Subcellular membrane topology and turnover of a rat hepatic binding protein specific for asialoglycoproteins

Development of siRNA Therapeutics for the Treatment of Liver Diseases

Small interfering RNA (siRNA)-based therapeutics holds the promise to treat a wide range of human diseases that are currently incurable using conventional therapies. Most siRNA therapeutic efforts to date have focused on the treatment of liver diseases due to major breakthroughs in the development of efficient strategies for delivering siRNA drugs to the liver. Indeed, the development of lipid nanoparticle-formulated and GalNAc-conjugated siRNA therapeutics has resulted in recent FDA approvals of the first siRNA-based drugs, patisiran for the treatment of hereditary transthyretin amyloidosis and givosiran for the treatment of acute hepatic porphyria, respectively. Here, we describe the current strategies for delivering siRNA drugs to the liver and summarize recent advances in clinical development of siRNA therapeutics for the treatment of liver diseases.

A liver-targeting Cu(i) chelator relocates Cu in hepatocytes and promotes Cu excretion in a murine model of Wilson's disease

A hepatocyte-targeting chelator promotes Cu biliary excretion, hence restoring the physiological Cu detoxification pathway in a murine Wilson's disease model.

GalNAc-siRNA Conjugates: Leading the Way for Delivery of RNAi Therapeutics

Short-interfering RNA (siRNA)-induced RNAi responses have great potential to treat a wide variety of human diseases from cancer to pandemic viral outbreaks to Parkinson's Disease. However, before siRNAs can become drugs, they must overcome a billion years of evolutionary defenses designed to keep invading RNAs on the outside cells from getting to the inside of cells. Not surprisingly, significant effort has been placed in developing a wide array of delivery technologies. Foremost of these has been the development of N-acetylgalactosamine (GalNAc) siRNA conjugates for delivery to liver. Tris-GalNAc binds to the Asialoglycoprotein receptor that is highly expressed on hepatocytes resulting in rapid endocytosis. While the exact mechanism of escape across the endosomal lipid bilayer membrane remains unknown, sufficient amounts of siRNAs enter the cytoplasm to induce robust, target selective RNAi responses in vivo. Multiple GalNAc-siRNA conjugate clinical trials, including two phase III trials, are currently underway by three biotech companies to treat a wide variety of diseases. GalNAc-siRNA conjugates are a simple solution to the siRNA delivery problem for liver hepatocytes and have shown the RNAi (and antisense oligonucleotide) field the path forward for targeting other tissue types.

Galactose Derivative-Modified Nanoparticles for Efficient siRNA Delivery to Hepatocellular Carcinoma

Successful siRNA therapy requires suitable delivery systems with targeting moieties such as small molecules, peptides, antibodies, or aptamers. Galactose (Gal) residues recognized by the asialoglycoprotein receptor (ASGPR) can serve as potent targeting moieties for hepatocellular carcinoma (HCC) cells. However, efficient targeting to HCC via galactose moieties rather than normal liver tissues in HCC patients remains a challenge. To achieve more efficient siRNA delivery in HCC, we synthesized various galactoside derivatives and investigated the siRNA delivery capability of nanoparticles modified with those galactoside derivatives. In this study, we assembled lipid/calcium/phosphate nanoparticles (LCP NPs) conjugated with eight types of galactoside derivatives and demonstrated that phenyl β-d-galactoside-decorated LCP NPs (L4-LCP NPs) exhibited a superior siRNA delivery into HCC cells compared to normal hepatocytes. VEGF siRNAs delivered by L4-LCP NPs downregulated VEGF expression in HCC in vitro and in vivo and led to a potent antiangiogenic effect in the tumor microenvironment of a murine orthotopic HCC model. The efficient delivery of VEGF siRNA by L4-LCP NPs that resulted in significant tumor regression indicates that phenyl galactoside could be a promising HCC-targeting ligand for therapeutic siRNA delivery to treat liver cancer.

Endotoxin-induced inflammation down-regulates L-type amino acid transporter 1 (LAT1) expression at the blood-brain barrier of male rats and mice

Background

We recently reported that bacterial lipopolysaccharide (LPS)-induced inflammation decreases the expression of the primary thyroid hormone transporters at the blood–brain barrier, organic anion-transporting polypeptide 1c1 (OATP1c1) and monocarboxylate transporter 8 (MCT8). l-type amino acid transporters 1 and 2 (LAT1 & LAT2) are regarded as secondary thyroid hormone transporters, and are expressed in cells of the blood–brain or blood-cerebrospinal fluid barrier and by neurons. The purpose of this study was to examine the effect of LPS-induced inflammation on the expression of LAT1 and LAT2, as these may compensate for the downregulation of OATP1c1 and MCT8.

Methods

LPS (2.5 mg/kg body weight) was injected intraperitoneally to adult, male, Sprague–Dawley rats and C57Bl/6 mice, which were euthanized 2, 4, 9, 24 or 48 h later. LAT1 and LAT2 mRNA expression were studied on forebrain sections using semiquantitative radioactive in situ hybridization. LAT1 protein levels in brain vessels were studied using LAT1 immunofluorescence. Statistical comparisons were made by the non-parametric Kruskal–Wallis and Dunn’s tests.

Results

In both species, LAT1 mRNA decreased in brain blood vessels as soon as 2 h after LPS injection and was virtually undetectable at 4 h and 9 h. During recovery from endotoxemia, 48 h after LPS injection, LAT1 mRNA in brain vessels increased above control levels. A modest but significant decrease in LAT1 protein levels was detected in the brain vessels of mice at 24 h following LPS injection. LPS did not affect LAT1 and LAT2 mRNA expression in neurons and choroid plexus epithelial cells.

Conclusions

The results demonstrate that LPS-induced inflammation rapidly decreases LAT1 mRNA expression at the blood–brain barrier in a very similar manner to primary thyroid hormone transporters, while changes in LAT1 protein level follow a slower kinetics. The data raise the possibility that inflammation may similarly down-regulate other blood–brain barrier transport systems at the transcriptional level. Future studies are required to examine this possibility and the potential pathophysiological consequences of inflammation-induced changes in blood–brain barrier transport functions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12987-015-0016-8) contains supplementary material, which is available to authorized users.

Carbohydrate clearance receptors in transfusion medicine

BACKGROUND

Complex carbohydrates play important functions for circulation of proteins and cells. They provide protective shields and refraction from non-specific interactions with negative charges from sialic acids to enhance circulatory half-life. For recombinant protein therapeutics carbohydrates are especially important to enhance size and reduce glomerular filtration loss. Carbohydrates are, however, also ligands for a large number of carbohydrate-binding lectins exposed to the circulatory system that serve as scavenger receptors for the innate immune system, or have more specific roles in targeting of glycoproteins and cells.

SCOPE OF REVIEW

Here we provide an overview of the common lectin receptors that play roles for circulating glycoproteins and cells, and present a discussion of ways to engineer glycosylation of recombinant biologics and cells to improve therapeutic effects.

MAJOR CONCLUSIONS

While the pharmaceutical industry has learned how to exploit carbohydrates to improve pharmacokinetic properties of recombinant therapeutics, our understanding of how to improve cell-based therapies by manipulation of complex carbohydrates is still at its infancy. Progress with the latter has recently been achieved with cold-stored platelets, where exposure of uncapped glycans lead to rapid clearance from circulation by several lectin-mediated pathways.

GENERAL SIGNIFICANCE

Understanding lectin-mediated clearance pathways is essential for progress in development of biological pharmaceuticals.

Regulation of protein breakdown in hepatocyte monolayers

Effects of potential modifiers on intracellular protein degradation have been measured in hepatocyte monolayers two days after the cells were isolated and plated. Modifiers were added after cells had been labelled with [3H]leucine and generally at the beginning of the degradation period. Protein degradation was inhibited by insulin, epidermal growth factor and serum as well as the protein synthesis inhibitors, leupeptin and weak bases. Degradation was stimulated by cyclic AMP, glucagon, beta-agonists, glucocorticoids and nutritional stepdown. The results from addition and competition experiments are consistent with all effectors acting on lysosomal proteolysis but differing in whether they alter amounts of proteinases, conditions favourable for proteinase function, or degree of autophagocytosis.

Direct identification of nonreducing GlcNAc residues on N-glycans of glycoproteins using a novel chemoenzymatic method

The mutant beta1,4-galactosyltransferase (beta4Gal-T1), beta4Gal-T1-Y289L, in contrast to wild-type beta4Gal-T1, can transfer GalNAc from the sugar donor UDP-GalNAc to the acceptor, GlcNAc, with efficiency as good as that of galactose from UDP-Gal. Furthermore, the mutant can also transfer a modified sugar, C2 keto galactose, from its UDP derivative to O-GlcNAc modification on proteins that provided a functional handle for developing a highly sensitive chemoenzymatic method for detecting O-GlcNAc post-translational modification on proteins. We report herein that the modified sugar, C2 keto galactose, can be transferred to free GlcNAc residues on N-linked glycoproteins, such as ovalbumin or asialo-agalacto IgG1. The transfer is strictly dependent on the presence of both the mutant enzyme and the ketone derivative of the galactose. Moreover, the PNGase F treatment of the glycoproteins, which cleaves the N-linked oligosaccharide chain, shows that the modified sugar has been transferred to the N-glycan chains of the glycoproteins and not to the protein portion. The application of the mutant galactosyltransferase, beta4Gal-T1-Y289L, to produce glycoconjugates carrying sugar moieties with reactive groups, is demonstrated. We envision a broad potential for this technology such as the possibilities to link cargo molecules to glycoproteins, such as monoclonal antibodies, via glycan chains, thereby assisting in the glycotargeting of drugs to the site of action or used as biological probes.

Transporters on demand: intrahepatic pools of canalicular ATP binding cassette transporters in rat liver

ABC transporter trafficking in rat liver induced by cAMP or taurocholate and [(35)S]methionine metabolic labeling followed by subcellular fractionation were used to identify and characterize intrahepatic pools of ABC transporters. ABC transporter trafficking induced by cAMP or taurocholate is a physiologic response to a temporal demand for increased bile secretion. Administration of cAMP or taurocholate to rats increased amounts of SPGP, MDR1, and MDR2 in the bile canalicular membrane by 3-fold; these effects abated after 6 h and were insensitive to prior treatment of rats with cycloheximide. Half-lives of ABC transporters were 5 days, which suggests cycling of ABC transporters between canalicular membrane and intrahepatic sites before degradation. In vivo [(35)S]methionine labeling of rats followed by immunoprecipitation of (sister of P-glycoprotein) (SPGP) from subcellular liver fractions revealed a steady state distribution after 20 h of SPGP between canalicular membrane and a combined endosomal fraction. After mobilization of transporters from intrahepatic sites with cAMP or taurocholate, a significant increase in the amount of ABC transporters in canalicular membrane vesicles was observed, whereas the decrease in the combined endosomal fraction remained below detection limits in Western blots. This observation is in accordance with relatively large intracellular ABC transporter pools compared with the amount present in the bile canalicular membrane. Furthermore, trafficking of newly synthesized SPGP through intrahepatic sites was accelerated by additional administration of cAMP but not by taurocholate, indicating two distinct intrahepatic pools. Our data indicate that ABC transporters cycle between the bile canaliculus and at least two large intrahepatic ABC transporter pools, one of which is mobilized to the canalicular membrane by cAMP and the other, by taurocholate. In parallel to regulation of other membrane transporters, we propose that the "cAMP-pool" in hepatocytes corresponds to a recycling endosome, whereas recruitment from the "taurocholate-pool" involves a hepatocyte-specific mechanism.

A 3D model for the human hepatic asialoglycoprotein receptor (ASGP-R)

The human hepatic Asialoglycoprotein Receptor (ASGP-R) consists of two different types of liver specific membrane glycoproteins that bind to terminal galactose and N-acetylgalactosamine residues of serum glycoproteins. The two different polypeptide chains are referred to as two receptor subunits, HH1 and HH2, which are both involved in the activity of the functional receptor. This receptor has served as a model for understanding receptor-mediated endocytosis and carbohydrate mediated recognition phenomena. Here models for the C-terminal extracellular region of both HH1 and HH2 subunit are presented. The standard homology building procedure was modified in order to make it suitable for the modeling problem at hand. The models for the extracellular regions of HH1 and HH2 were initially constructed by exploiting several fragments, belonging to proteins of known 3D structure, and showing high local sequence similarity with respect to the glycoproteins of interest. Putative binding sites were first hypothesized on the basis of the comparison with other complexes of lectins, the crystal structure of which was available in the Protein Data Bank. A model for the complex involving the HH2 subunit and the typical high affinity ligand N-acetylgalactosamine (NacGal) was refined as the first by a suitable combination of MD simulations and Energy Minimization calculations, since it seemed to quickly converge to a plausible structure. An intermediate model for HH1 was then rebuilt on the basis of the refined model for HH2. It was then submitted to a sequence of molecular dynamics simulations with templates which took into account the secondary structure prediction for a final refinement. The structures of small regions of the models, located around the binding sites, were compared with more recent crystallographic data regarding a complex involving the mutant of Mannose Binding Protein QPDWGH (1BCH entry in the Protein Data Bank) and NacGal. This mutant shows high local sequence similarity with HH1 and HH2 at the binding sites. On the basis of the above comparison, different locations of the binding sites were also considered. In addition to other expected interactions, two hydrophobic interactions were observed in the models with Trp residues (positions 243 in HH1 and 181 or 267 in HH2 respectively) and His residues (positions 256 in HHI and 184 in HH2.respectively). The quality of the models was evaluated by the Procheck program and they seemed plausible. This observation together with analogies found between binding sites of the models and IBCH supported the validity of the models. A further validation element arose by comparison between experimental binding data available in the literature about the homologous rat receptor subunits and theoretical interaction energies evaluated, by means of the DOCK 3.5 program, in models for the rat subunits obtained from the corresponding human ones. The new modeling procedure used here appears to be a well-suited method for structural analysis of small regions, located around the ligands, in proteins of unknown 3D structure.

The major form of the murine asialoglycoprotein receptor: cDNA sequence and expression in liver, testis and epididymis

Northern blot analysis of poly(A)+RNAs isolated from mouse liver or mouse testis (Te)/epididymis (Ep) reveals that both tissues express 1.5- and 7.5-kb transcripts which have extensive homology to the major form of the rat asialoglyco-protein receptor (ASGP-R). In situ hybridization studies have localized the expression of this ASGP-R-like transcript to late-stage sperm from Te and Ep of several different strains of mice. Swiss Webster mice express this ASGP-R-like transcript in late-stage spermatids at the time of release into the seminiferous tubule and in Ep sperm, while Balb/C, NIH Swiss and C57Bl/6 mice express this ASGP-R-like transcript predominantly in Ep sperm. cDNAs containing the entire coding region for this ASGP-R-like transcript have been cloned from mouse liver and mouse Te/Ep. These cDNAs are 100% identical in the coding region and 3'-untranslated region (UTR), but differ in the 5'-UTR. The gene encoding these cDNAs is called MHL-1, designating the major form of the mouse ASGP-R. The deduced amino acid (aa) sequence of MHL-1 shares 88% homology to the rat hepatic (He) lectin form 1 (RHL-1) and 78% homology to the human asialoglycoprotein receptor form 1 (H1). The three sites for N-linked glycosylation in the RHL-1 sequence are all conserved in the deduced MHL-1 sequence. Taken collectively, these data describe the cloning and sequencing of the MHL-1 cDNA and illustrate its deduced aa homology to RHL-1 and H1.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the "hepatic" asialoglycoprotein receptor in rat late-stage spermatids and epididymal sperm

Northern blot analysis of rat testicular (Te) poly(A)+RNA reveals that a transcript homologous to the major form of the asialoglycoprotein receptor (ASGP-R), designated RHL-1, is expressed as early as one week postnatally and that steady-state levels are approx. 8-times higher in the Te of an 8-week-old rat (sexually mature) as compared to an 84-week-old rat (aged). Partial cDNAs encoding RHL-1 and the minor form of the ASGP-R, designated RHL-2/3, have been cloned from two rat Te/epididymal (Ep) cDNA libraries and rat Te poly(A)+RNA. Sequence analysis of the Te/Ep RHL-1 cDNA and the Te/Ep RHL-2/3 cDNA indicates that these cDNAs are identical to the forms expressed in rat liver. Western blot analysis demonstrates the presence of a 49-kDa Te/Ep RHL-1-related protein band and a 54-kDa Te/Ep RHL-2/3-related protein band in both rat Te membrane fractions (MF) and rat Ep sperm MF. The RHL-1-related protein has been localized to late-stage Te spermatids at the time of release from the seminiferous tubules and to Ep sperm in the region of the sperm tail, referred to as the middle piece. Taken collectively, these data indicate that the authentic RHL-1 and RHL-2/3 genes of the ASGP-R are expressed in late-stage spermatids; however, the Te/Ep RHL-1-related protein differs in size from the hepatic RHL-1 polypeptide, possibly indicating a specific function of the RHL-1-related protein in spermatogenesis.

Preparation and biological evaluation of radiolabelled antibodies with selected carbohydrate modifications

Two carbohydrates, N-acetylgalactosamine (GalNAc) and galactose-beta-1,3-GalNAc have been attached to human IgG (hIgG) by a novel linking reagent, hexafluoroglutaric acid dimethyl ester. Fluorine-19 NMR signals were used for the determination of the conjugation ratio. A third carbohydrate, sialic acid, was conjugated via reductive amination and the conjugation ratio determined by a resorcinol assay. The biological behaviour of these radioiodinated antibodies with carbohydrate modifications in normal mice indicates an enhanced liver uptake at 15 min post-injection with an associated change in circulating blood levels occurs for the galactose-based hIgG preparations. However, no significant differences in the biodistribution were observed for the sialic acid conjugate. These studies confirm the potential of carbohydrate-antibody conjugation for modifying the behaviour of antibodies in immunoscintigraphy and radioimmunotherapy.

Distribution of asialoglycoprotein receptor in human hepatocellular carcinoma

Altered expression of asialoglycoprotein (ASGP) receptors on hepatocytes has been reported during hepatic neoplasia mostly in animal models. In this study, we examined immunohistochemically the distribution of the ASGP receptor in humans with various liver diseases, including ten cases of hepatocellular carcinoma (HCC). In livers of acute hepatitis, chronic hepatitis, cirrhosis and the non-cancerous tissues (mostly cirrhosis) adjacent to HCC, the receptor was present in its normal distribution, i.e. mostly along the sinusoidal margin and partly on the lateral surface of hepatocytes. In four of six well-differentiated HCCs, the receptor was also normally distributed on the plasma membrane; by immunoelectron microscopy, it was seen in the endoplasmic reticulum and in pits in the plasma membrane but not on bile canaliculus-like structures, suggesting that it was synthesized, transported, and integrated into the plasma membrane in a polar manner. In contrast, there was no surface expression of the ASGP receptor in the remaining six HCCs (two well-differentiated and four poorly differentiated). In two of the poorly differentiated HCCs, the receptor, although absent from the cell surface, was prominent in the endoplasmic reticulum, suggesting disturbed transport of the ASGP receptor to the cell surface. When we examined proliferative activity of HCCs by immunohistochemical labeling of DNA polymerase alpha, HCCs with high percentages (above 30%) of DNA polymerase alpha-positive cells had lost the cell-surface expression of the receptor. Thus, the expression of the ASGP receptor in human HCC appears to be closely related to differentiation and proliferative activity of the tumor cells.

Modification of blood group A antigen expression in a pancreatic cancer cell line (PC-1) by inhibitors of N-glycan processing

Pancreatic adenocarcinomas induced in Syrian hamsters by treatment with N-nitrosobis(2-oxopropyl) amine express blood group A antigen, which is absent in normal pancreatic cells. On membrane glycoproteins purified from tumors, blood group A antigen has been found to be expressed on multiantennary Asn-linked complex glycans. In this study, we investigated the effect of inhibitors of Asn-glycan processing on blood group A antigen bearing glycan structures in a cell line (PC-1) established from a primary induced pancreatic cancer. Expression of blood group A antigen on cells and in membrane preparations was blocked by treatment with 1-deoxymannojirimycin, an inhibitor of mannosidase I, but was retained after treatment with swainsonine, an inhibitor of mannosidase II. However, swainsonine treatment altered the glycan structure associated with blood group A antigen from an endoglycosidase H resistant type to a sensitive type, indicating that the blood group A structure might shift from a complex type to a hybrid type glycan by this treatment. These results demonstrate that Asn-linked glycans carry the major blood group A antigens in PC-1 cells.

Abnormal surface distribution of the human asialoglycoprotein receptor in cirrhosis

Serum concentrations of asialoglycoproteins are increased in cirrhosis. We hypothesized that this increase results from abnormalities in the asialoglycoprotein receptor, which is located on the sinusoidal and lateral membrane of hepatocytes. Therefore we searched for morphological alterations in the distribution of the asialoglycoprotein receptor in human liver, using a light microscopic immunoperoxidase method in autopsy livers. In 24 of 25 (96%) of patients without liver disease, the asialoglycoprotein receptor was located on the sinusoidal and, less prominently, the lateral surface of hepatocytes but not the canalicular surface. In contrast, in 12 of 18 (67%) patients with cirrhosis of various causes, the receptor also was localized strikingly along the canalicular surface, with a corresponding decrease on the sinusoidal and lateral surfaces. We conclude that an abnormal cell-surface distribution of the asialoglycoprotein receptor commonly occurs in cirrhosis. This abnormality might result in impaired clearance of desialylated glycoproteins from plasma.

The human hepatic asialoglycoprotein receptor is a target antigen for liver-infiltrating T cells in autoimmune chronic active hepatitis and primary biliary cirrhosis

Autoantibodies to the human hepatic asialoglycoprotein receptor have been found in nearly 50% of the sera of patients with autoimmune chronic active hepatitis and in 15% of patients with primary biliary cirrhosis. In this study we demonstrate that the human hepatic asialoglycoprotein receptor is also a target antigen for T cell-mediated immune responses. Peripheral blood lymphocytes of 37% (7 of 19) of patients with autoimmune chronic active hepatitis and 33% (2 of 6) of patients with primary biliary cirrhosis showed a proliferative response to highly purified human hepatic asialoglycoprotein receptor, whereas no proliferation was found with peripheral blood lymphocytes of patients with chronic viral hepatitis (0 of 13) and healthy blood donors (0 of 4). Moreover, we isolated T-cell clones from liver biopsy samples of two patients with autoimmune chronic active hepatitis and two patients with peripheral blood lymphocytes. Between 2.8% and 14.3% of these clones showed a specific proliferative response to purified human hepatic asialoglycoprotein receptor. The response was restricted to autologous antigen-presenting cells and could be blocked by monoclonal antibodies against human leukocyte antigen-DR molecules. The response of T cells to the human hepatic asialoglycoprotein receptor did not require the lectinlike activity of the asialoglycoprotein receptor. Thus the human hepatic asialoglycoprotein receptor could be identified as a major target antigen of humoral and cellular immune reactions in autoimmune-mediated liver diseases.

Autoantibodies to human asialoglycoprotein receptor in autoimmune-type chronic hepatitis

Autoantibodies to the human asialoglycoprotein receptor (anti-h-ASGPR) were studied with a solid-phase ELISA in the sera of 421 patients with inflammatory liver diseases, 288 patients with various other disorders and 31 controls. Anti-h-ASGPR were found predominantly in autoimmune chronic active hepatitis (44 of 88, 50%) and were closely related to inflammatory activity. In a subpopulation of these patients with untreated, biopsy-proven active disease or relapse, 15 of 17 were positive (88%). In contrast, only 11 of 204 patients (5.3%) with viral hepatitis were anti-h-ASGPR receptors-positive (chi 2 analysis; p less than 0.001). We also compared the occurrence of anti-h-ASGPR with antibodies to rabbit and rat asialoglycoprotein receptors in 352 sera. In contrast to the anti-human asialoglycoprotein receptor antibodies (3 of 107), anti-rabbit- or anti-rat-asialoglycoprotein receptor antibodies were found in 21 and 28 of 107 cases of viral hepatitis, indicating that different epitopes were recognized by these sera. In various other diseases anti-human asialoglycoprotein receptor antibodies were rarely found. Some sera from patients with connective-tissue diseases (8 of 73) and primary or secondary liver malignancies (6 of 55) exhibited anti-h-ASGPR. In autoimmune chronic active hepatitis the presence of anti-human asialoglycoprotein receptors did not correlate to other established autoantibody systems. Thus we conclude that anti-human asialoglycoprotein receptor antibodies can serve as diagnostic markers for inflammatory active cases of autoimmune chronic active hepatitis. Immune reactions to the asialoglycoprotein receptor, which is expressed on the hepatocellular membrane as a liver-specific antigen, might contribute to the pathogenesis of autoimmune chronic active hepatitis.

99mTc-neoglycoalbumin (NGA)-binding to human hepatic binding protein (HBP) in vitro

1 Neoglycoalbumin (NGA) was synthesised by covalent coupling of 2-imino-2-methoxyethyl-1-thio-beta-D-galactopyranoside (IME-thiogalactose) to the primary amino groups of human serum albumin (HSA). NGA was purified by ultrafiltration and size exclusion h.p.l.c. (SEC). 99mTc-labelling was performed with and without SEC purification. 2 Estimation of 99mTc-NGA-binding to human hepatic binding protein (HBP) revealed a complex behaviour indicating saturable high- and low-affinity sites. The high-affinity binding capacity was 1.1 +/- 0.4 pmol mg-1 human liver plasma membrane protein, the low-affinity binding capacity was 6.2 +/- 1.8 pmol mg-1 liver plasma membrane protein. The apparent equilibrium dissociation constants were 2.4 +/- 1.2 and 18.4 +/- 4.8 nM, respectively. 3 Specific binding of 99mTc-NGA to human HBP in the presence of 100 microM unlabelled NGA, Ca++ and Mg++ at pH 7.5 and 37 degrees C reached 85 +/- 5% at equilibrium. The amount of ligand specifically bound increased with the amount of human liver membrane protein added. The concentration of unlabelled agonist necessary to displace 50% of ligand bound amounted to 100 nM.

Diacytosis of asialoglycoprotein in isolated hepatocytes is dependent on the structure of ligand and cellular distribution of the receptors

Diacytosis, degradation and retention of 125I-labeled asialoorosomucoid (ASOR), its reduced and carboxymethylated N-terminal cyanogen bromide-cleaved fragment (RC-ASCNBr-I) and asialofetuin preloaded into isolated rat or rabbit hepatocytes for various periods of time were compared. In rat hepatocytes preloaded with a saturating concentration (3.10(-8) M) of the ligands, the proportion of the preloaded ligands distributed to degradation and diacytosis was fairly constant during 4 h of preincubation. In addition, a small portion of the preloaded ligands was neither diacytosed nor degraded, but was retained intracellularly. Diacytosis of 125I-ASOR (29%) was greater than that of either 125I-RC-ASCNBr-I (23%) or 125I-asialofetuin (15%). Diacytosis of 125I-ASOR, when preloaded in the presence of 5 microM colchicine, was significantly enhanced by 79% (increasing from 29% to 52%), whereas those of 125I-RC-ASCNBr-I and 125I-asialofetuin were not significantly altered (with average increases of 14% and 19%, respectively). The fraction of the preloaded 125I-asialofetuin (69%) and 125I-RC-ASCNBr-I (68.6%) that was degraded was slightly higher than that of 125I-ASOR (64%) and all was decreased by colchicine. The fraction of all three ligands retained by the cells was increased 2- to 4-fold by colchicine. The extents of retention of 125I-asialofetuin and 125I-ASCNBr-I were greater than that of 125I-ASOR, particularly after preloaded for more than 2 h. Preloading of the cells with ligands at a non-saturating concentration (6.5.10(-10) M) did not change these patterns of ligand distribution. Conjugation of diphtheria toxin fragment A (DTA) to ASOR or RC-ASCNBr-I also did not significantly alter the pattern of ligand distribution. In rabbit hepatocytes containing more asialoglycoprotein receptors than rat cells, 125I-ASOR was diacytosed to a greater extent (50%, -colchicine; 61%, + cholchicine) but degraded to a lesser extent (33%, -colchicine; 13%, + colchicine) than was observed in rat cells. The extent of retention of 125I-ASOR in rabbit cells was also greater than that in rat cells. A similar pattern of differences between rabbit and rat hepatocytes was observed for 125I-DTA-ASOR. These results indicate that intracellular sorting of internalized asialglycoproteins between diacytosis and degradation is dependent on both the structure of the ligand and the distribution of the cellular receptors.

Rapid intramolecular turnover of N-linked glycans in plasma membrane glycoproteins. Extension of intramolecular turnover to the core sugars in plasma membrane glycoproteins of hepatoma

Plasma membrane glycoproteins of rat hepatocytes undergo a rapid terminal deglycosylation in that the terminal sugars of the oligosaccharide side chains are rapidly removed from the otherwise intact glycoproteins [Tauber, R., Park, C.S. & Reutter, W. (1983) Proc. Natl Acad. Sci. USA 80, 4026-4029]. The present paper demonstrates that this rapid intramolecular turnover of plasma membrane glycoproteins is not restricted to peripheral sugars but, in contrast to liver, in hepatoma the core sugars of the oligosaccharide chains are also involved. Intramolecular turnover was measured in Morris hepatoma 7777 in five plasma membrane glycoproteins with Mr of 85,000 (hgp85), 105,000 (hgp105), 115,000 (hgp115), 125,000 (hgp125), 175,000 (hgp175) (hgp = hepatoma glycoprotein) that were isolated and purified to homogeneity by concanavalin-A--Sepharose affinity chromatography and semipreparative SDS gel electrophoresis. Analysis of the carbohydrates of hgp85, hgp105, hgp115 and hgp125 revealed the presence of N-linked oligosaccharides containing L-fucose, D-galactose, D-mannose and N-acetyl-D-glucosamine, but only of trace amounts of N-acetyl-D-galactosamine; hgp175 additionally contained significant amounts of N-acetyl-D-galactosamine, indicating the presence of both N- and O-linked oligosaccharides. As shown by digestion with endoglucosaminidase H, the N-linked oligosaccharides of hgp105, hgp115, hgp125 and hgp175 were of the complex type, whereas hgp85 also contained oligosaccharides of the high-mannose type. Half-lives of the turnover of the oligosacharide chains and of the protein backbone of the five glycoproteins were measured in the plasma membrane in pulse-chase experiments in vivo, using L-[3H]fucose as a marker of terminal sugars, D-[3H]mannose as marker of a core sugar and L-[3H]leucine for labelling the protein backbone. Protein backbones of the five glycoproteins were degraded with individual half-lives ranging over 41-90 h with a mean of 66 h. Compared to the degradation of the polypeptide backbone, both the terminal sugar L-fucose and the core sugar D-mannose turned over with much shorter half-lives averaging about 20 h in the five glycoproteins. The data show that, conversely to liver, within plasma membrane glycoproteins of hepatoma not only peripheral sugars but also core sugars of the oligosaccharides are split off during the life-span of the protein backbone. It may therefore be assumed that this reprocessing of plasma membrane glycoproteins is sensitive to malignant transformation.

Decreased intramolecular turnover of L-fucose in membrane glycoproteins of rat liver during liver regeneration

In plasma membrane glycoproteins of rat liver L-fucose undergoes a rapid intramolecular turnover in that fucose residues are removed from the glycoproteins (Tauber, R., Park, C.S. & Reutter, W. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 4026-4029). The present paper demonstrates that the intramolecular turnover of L-fucose is markedly decreased during liver regeneration. Turnover half-lives of L-fucose were measured in regenerating liver by pulse-chase experiments in five plasma membrane glycoproteins (Mr 60,000 (gp60), 80,000 (gp80), 120,000 (gp120), 140,000 (gp140), and 160,000 (gp160). The glycoproteins were isolated from plasma membranes by concanavalin A-Sepharose affinity chromatography and semipreparative NaDodSO4 polyacrylamide gel electrophoresis. L-Fucose turned over in the five glycoproteins with heterogeneous half-lives ranging from 22 h (gp160) to 49 h (gp120). The protein moieties of the glycoproteins were degraded with half-lives ranging from 56 h (gp80) to 107 h (gp140). Relative to the half-life of the protein backbone the half-live of L-fucose was increased in the five membrane glycoproteins by 70% (gp60), 150% (gp80), 182% (gp120), 60% (gp140) and 16% (gp160) during liver regeneration when compared to normal liver. The data show that L-fucose turns over in different membrane glycoproteins with individual rates, and that loss of L-fucose from plasma membrane glycoproteins is reduced in rapidly proliferating liver after partial hepatectomy.

Carcinoma autoantigens T and Tn and their cleavage products interact with Gal/GalNAc-specific receptors on rat Kupffer cells and hepatocytes

We studied interactions of isolated Thomsen-Friedenreich (T)- and Tn-specific glycoproteins with the Gal/GalNAc-specific receptors on rat Kupffer cells and compared them to those with rat hepatocytes. Immunoreactive T and Tn are specific pancarcinoma epitopes. Electron microscopy of gold-labelled T and Tn antigens revealed their specific binding to Kupffer cells, followed by their uptake via the coated pit/vesicle pathway of receptor-mediated endocytosis. Preincubation of Kupffer cells with GalNAc and GalNAc-BSA, but not GlcNAc or GlcNAc-BSA specifically inhibited binding of the T and Tn glycoproteins. Desialylated, isologous erythrocytes (T RBC) are known to bind to the Gal/GalNAc receptors of rat Kupffer cells and hepatocytes. This attachment was specifically inhibited by T and Tn in a concentration-dependent manner: 50% T RBC-Kupffer cell contacts were inhibited at 8.5.10(-6) mM T and 8.5.10(-5) mM Tn antigen concentrations, respectively. The corresponding figures for hepatocytes were 6.10(-6) mM T and 1.2.10(-6) mM Tn antigen. Amino-terminal cleavage products of the T glycoprotein, possessing clusters terminating in non-reducing Gal/GalNAc, inhibited T RBC binding to Kupffer cells and hepatocytes usually at 10(-2) to 10(-5) mM concentrations, whereas GalNAc, galactose and galactose glycosides inhibited at millimolar concentrations. Galactose-unrelated carbohydrates were inactive at concentrations greater than or equal to 50 mM.

Isolation and some properties of mammalian hepatic membrane lectins

Membrane lectins were isolated from sheep, goat, and buffalo liver by chromatography on an asialofetuin (ASF)-Sepharose 4B column. The lectins moved as a single protein band in SDS-PAGE with molecular masses of 42, 54 and 50 kDa, respectively, for sheep, goat and buffalo lectins. The molecular masses remained unchanged in 0.2 M 2-mercaptoethanol. As judged from the inhibition of binding of the lectin to ASF gel, the three lectins were beta-galactoside-specific. Sheep, goat and buffalo lectins were found to be sialoglycoproteins containing 18.6, 27 and 38.8 mol/mol lectin of neutral hexose, respectively; the corresponding values for the sialic acid content being 5.3, 8.7 and 11.8 mol/mol lectin. Thus goat and buffalo lectins are physico-chemically different from many mammalian hepatic lectins described so far.

The cyanogen bromide fragment I of asialoorosomucoid is transported more efficiently than asialoorosomucoid in rat hepatocytes

Cultured rat hepatocytes internalized and degraded 125I-labeled asialoorosomucoid (125I-ASOR) through asialoglycoprotein receptor at rates about half that of its cyanogen bromide fragment I (125I-ASCNBr-I). Reduction and carboxymethylation of the fragment resulted in decreased rates of internalization and degradation which were still greater than those of 125I-ASOR. In the presence of 5 microM colchicine, degradation of all three ligands was inhibited. However, the intracellular level of 125I-ASOR at steady state remained unchanged, while those of the fragments increased continuously. Study of the binding of these ligands to hepatocytes at 4 degrees C indicated that there was no significant difference in binding parameters between ASOR, ASCNBr-I and RC-ASCNBr-I (reduced and carboxymet ASCNBr-I). Studies of the fate of these ligands preloaded in the cell at 37 degrees C indicated that a higher fraction of the internalized ASOR than of the fragments was released by diacytosis. In contrast to ASOR, diacytosis of the fragments was not enhanced by colchicine. Studies of the distribution of intracellular ligands by Percoll density gradient centrifugation indicated that they were internalized initially into two early endosomal compartments of d = 1.037 g/ml and d = 1.045 g/ml. In the presence of colchicine, accumulation of the ligands in a third endosomal compartment of d = 1.08-1.095 g/ml was revealed, while in the presence of leupeptin accumulation of the ligands in lysosomes was observed. The results of a kinetic analysis indicated that both cyanogen bromide fragments were transported to all these compartments more rapidly than was ASOR. It appears that they are internalized and degraded more rapidly than ASOR due to a more efficient sorting of the internalized ligand into the pathway of lysosomal degradation.

Effects of partial hepatectomy on microtubules and hepatocellular transport of indocyanine green in rats

The effects of partial hepatectomy on plasma disappearance and biliary excretion of indocyanine green (ICG) have been studied in rats and correlated with morphometric changes of hepatocellular microtubules. The plasma disappearance rate of ICG was in good accord with recovery of liver weight after partial hepatectomy. Biliary excretion of ICG per 100 g liver significantly increased between 3 h and 7 days postoperatively. Colchicine significantly reduced plasma disappearance and biliary excretion of ICG, with no reduction in bile flow, in both intact and hepatectomized rats. Morphometrically, microtubules significantly increased from 3 h following partial hepatectomy and reached a maximum at 24 h with a gradual return to preoperative values at 5 days. These observations suggest that the increased hepatocellular transport of ICG after partial hepatectomy is related to an increase in the number of microtubules.

Structural similarity between the macrophage lectin specific for galactose/N-acetylgalactosamine and the hepatic asialoglycoprotein binding protein

The rat peritoneal macrophage lectin specific for galactose/N-acetylgalactosamine was shown to be a homologue of the hepatic asialoglycoprotein binding protein (rat hepatic lectin, RHL). The macrophage lectin was immunochemically crossreactive with the major form of RHL (RHL-1) but not with the minor forms (RHL-2 and -3). The overall homology between the macrophage lectin and RHL-1 was confirmed by peptide maps of their lysyl endopeptidase digests on reverse-phase HPLC. Despite these similarities, however, the macrophage lectin was distinct from HRL-1 as revealed by the differences in the NH2-terminal 20 amino acid sequences of these two lectins.

Hepatic uptake of asialoglycoprotein is different among mammalian species due to different receptor distribution

Isolated hepatocytes of rat, rabbit and guinea pig were found to take up and degrade 125I-labelled asialoorosomucoid at different rates with the rank order: rabbit greater than rat greater than guinea pig. Measurement of 125I-asialoorosomucoid binding at 4 degrees C to these hepatocytes revealed that all these cells had two classes of receptors with a major difference occurring in the number of high-affinity binding sites. The average binding affinity constants (K) and receptor concentration (N) calculated from a least-square analysis of the Scatchard plots were K1 = 1.15.10(9) M-1, K2 = 0.93.10(7) M-1, N1 = 0.049 pmol/mg cell protein and N2 = 0.27 pmol/mg cell protein for the rat; K2 = 3.16.10(7) M-1, N1 = 0.027 pmol/mg cell protein and N2 = 0.13 pmol/mg cell protein for the guinea pig and K1 = 0.74.10(9) M-1, K2 = 3.85.10(7) M-1, N1 = 0.205 pmol/mg cell protein and N2 = 0.37 pmol/mg cell protein for the rabbit hepatocytes, respectively. Measurement of the total number of cellular receptors after solubilization with Triton X-100 also revealed the same receptor concentration rank order of rabbit (5.8 pmol/mg cell protein) greater than rat (0.55 pmol/mg cell protein) greater than guinea pig (0.18 pmol/mg cell protein). Intravenous injection of 125I-asialoorosomucoid into anesthetized animals of matched body weight also indicated that the rate of plasma clearance and the rate of appearance of the degraded product of the tracer were different among these species with the same rank order as that observed with isolated hepatocytes. Thus there is a fundamental difference in the number of asialoglycoprotein receptors both on the cell surface and inside hepatocytes of these mammalian species.

Identification and characterization of a murine receptor for galactose-terminated glycoproteins

The asialoglycoprotein receptor, the hepatic binding lectin for galactose-terminated glycoproteins, has been isolated and characterized from human, rabbit and rat liver. Several recent studies have shown the existence of the same receptor in murine liver. However, the biochemical structure of the receptor in murine liver has not been resolved. In this paper, we describe the identification and purification of the receptor for asialoglycoproteins from murine liver. The purified receptor has three polypeptides with apparent molecular weights of 42,000, 45,000 and 51,000, respectively, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Furthermore, our studies suggest that the receptor from murine liver is very similar to its counterpart in rat liver, although some potential interesting differences have also been observed. Initial studies indicate that this receptor is well conserved in different mouse strains.

Drug delivery using vesicles targeted to the hepatic asialoglycoprotein receptor

We assessed the utility of liver-targeted vesicles as a drug delivery system for the treatment of liver diseases. Small, unilamellar vesicles (mean diameter, 60-80 nm) composed of dipalmitoylphosphatidylcholine, cholesterol, dipalmitoylphosphatidylglycerol and digalactosyldiacylglycerol (mol ratios, 40:40:5:15) are rapidly cleared from the blood in rats after intravenous injection. In vivo organ distribution shows that the liver is the major site of vesicle accumulation, with roughly 60-80% of the vesicle contents delivered to the liver. Isolated, perfused rat liver experiments show that the uptake is due to the hepatic asialoglycoprotein receptor, and the uptake process occurs with minimal vesicle leakage. At low doses of the vesicles, the single pass extraction by the liver is around 50%, which means that this vesicle formulation operates close to optimal efficiency as a drug delivery system to the liver. Binding of vesicles to the liver was determined to saturate at 6.5 mg total lipid/kg body weight, with a maximum steady-state turnover rate of vesicles at 37 degrees C of 79 micrograms lipid/min per kg body weight. This gives a receptor recycling time of around 80 min. We have incorporated this information into a pharmacokinetic model of vesicle distribution which quantitatively predicts the kinetics and dose dependence of vesicle uptake by the liver in vivo. This information can be used to optimize vesicle-mediated drug delivery to the liver.

Effect of colchicine and phalloidin on the distribution of three plasma membrane antigens in rat hepatocytes: comparison with bile duct ligation

The hepatocyte plasma membrane presents a morphological and functional regionalization into three domains: the sinusoidal; the lateral, and the canalicular. The mechanisms responsible for the biogenesis and maintenance of this regionalization are poorly understood. In this work, we have used colchicine and phalloidin, two drugs known to interfere with the secretory processes in hepatocytes, to study whether they also affect the transport of membrane proteins. The localization of three plasma membrane antigens was studied by light and electron microscopy using monoclonal antibodies identifying either the sinusoidal (A39) or the lateral (B1) or the canalicular (B10) domains in normal hepatocytes. In rats injected with colchicine (0.25 mg per 100 gm), A39 moved from the sinusoidal membrane to the lateral and canalicular ones, whereas B10 was displaced from the canalicular to the sinusoidal and lateral membranes, resulting after 8 hr in an almost equal labeling of the three domains with both antibodies. In rats injected daily for 7 days with phalloidin (50 micrograms per 100 gm), A 39 became mainly localized on the bile canalicular membrane instead of the sinusoidal one; B10 predominated on the canalicular membrane as in controls but in places it labeled the sinusoidal and lateral domains as well. In bile duct-ligated rats studied for comparison for 4, 10 or 21 days, A39 and B10 localizations evolved as after phalloidin, but the changes were more marked. B1 was not affected by any of the treatments. In conclusion, colchicine, phalloidin and bile duct ligation do not seem to hinder the antigens in reaching the plasma membrane, but induce a redistribution of two of them, suggesting a disturbance in the biogenesis and/or control of the plasma membrane regionalization. Such an abnormal distribution could be involved in--or contribute to--the initiation of cholestasis.

Monoclonal antibodies identifying antigens on distinct domains of rat hepatocytes

To aid in evaluating the functional significance of various domains of the hepatocyte plasma membrane, we have developed monoclonal antibodies that react with three different antigens on the surface of rat hepatocytes. One antigen is present on the sinusoidal-lateral plasma membrane but is absent from the bile canalicular membrane. On the basis of apparent molecular weight and ability to bind a desialylated glycoprotein, this antigen may be the asialoglycoprotein receptor. It was also identified immunocytochemically on the sinusoidal-lateral plasma membrane of human hepatocytes. The second antigen is present only on the bile canalicular membrane, whereas the third is present on the entire cell surface of rat hepatocytes. Our monoclonal antibodies may be useful in investigations of the sorting mechanisms of hepatocyte membrane proteins and the pathogenesis of certain liver diseases.

Transformation-related changes in the expression of endogenous cell lectins

Endogenous lectins purified from UV-2237-IP3 murine fibrosarcoma cells by affinity chromatography consisted of two polypeptide species of Mr 14,500 (L-14.5) and Mr 34,000 (L-34). Antibodies against this material immunoprecipitated the lectins from cells radiolabelled on the cell surface as well as in the cytoplasmic compartment. Similar analyses of normal rat embryonal fibroblasts revealed the presence of only the L-14.5. In contrast, both L-14.5 and L-34 were found in oncogene-transfected, immortalized cell clones derived from the normal rat cells, as well as in untransformed BALB/c-3T3 clone A31 and its descendant subclones selected for expression of the transformed, the tumorigenic and the metastatic phenotypes. Among the cells constituting the latter system, a marked increase in the amount of cellular and cell-surface lectins was observed upon progression to the metastatic phenotype. These results suggest that the expression of endogenous tumor-cell-surface lectins is associated with transformation and metastasis.

Preparation and disposition of asialofetuin-labelled liposome

A liposome labelled with asialofetuin(AF-liposome) was prepared by the detergent dialysis method. Asialofetuin in the amount of 34-68 micrograms were imposed among the 1 microM of phospholipids constituting the outer surface of liposome. The AF-liposome containing [14C]-cholesterol in the lipid layer and [3H]-sucrose in the aqueous layer was intravenously administered to rats. Both radioactivities were biphasically cleared from systemic circulation at rates faster than that of the fetuin-labelled liposome. These radioactivities were recovered in the liver by about 90 per cent within 60 min, and per gram of tissue, were highest in this organ. In an in vitro experiment using isolated rat hepatocytes, the high affinity of the AF-liposome toward hepatocytes was observed from scanning electron micrographs. Based on these findings, the AF-liposome may possibly be stable in circulation and function as a carrier of encapsulated drugs to hepatocytes.

Isolation and characterization of a receptor lectin specific for galactose/N-acetylgalactosamine from macrophages

Rat-peritoneal macrophages are shown to be able to take up glycoproteins terminated by galactose as well as those by mannose and/or N-acetylglucosamine. A lectin responsible for the uptake of galactose-terminated glycoproteins was isolated by affinity chromatography on a column of Sepharose 4B-asialoorosomucoid. The macrophage lectin isolated shared many properties in common with the well-established hepatic lectin specific for galactose/N-acetylgalactosamine. Thus, the lectin bound to asialoglycoproteins specifically, and the binding was inhibited by galactose and N-acetylgalactosamine. The lectin had a single major component of mol. wt. 42,000 as well as two minor components of 60,000 and 65,000, and required calcium for binding. In addition, the macrophage lectin was immunologically crossreactive with the hepatic lectin. Despite these similarities, however, the macrophage lectin was differentiated from the hepatic lectin in molecular size, relative preponderance of minor components, and titration profile with the antibodies raised against the hepatic galactose/N-acetylgalactosamine-specific lectin.

Liver endothelium mediates the uptake of iron-transferrin complex by hepatocytes

We have previously shown that in the liver, transferrin (TF) receptors are limited to endothelial cells, and hepatocytes and Kupffer cells do not have TF receptors. To study the transport of iron into hepatocytes, we fractionated liver cell suspensions into endothelium and hepatocyte fractions. At 4 degrees C liver (but not umbilical cord) endothelium bound Fe-TF with a saturable kinetics. At 37 degrees C, the endothelial uptake was followed by its gradual release. Transendothelial transport of TF was visually demonstrated by perfusion of liver using colloidal gold-labeled TF. The released Fe-TF acquired the potential for binding to fresh target hepatocytes and binding was not inhibited by excess cold TF but was inhibitable by asialofetuin, suggesting galactosyl receptors and not TF receptors as a recognition mechanism. Isoelectrofocusing of the supernate after preincubation for 90 min at 37 degrees C with endothelial cells, demonstrated the presence of a newly generated band which co-migrated with asialotransferrin. We conclude that Fe-TF is initially removed by liver endothelium where it is modified probably by desialation to expose the galactosyl residues of the glycoproteins. The modified molecule is subsequently released and recognized by hepatocytes through a TF receptor-independent mechanism which may involve galactosyl receptors of hepatocytes. The findings indicate a key role for endothelium in the transport of Fe-TF into the liver and may suggest a physiological function for galactosyl receptors on hepatocyte surface.

Development of a monoclonal antibody identifying an antigen which is segregated to the sinusoidal and lateral plasma membranes of rat hepatocytes

We developed a monoclonal antibody to the plasma membrane of rat hepatocytes. Immunoelectron microscopic characterization of an antigen identified by our antibody revealed that the antigen was present diffusely on the sinusoidal and lateral plasma membranes of hepatocytes but absent from the bile canalicular membrane. Sinusoidal lining cells (Kupffer cells and endothelial cells) were devoid of the antigen. Within hepatocytes, the antigen was present in the Golgi complexes, segments of endoplasmic reticulum and small vesicle-like structures. The development of the monoclonal antibody to the segregated membrane antigen of the hepatocyte in this study provides a reliable marker of specific membrane domains for use of isolation of plasma membrane surfaces and is a useful tool for investigation of the transferring mechanisms of membrane proteins to their destinations.