the previously characterized State 2 ASGP-Rs were inactivated. We conclude that both HHL1 and HHL2 are covalently modified by fatty acylation, which may regulate the ligand-binding activity of human State 2 ASGP-Rs. We propose that fatty acylation/deacylation of cytoplasmic domains is a general mechanism by which extracellular ligand-binding activity of oligomeric transmembrane receptors can be regulated.

A hepatocyte-targeted conjugate capable of delivering biologically active colchicine in vitro

A derivative of colchicine was synthesized, in a manner that preserved its important structural features, and conjugated to an asialoglycoprotein. The conjugate was characterized by ultraviolet-visible spectrophotometry and protein analysis. An average coupling ratio of 2 mol of colchicine per mole of asialoglycoprotein was achieved. The conjugate was stable to incubation in serum but was split into its separate components under chemically reducing conditions. Incubation with cells in culture revealed that the conjugate had antiproliferative activity similar to that of colchicine, but only in asialoglycoprotein receptor-containing cells. There was no effect at all on asialoglycoprotein receptor (-) cells. Furthermore, the antiproliferative effect of the conjugate on receptor (+) cells was blocked by addition of a large molar excess of free asialoglycoprotein. Immunofluorescence microscopy revealed disruption of microtubules in cell cultures that were pretreated with the conjugate. These results indicate that a colchicine conjugate that is taken up specifically into cells by asialoglycoprotein receptors and released intracellularly in a biologically active form can be prepared.

Three-dimensional organization of rat hepatocyte cytoskeleton: relation to the asialoglycoprotein endocytosis pathway

Analysis by confocal microscopy has revealed features of the microtubule network of rat hepatocytes in culture, establishing the three-dimensional disposition of the microtubule-based cytoskeleton, its relation to the actin-based cytoskeleton and to ligand-containing endosomes during receptor-mediated endocytosis and the alterations in its structure and disposition by the microtubule pertubant, Taxol. By co-localization studies, we have been able to demonstrate that the microtubules have a significant role in receptor-mediated endocytosis of asialoglycoproteins in this cell. Asialoorosomucoid-containing endosomes attach to widely spaced arrays of microtubules running under the baso-lateral surface of the hepatocytes 5–15 minutes after the initiation of endocytosis and then travel along microtubule paths to become concentrated with microtubules near the centrosome and at bile canaliculi after 30–60 minutes of receptor-mediated endocytosis. Receptor-mediated endocytosis is affected, but not abolished by Taxol, which inhibits the rate of asialoorosomucoid degradation at the same concentrations as those that disrupt microtubule and cytoplasmic dynein distribution, and that prevent the concentration of endosomes centrally. The results support suggestions that asialoorosomucoid-containing endosomes are captured by microtubules just below the actin layer at the cell periphery and these are actively transported centrally along microtubules, possibly by cytoplasmic dynein, so that the concentration of endosomes near the centrosome, and the subsequent efficient lysosomal degradation of ligand, are consequences of the confluence of microtubules in this region.

Effects of biliary obstruction on the endocytic activity in hepatocyte and liver sinusoidal endothelial cells in rats

Obstructive jaundice impairs function in liver parenchymal and sinusoidal cells. In this study, the endocytic activity in sinusoidal endothelial cells in the rats with biliary obstruction was measured by plasma clearance of radiolabeled formaldehyde-treated bovine serum albumin. The endocytic activity in hepatocytes was also measured with asialofetuin. The clearance of asialofetuin significantly decreased after 1 week of biliary obstruction and the clearance was reduced to 42% of the controls at 4 weeks. In contrast, the clearance of formaldehyde-treated bovine serum albumin was essentially unchanged until 4 weeks of biliary obstruction. The maximal removal rate which was assessed by kinetic analysis of injected protein for formaldehyde-treated bovine serum albumin showed no significant decrease at 2 weeks compared with the controls, while that for asialofetuin was significantly decreased to 50% of the controls. These results suggest that the endocytosis of formaldehyde-treated bovine serum albumin in endothelial cells is maintained until the advanced stage of biliary obstruction, whereas the endocytic activity for asialofetuin in hepatocytes is impaired earlier.

Effect of ligand orientation on hepatocyte attachment onto the poly(N-p-vinylbenzyl-o-beta-D-galactopyranosyl-D-gluconamide) as a model ligand of asialoglycoprotein

The orientation effect of galactose ligand on hepatocyte attachment was investigated. Poly(N-p-vinylbenzyl-o-beta-D-galactopyranosyl-D-gluconamide )(PVLA), a beta-galactose-carrying styrene homopolymer, was used as a model ligand for the asialoglycoprotein receptors on hepatocytes. PVLA was transferred onto the poly(gamma-benzyl L-glutamate) (PBLG) or PBLG/poly(ethylene glycol) (PEG)PBLG Langmuir-Blodgett (LB) films as the monolayer level. The dichroic fluorescence values of the confocal microscope indicated that the PVLA transferred onto the LB films was located with a preferential orientation of its molecular axes with regard to the direction of the alpha-helix of polypeptide. Hepatocyte recognized well-oriented galactose moieties of the surface of PVLA through asialoglycoprotein receptors.

Transmembrane orientation of signal-anchor proteins is affected by the folding state but not the size of the N-terminal domain

Upon insertion of a signal-anchor protein into the endoplasmic reticulum membrane, either the C-terminal or the N-terminal domain is translocated across the membrane. Charged residues flanking the transmembrane domain are important determinants for this decision, but are not necessarily sufficient to generate a unique topology. Using a model protein that is inserted into the membrane to an equal extent in either orientation, we have tested the influence of the size and the folding state of the N-terminal domain on the insertion process. A small zinc finger domain or the full coding sequence of dihydrofolate reductase were fused to the N-terminus. These stably folding domains hindered or even prevented their translocation. Disruption of their structure by destabilizing mutations largely restored transport across the membrane. Translocation efficiency, however, did not depend on the size of the N-terminal domain within a range of 40-237 amino acids. The folding behavior of the N-terminal domain is thus an important factor in the topogenesis of signal-anchor proteins.

Immunoadsorption of hepatic vesicles carrying newly synthesized dipeptidyl peptidase IV and polymeric IgA receptor

Hepatocytes must transport newly synthesized apical membrane proteins from the basolateral to the apical plasma membrane. Our earlier morphological study showed that the apical proteins share a late (subapical) part of the transcytotic pathway with the well characterized polymeric immunoglobulin A receptor (Barr, V. A., and Hubbard, A. L. (1993) Gastroenterology 105, 554-571). Starting with crude microsomes from the livers of [35S]methionine-labeled rats, we sequentially immunoadsorbed first vesicles containing the endocytic asialoglycoprotein receptor and then (from the depleted supernatant) vesicles containing the polymeric IgA receptor. Biochemical characterization indicated that early basolateral and late endosomes were present in the first population but not in the second. Neither Golgi-, apical plasma membrane (PM)-, nor basolateral PM-derived vesicles were significant contaminants of either population. Both vesicle populations contained 35S-labeled receptor and 35S-labeled-dipeptidyl peptidase IV. Importantly, the elevated relative specific activity of the dipeptidyl peptidase (% of 35S-labeled/% immunoblotted) in the second population indicated that these vesicles must transport newly synthesized dipeptidyl peptidase IV. A distinct kind of vesicle was immunoadsorbed from a "carrier-vesicle fraction"; surprisingly, these vesicles contained little 35S-receptor and virtually no dipeptidyl peptidase IV. These results, together with previous kinetic data from in vivo experiments, are consistent with a computer-generated model predicting that newly synthesized dipeptidyl peptidase IV is delivered to basolateral endosomes, which also contain newly synthesized polymeric immunoglobulin A receptor. The two proteins are then transcytosed together to the subapical region.

IL2-ricin fusion toxin is selectively cytotoxic in vitro to IL2 receptor-bearing tumor cells

Fusion toxins consist of peptide ligands linked through amide bonds to polypeptide toxins. The ligand directs the molecule to the surface of target cells and the toxin enters the cytosol and induces cell death. Ricin toxin is an excellent candidate for use in fusion toxins because of its extreme potency, the extensive knowledge of its atomic structure, and the years of experience with RTA chemical conjugates in clinical trials. We synthesized a baculovirus transfer vector with the polyhedrin promoter followed sequentially from the 5' end with DNA encoding the gp67A leader sequence, the tripeptide ADP, IL2, another ADP tripeptide, and RTB. Recombinant baculovirus was generated in Sf9 insect cells and used to infect Sf9 cells. Recombinant IL2-RTB protein was recovered at high yields from day 5 insect cell supernatants, partially purified by affinity chromatography, and characterized. The recombinant product was soluble and immunoreactive with antibodies to RTB and IL2, bound asialofetuin and lactose, and reassociated with RTA. In the presence of lactose to block galactose-binding sites on RTB, the IL2-RTB-RTA heterodimer was selectively cytotoxic to IL2 receptor, bearing cells. Specific cytotoxicity could be blocked with IL2. Thus, we report a novel targeted plant toxin fusion protein with full biological activity.

Biosynthetic transport of the asialoglycoprotein receptor H1 to the cell surface occurs via endosomes

Signals for endocytosis and for basolateral and lysosomal sorting are closely related in a number of membrane proteins, suggesting similar sorting mechanisms at the plasma membrane and in the trans-Golgi network (TGN). We tested the hypothesis that basolateral membrane proteins are transported to the cell surface via endosomes for the asialoglycoprotein receptor H1. This protein was tagged with a tyrosine sulfation site (H1TS) to allow specific labeling with [35S]sulfate in the TGN. Madin-Darby canine kidney cells expressing H1TS were pulse-labeled and chased for a period of time insufficient for labeled H1TS to reach the cell surface. Upon homogenization and gradient centrifugation, fractions devoid of TGN were subjected to immunoisolation of compartments containing mannose 6-phosphate receptor, which served as an endosomal marker. H1TS in transit to the cell surface was efficiently coisolated, whereas a labeled secretory protein and free glycosaminoglycan chains were not. This indicates an indirect pathway for the asialoglycoprotein receptor to the plasma membrane via endosomes and has important implications for protein sorting in the TGN and endosomes.

[Elimination of asialoorosomucoid and its catabolism in liver of rats with toxic CCl4-induced cirrhosis]

Human 125I-acyaloorosomucoid was intravenously injected to male Wistar rats. Hepatic cirrhosis was induced by oily CCl4 solution (0.04 ml toxin per 100 g body weight, twice a week for 2 months). Cirrhosis was found not to prevent blood elimination of 125I-acyaloorosomucoid (t1/2 = 2.53 versus 2.50 in the control). Ten minutes after injection, there was 75.2% of the administered radioactive dose in the liver versus 77.1% in the control. The 60-minute hepatic elimination of substrate in rats with cirrhosis was even more intensive than in the control animals, which is in agreement with the high levels of acid-soluble radioactivity (for 10 minutes). By large, CCl4-induced hepatic cirrhosis did not lead to disturbances of the hepatocytic function aimed at endocytosis and catabolism of acyaloorosomucoid.

Interaction of native and asialo rat sublingual glycoproteins with lectins

The binding properties of the rat sublingual glycoprotein (RSL) and its asialo product with lectins were characterized by quantitative precipitin(QPA) and precipitin inhibition(QPIA) assays. Among twenty lectins tested for QPA, native RSL reacted well only with Artocarpus integrifolia (jacalin), but weakly or not at all with the other lectins. However, its asialo product (asialo-RSL) reacted strongly with many Gal and GalNAc specific lectins-it bound best to three of the GalNAc alpha 1-->Ser/Thr (Tn) and/or Gal beta 1-->4GlcNAc (II) active lectins [jacalin, Wistaria floribunda and Ricinus communis agglutinins] and completely precipitated each of these three lectins. Asialo-RSL also reacted well with Abrus precatorius, Glycine max, Bauhinia purpurea alba, and Maclura pomifera agglutinins, and abrin-a, but not with Arachis hypogeae and Dolichos biflorus agglutinins. The interaction between asialo-RSL and lectins were inhibited by either Gal beta 1-->4GlcNAc, p-NO2-phenyl alpha-GalNAc or both. The mapping of the precipitation and inhibition profiles leads to the conclusion that the asialo rat sublingual glycoprotein provides important ligands for II (Gal beta 1-->4GlcNAc beta 1-->) and Tn (GalNAc alpha 1-->Ser/Thr) active lectins.

Inhibition of asialoglycoprotein endocytosis and degradation in rat hepatocytes by protein phosphatase inhibitors

In isolated rat hepatocytes, a radiolabelled tyramine-cellobiose conjugate of asialo-orosomucoid, 125I-TC-AOM, was rapidly taken up by receptor-mediated endocytosis and proteolytically degraded in the lysosomes, where radioactive degradation products accumulated. Okadaic acid and other protein phosphatase inhibitors (microcystin-LR, calyculin A) strongly reduced the fraction of asialoglycoprotein (ASGP) receptors localized to the cell surface, and correspondingly inhibited the uptake of 125I-TC-AOM. In addition, the inhibitors suppressed 125I-TC-AOM degradation strongly (90% at 150 nM) and potently (half-maximal effect at 20 nM okadaic acid), indicating an involvement of protein phosphorylation, and of a protein phosphatase of type 2A, in the regulation of intracellular endocytic flux. The effects of okadaic acid on 125I-TC-AOM accumulation, as well as on degradation, could be eliminated by the protein kinase inhibitor genistein. Okadaic acid prevented the transfer of 125I-TC-AOM to a non-recycling endocytic compartment, causing its retention in a recycling compartment from which about one-third of the endocytosed 125I-TC-AOM could be returned to the cell surface and detached from its receptor in the presence of EGTA. ASGP receptors recycled extensively both in the presence and absence of okadaic acid, as indicated by a sustained uptake of 125I-TC-AOM. Sucrose density gradient analysis and sedimentation studies indicated that okadaic acid caused accumulation of 125I-TC-AOM in light endosomes (1.11 g/ml), preventing its transfer to dense endosomes (1.14 g/ml) and lysosomes (1.18 g/ml). The lysosomes could be identified in density gradients by their contents of lysosomal marker enzymes and acid-soluble radioactivity, and by their sensitivity towards the lysosome-disrupting agent glycyl-L-phenylalanine-2-naphthylamide. By using endocytosed AOM-gold particles as an ultrastructural endocytic marker, it could be shown that the light endosomes accumulating ASGP in the presence of okadaic acid had the morphological appearance of small endocytic vesicles/tubules and multivesicular endosomes. Whereas in control cells 4% of the AOM-gold was in small vesicles/tubules, 55% in multivesicular endosomes and 41% in lysosomes, the corresponding figures for okadaic acid-treated cells were 17%, 73% and 11%. Our results thus indicate that protein phosphatase inhibitors have two effects on ASGP endocytosis: (1) an early inhibition of ligand uptake, due to a reduction in the fraction of ASGP receptors at the cell surface, and (2) an inhibition of ASGP transfer from a recycling compartment consisting of light, small endocytic vesicles and multivesicular endosomes, to a non-recycling compartment consisting of dense multivesicular endosomes.(ABSTRACT TRUNCATED AT 400 WORDS)

Interactions of gelatinases with soluble and immobilized fetuin and asialofetuin

We have analyzed the interactions of human and mouse gelatinases with fetuin and asialofetuin. The data showed that recombinant human gelatinase A (MMP-2) and B (MMP-9) were both specifically bound to asialofetuin and fetuin immobilized to activated agarose (affigel) with subsequent cleavage of the enzymes to lower molecular weight forms, which were likewise bound to asialofetuin/fetuin. The binding of gelatinases to immobilized forms of asialofetuin and fetuins was abrogated in the presence of either soluble fetuin or asialofetuin. Endogenous mouse macrophage gelatinases (mol wt 92 and approximately 52 kDa) were also specifically bound to immobilized asialofetuin upon which the two forms of the gelatinases were reduced to a approximately 45-kDa fragment. The binding of the approximately 45-kDa fragment to asialofetuin was also abrogated in the presence of either soluble fetuin or asialofetuin. Whereas only the activated MMP-2 bound to immobilized asialofetuin had significant gelatinolytic activity, both the zymogen and the activated forms of MMP-9 hydrolyzed soluble [3H]gelatin to the same extent while still bound to asialofetuin. Our data suggest that cell surface bound fetuin/asialofetuin could perform two functions: they could (a) act as cell surface receptors or anchors for MMP-2 and MMP-9 and (b) bind and activate MMP-9 on the cell surface.

Hydroxylamine treatment differentially inactivates purified rat hepatic asialoglycoprotein receptors and distinguishes two receptor populations

We previously showed that two subpopulations of asialoglycoprotein receptors (ASGP-Rs), designated State 1 and State 2 ASGP-Rs, are present in intact cells and that State 2 ASGP-Rs can be inactivated in permeable rat hepatocytes in a temperature- and ATP-dependent manner. These inactivated ASGP-Rs can be quantitatively reactivated by the addition of palmitoyl-CoA (Weigel, P. H., and Oka, J. A. (1993) J. Biol. Chem. 268, 27186-27190). Here we show that approximately 50% of purified rat ASGP-Rs are inactivated by treatment with hydroxylamine under mild conditions. The activity of affinity-purified ASGP-Rs was assessed by measuring the specific binding of 125I-asialo-orosomucoid (ASOR) in a dot-blot assay after immobilization onto nitrocellulose. Treatment of ASGP-Rs in solution with 0.0125-1.0 M NH2OH, pH 7.4, at 4 degrees C for 4 h resulted in a progressive loss of ASOR binding activity. ASGP-R inactivation with NH2OH occurred more readily at basic pH or at room temperature. Similar treatment with Tris had no effect on ASGP-R activity. The kinetics of ASGP-R activity loss and the dose-response for this inactivation were both biphasic, indicating the presence of two equal populations of ASGP-Rs with different sensitivities to NH2OH. The more sensitive population of ASGP-Rs (approximately 50%) was inactivated by treatment with 0.2 M NH2OH (4 degrees C, 4 h) or with 1.0 M NH2OH (4 degrees C, 1 h) without detectable peptide cleavage as assessed by SDS-polyacrylamide gel electrophoresis. State 1 ASGP-Rs, purified from chloroquine- or monensin-treated hepatocytes, showed significantly less sensitivity to NH2OH treatment (both in kinetics and dose dependence). Furthermore, under mild conditions NH2OH caused dissociation and inactivation of approximately 50% of the total ASGP-Rs (State 1 and State 2) that were prebound to ASOR-Sepharose, whereas the same treatment caused dissociation of only < 20% of State 1 ASGP-Rs from such preformed complexes. As shown in the accompanying paper (Zeng, F. Y., Kaphalia, B. S., Ansari, G. A. S., and Weigel, P. H. (1995) J. Biol. Chem. 270, 21382-21387) all three RHL subunits of active ASGP-Rs, in fact, contain covalently attached palmitate and stearate. In cultured cells, [3H]palmitic acid is metabolically incorporated into all three subunits. These radiolabeled fatty acids are completely released from purified ASGP-Rs by mild NH2OH treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Fatty acylation of the rat asialoglycoprotein receptor. The three subunits from active receptors contain covalently bound palmitate and stearate

Rat hepatic asialoglycoprotein receptors (ASGP-Rs) are hetero-oligomers composed of three homologous glycoprotein subunits, designated rat hepatic lectins (RHL) 1, 2, and 3. ASGP-Rs mediate the endocytosis and degradation of circulating glycoconjugates containing terminal N-acetylgalactosamine or galactose, including desialylated plasma glycoproteins. We have shown in permeable rat hepatocytes that the ligand binding activity of one subpopulation of receptors (designated State 2 ASGP-Rs) can be decreased or increased, respectively, by ATP and palmitoyl-CoA (Weigel, P. H., and Oka, J. A. (1993) J. Biol. Chem. 268, 27186-27190). We proposed that a reversible and cyclic acylation/deacylation process may regulate ASGP-R activity during endocytosis, receptor-ligand dissociation, and receptor recycling. In the accompanying paper (Zeng, F-Y., and Weigel, P. H. (1995) J. Biol. Chem. 270, 21388-21395), we show that the ligand binding activity of affinity-purified State 2 ASGP-Rs is decreased by treatment with hydroxylamine under mild conditions consistent with these ASGP-Rs being fatty acylated in vivo. In this study, we used a chemical method to determine the presence of covalently-bound fatty acids in individual ASGP-R subunits. The affinity-purified ASGP-R preparations were separated by SDS-polyacrylamide gel electrophoresis under nonreducing conditions, and the gel slices containing individual RHL subunits were treated with alkali to release covalently bound fatty acids, which were subsequently analyzed by gas chromatography and confirmed by gas chromatography-mass spectrometry. Both stearic and palmitic acids were detected in all three receptor subunits. Pretreatment of ASGP-Rs with hydroxylamine before SDS-polyacrylamide gel electrophoresis reduced the content of both fatty acids by 66-80%, indicating that most of these fatty acids are attached to cysteine residues via thioester linkages. Furthermore, when freshly isolated hepatocytes were cultured in the presence of [3H]palmitate, all three RHL subunits in affinity-purified ASGP-Rs were metabolically labeled. We conclude that RHL1, RHL2, and RHL3 are modified by fatty acylation in intact cells.

Enhanced folding and processing of a disulfide mutant of the human asialoglycoprotein receptor H2b subunit

Unfolded forms of the H2b subunit of the human asialoglycoprotein receptor, a galactose-specific C-type lectin, are degraded in the endoplasmic reticulum (ER), whereas folded forms of the protein can mature to the cell surface (Wikström, L., and Lodish, H. F. (1993) J. Biol. Chem. 268, 14412-14416). There are eight cysteines in the exoplasmic domain of the protein, forming four disulfide bonds in the folded protein. We have constructed double cysteine to alanine mutants for each of the four disulfide bonds and examined the folding and metabolic fate of each of the mutants in transfected 3T3 fibroblasts. We find that mutation of the two cysteines nearest to the transmembrane region (C1) does not prevent proper folding of the protein, whereas mutations of the other three disulfides prevent proper folding of the protein and all of the mutant proteins are degraded in the ER. A normal (approximately 20%) fraction of the C1 mutant protein exists the endoplasmic reticulum and is processed in the Golgi complex, and it does so at a faster rate compared to the wild-type. Furthermore, the folded form of this mutant protein is more resistant to unfolding by dithiothreitol than the wild-type. The C1 mutant protein is expressed on the cell surface and can form a functional receptor with the H1 subunit with similar binding affinities for natural ligands as that of the wild-type receptor. The same fraction of newly made mutant and wild-type proteins (approximately 80%) remain in the ER, but the mutant protein is degraded more quickly. Thus, the presence of the C1 disulfide bond in the wild-type receptor both reduces the rate of protein folding and exit to the Golgi and slows the rate of ER degradation of the portion (approximately 80%) of the receptor that never folds properly.

Asparagine-linked oligosaccharide structures determine clearance and organ distribution of pituitary and recombinant thyrotropin

The recombinant human TSH (rhTSH) with highly sialylated oligosaccharide chains showed higher in vivo bioactivity and a lower MCR than the predominantly sulfated pituitary human TSH (phTSH). The aim of the present study was to investigate the role of terminal carbohydrate residues in organ distribution and metabolic clearance of TSH using an in vivo rat model. The different 125I-labeled TSH preparations with distinct carbohydrate composition were injected i.v. At various time points (5-180 min) after bolus TSH injection, blood, liver, kidney, spleen, lung, heart, and thyroid samples were collected. TSH uptake was determined by trichloroacetic acid precipitation of [125I]TSH in the organ homogenates. The rhTSH (solely sialylated) was distributed predominantly to the kidneys 5, 15, and 30 min after injection. In contrast, phTSH (sulfated/sialylated) and bovine TSH (bTSH; solely sulfated) were cleared predominantly by the liver (at 5 min), with a later renal phase of clearance (at 30 min). Asialo-rhTSH was cleared by the liver with only minor involvement of other organs. The early liver uptake (at 5 min) was proportionally highest for the asialo-rhTSH and bTSH preparations and lowest for rhTSH, which correlated inversely with the serum levels and the degree of sialylation. Blockade of the N-acetylgalactosamine (GalNAc) sulfate receptors by injection of bovine LH resulted in a significant decrease in liver uptake of phTSH. Similarly, liver uptake of asialo-rhTSH was significantly inhibited by injection of asialo-fetuin. Thus, phTSH and bTSH preparations containing sulfated oligosaccharide chains are cleared at least in part by the GalNAc sulfate-specific receptors in the liver. In contrast, rhTSH with highly sialylated oligosaccharides in both subunits accumulates predominantly in the kidneys, even at the early phase of clearance, indicating that sialylated glycoprotein hormones escape from specific receptor-mediated clearance mechanisms in the liver. These data indicate that terminal sialic acid and GalNAc sulfate residues, each to a different extent, determine glycoprotein hormone distribution and thereby plasma level, which as we have shown previously is a major factor in determining the in vivo potency of TSH.

Receptor-mediated transfer of pSV2CAT DNA to a human hepatoblastoma cell line HepG2 using asialofetuin-labeled cationic liposomes

Asialofetuin-labeled liposomes (AF-lps) were developed as a vector for gene transfer to hepatocytes. Plasmid pSV2CAT DNA which encodes bacterial chloramphenicol acetyltransferase (CAT) was associated with (meaning, in this report, the sum of 'to be adsorbed on the surface of' and 'to be encapsulated into the internal phase of') AF-lps (AF-lps-pSV2CAT) prepared by a tandem combination of the detergent removal and freeze-thaw methods. Ninety-six percent of input pSV2CAT was associated with AF-lps containing N-(alpha-trimethylammonioacetyl)-didodecyl-D-glutamate chloride, and approx. two-thirds of the associated DNA was encapsulated into the internal phase. The uptake of AF-lps by the cultured human hepatoblastoma cell line HepG2, having asialoglycoprotein receptors (AGPR) on their plasma membrane, was decreased by the addition of free AF and cytochalasin B. AF-lps bound to HepG2 cells through specific interaction with AGPR, and were internalized into the cells by the receptor-mediated endocytotic pathway. HepG2 cells transfected by AF-lps-pSV2CAT showed a significantly higher CAT activity than those transfected by pSV2CAT associated with non-labeled control lps (N-lps-pSV2CAT) or a mixture of pSV2CAT and empty AF-lps. Pretreatment with EDTA-encapsulated AF-lps increased the transfection efficiency of AF-lps-pSV2CAT. The CAT activity in A431 and Swiss/3T3 cells transfected with AF-lps-pSV2CAT was low and almost the same as those transfected with N-lps-pSV2CAT. Since DNA encapsulated in lps is likely to be protected against digestion by nucleases in the blood circulation, AF-lps could be used as a gene transfer vector targeting the hepatocytes in vivo.

trans-Golgi retention of a plasma membrane protein: mutations in the cytoplasmic domain of the asialoglycoprotein receptor subunit H1 result in trans-Golgi retention

Unlike the wild-type asialoglycoprotein receptor subunit H1 which is transported to the cell surface, endocytosed and recycled, a mutant lacking residues 4-33 of the 40-amino acid cytoplasmic domain was found to be retained intracellularly upon expression in different cell lines. The mutant protein accumulated in the trans-Golgi, as judged from the acquisition of trans-Golgi-specific modifications of the protein and from the immunofluorescence staining pattern. It was localized to juxtanuclear, tubular structures that were also stained by antibodies against galactosyltransferase and gamma-adaptin. The results of further mutagenesis in the cytoplasmic domain indicated that the size rather than the specific sequence of the cytoplasmic domain determines whether H1 is retained in the trans-Golgi or transported to the cell surface. Truncation to less than 17 residues resulted in retention, and extension of a truncated tail by an unrelated sequence restored surface transport. The transmembrane segment of H1 was not sufficient for retention of a reporter molecule and it could be replaced by an artificial apolar sequence without affecting Golgi localization. The cytoplasmic domain thus appears to inhibit interaction(s) of the exoplasmic portion of H1 with trans-Golgi component(s) for example by steric hindrance or by changing the positioning of the protein in the membrane. This mechanism may also be functional in other proteins.

The asialoglycoprotein receptor: relationships between structure, function, and expression

Transport of macromolecules into the cell by receptor-mediated endocytosis follows a complex series of intracellular transfers, passing through distinct environments. The asialoglycoprotein receptor is a prototype of the class of receptors that constitutively enters cells via coated pits and delivers ligand to these intracellular compartments. In addition to being a model of receptor-mediated endocytosis, the presence of the receptor on hepatocytes provides a membrane-bound active site for cell-to-cell interactions, has made possible the selective targeting of chemotherapeutic agents and foreign genes, and has also been implicated as a site mediating hepatitis B virus uptake. Regulated expression of receptor subunits and their intracellular trafficking during biosynthesis and endocytosis has provided insights into the relationship of receptor structure to its overall function. As a marker of hepatocellular differentiation, its study has uncovered a unique response to intracellular guanosine 3',5'-cyclic monophosphate and translational regulation of the receptor. In this review, an overview of these diverse findings is provided in an attempt to relate the various aspects of structure and function as they impact on receptor expression.