The hepatic asialoglycoprotein receptor

Electron microscopic evidence for endocytosis of superoxide dismutase by hepatocytes using protein-gold adducts

Cu, Zn superoxide dismutase, an intracellular, carbohydrate-free protein, was found to bind to hepatocytes and endothelial cells by electron microscopy of liver treated with enzyme-gold conjugates. In cultured hepatocytes, the free enzyme, but not bovine serum albumin, competed for the binding, and at 37 degrees C, the gold-protein complexes were internalized in a typical process of receptor-mediated endocytosis.

Differential effects of leupeptin, monensin and colchicine on ligand degradation mediated by the two asialoglycoprotein receptor pathways in isolated rat hepatocytes

We have shown that degradation of asialo-orosomucoid (ASOR) in isolated rat hepatocytes occurs by two different intracellular pathways [Clarke, Oka & Weigel (1987) J. Biol. Chem. 262, 17384-17392] mediated by two subpopulations of cell surface galactosyl (Gal) receptors, designated State 1 or State 2 receptors. In the present study, several inhibitors were tested for their effects on ligand degradation by the State 1 or State 2 pathway. Leupeptin, monensin and chloroquine completely inhibited degradation of 125I-labelled ASOR in both pathways. Dose-response studies showed, however, that the State 2 pathway was more sensitive to leupeptin or monensin than the State 1 pathway. No differences were observed with chloroquine. For example, the onset of inhibition in the State 2 and State 1 pathways occurred at about 0.05 and 0.3 microM-leupeptin respectively, a 6-fold difference. At 3.5 microM-monensin, 125I-ASOR degradation in the State 2 pathway was completely blocked, whereas degradation in the State 1 pathway was essentially unaffected. Colchicine was observed to give the largest differential sensitivity between the two pathways. The State 2 degradation pathway was about 30-fold more sensitive to colchicine than the State 1 pathway. Lumicolchicine had no affect. The onset of inhibition of the rate of 125I-ASOR degradation in the State 2 and State 1 pathways occurred at approximately 0.1 and 3.0 microM-colchicine respectively. At very high concentrations (greater than 0.1 mM), the State 1 pathway could be completely inhibited. We conclude that intracellular 125I-ASOR processing or delivery to degradative compartments in both the State 1 and State 2 Gal receptor pathways requires low pH. Ligand delivery to the degradative compartment does not require microtubules in the State 1 pathway, consistent with the very rapid onset of degradation in this pathway. The State 2 degradation pathway does require microtubules.

Immunohistochemical demonstration of the asialoglycoprotein receptor in rat liver by a sensitive avidin-biotin technique

An immunohistochemical technique for the detection of the hepatic asialoglycoprotein receptor (ASGP-R) in cryostat sections of liver by polyclonal and monoclonal anti-ASGP-R antibodies is described. The procedure is based on the alkaline-phosphatase-avidin-biotin complex (ABC-AP) system and important features include fixation of the sections with periodate-lysine-paraformaldehyde (with or without dichromate) and an absolute requirement for blocking of endogenous biotin activity. The sensitivity of the technique is such that binding to ASGP-R can be detected with femtomolar concentrations of monoclonal anti-ASGP-R antibodies and, with polyclonal antisera, approaches that of a radioimmunoassay.

Relations between the intracellular pathways of the receptors for transferrin, asialoglycoprotein, and mannose 6-phosphate in human hepatoma cells

We compared the intracellular pathways of the transferrin receptor (TfR) with those of the asialoglycoprotein receptor (ASGPR) and the cation-independent mannose 6-phosphate receptor (MPR)/insulin-like growth factor II receptor during endocytosis in Hep G2 cells. Cells were allowed to endocytose a conjugate of horseradish peroxidase and transferrin (Tf/HRP) via the TfR system. Postnuclear supernatants of homogenized cells were incubated with 3,3'-diaminobenzidine (DAB) and H2O2. Peroxidase-catalyzed oxidation of DAB within Tf/HRP-containing endosomes cross-linked their contents to DAB polymer. The cross-linking efficiency was dependent on the intravesicular Tf/HRP concentration. The loss of detectable receptors from samples of cell homogenates treated with DAB/H2O2 was used as a measure of colocalization with Tf/HRP. To compare the distribution of internalized plasma membrane receptors with Tf/HRP, cells were first surface-labeled with 125I at 0 degrees C. After uptake of surface 125I-labeled receptors at 37 degrees C in the presence of Tf/HRP, proteinase K was used at 0 degrees C to remove receptors remaining at the plasma membrane. Endocytosed receptors were isolated by means of immunoprecipitation. 125I-TfR and 125I-ASGPR were not sorted from endocytosed Tf/HRP. 125I-MPR initially also resided in Tf/HRP-containing compartments, however 70% was sorted from the Tf/HRP pathway between 20 and 45 min after uptake. To study the accessibility of total intracellular receptor pools to endocytosed Tf/HRP, nonlabeled cells were used, and the receptors were detected by means of Western blotting. The entire intracellular TfR population, but only 70 and 50% of ASGPR and MPR, respectively, were accessible to endocytosed Tf/HRP. These steady-state levels were reached by 10 min of continuous Tf/HRP uptake at 37 degrees C. We conclude that 30% of the intracellular ASGPR pool is not involved in endocytosis (i.e., is silent). Double-labeling immunoelectron microscopy on DAB-labeled cells showed a considerable pool of ASGPR in secretory albumin-positive, Tf/HRP-negative, trans-Golgi reticulum. We suggest that this pool represents the silent ASGPR that has been biochemically determined. A model of receptor transport routes is presented and discussed.

Decreased number of asialoglycoprotein receptors in diabetic BB Wistar rat

The binding of asialoglycoproteins by hepatic binding protein was studied in freshly isolated hepatocytes from genetically diabetic BB Wistar rats. The number of cell surface asialoglycoprotein receptors was dramatically decreased (58,000 +/- 38,000 for diabetic rats compared to 267,000 +/- 70,000 for normal rats), while the association equilibrium constant was not changed. These results parallel those obtained with streptozotocin-diabetic rats and support the hypothesis that insulin deprivation is responsible for the decrease in the receptor number.

Identification of receptors in the liver that mediate endocytosis of circulating tissue kallikreins

The liver plays an important role in the clearance, by receptor-mediated endocytosis, of circulating glycoproteins. It has been demonstrated that tissue kallikreins, which are acid glycoproteins, circulate in plasma, where they are poorly inhibited by plasma proteins. We have shown that the liver is the main organ that clears tissue kallikreins from the circulation. We now report the identification of receptors involved in this clearance. Using a perfused rat-liver system, and as models, pig pancreatic (PPK) and horse urinary (HoUK) kallikreins, we have found that: (a) the binding of PPK to the perfused liver was inhibited by 50 mM methyl alpha-D-mannoside and 20 microM mannan, was partially inhibited by 50 mM mannose and was unaffected by 1.5 microM asialofetuin; (b) binding of HoUK to the perfused liver was inhibited by 1.5 microM asialofetuin, 50 mM galactose and 50 mM lactose and was unaffected by 50 mM mannose; (c) the clearance rate of both kallikreins followed the equation y = a.xb; (d) their binding was Ca2+-dependent and their clearance was inhibited by 3 mM chloroquine and 10 mM methylamine. Using isolated liver cells and tritiated HoUK, we calculated that 500,000 receptors/cell were present and the Scatchard plot showed that there were two apparent affinity constants: 0.24.10(9) l/M) (high-affinity) and 0.3.10(8) l/M (low-affinity). These results show that PPK is recognized by a liver mannose receptor and HoUK by the galactose receptor. The liver uptake of native and circulating tissue kallikreins thus emerges as a mechanism by which their levels in plasma are regulated.

Receptor-mediated endocytosis of asialoglycoproteins and diferric transferrin is independent of second messengers

The intracellular concentrations of cyclic AMP, polyphosphoinosides and free Ca2+ were unaffected during receptor-mediated endocytosis of the neoglycoprotein beta-D-galactosyl-bovine serum albumin (D-Gal-BSA) by isolated hepatocytes. Elevation of either intracellular cyclic AMP by glucagon or inositol phosphates and Ca2+ by vasopressin were without effect on the binding and internalization of D-Gal-BSA. The normal response of this cell to glucagon- and vasopressin-mediated mobilization of these second messengers was not modified in the presence of saturating concentrations of D-Gal-BSA. Receptor-mediated endocytosis of diferric transferrin (Fe3+-TRF) by both hepatocytes and HL60 cells was also shown to be independent of second messengers, although the unequivocal expression of the transferrin receptor by hepatocytes could not be satisfactorily demonstrated. The results of the present study are at variance with a suggested regulatory role for second messengers in receptor-mediated endocytosis of serum-derived ligands such as asialoglycoproteins and Fe3+-TRF. Receptor phosphorylation by protein kinase C in particular has been proposed to regulate the distribution and recycling of these receptors in actively endocytosing cells. We would suggest that if receptor phosphorylation has a regulatory role during endocytosis, it is likely to be mediated by a second-messenger-independent protein kinase analogous to casein kinase II. An alternative interpretation is that phosphorylation has no physiological significance and receptor-mediated endocytosis is a constitutive event coupled to membrane turnover.

The two subunits of the human asialoglycoprotein receptor have different fates when expressed alone in fibroblasts

Two related polypeptides, H1 and H2, comprise the human asialoglycoprotein receptor (ASGP-R). Stable lines of murine NIH 3T3 fibroblasts expressing H1 alone or H2 alone do not bind or internalize the ligand asialoorosomucoid (ASOR), which contains triantennary oligosaccharides. In contrast, cells expressing H1 and H2 together bind and degrade ASOR with properties indistinguishable from those of the ASGP-R in human hepatoma HepG2 cells. Whether or not H2 is coexpressed, H1 is synthesized as a 40-kDa precursor bearing high-mannose oligosaccharides, processed to its mature 46-kDa form, and transported to the cell surface. In cells expressing only H1, homodimers and -trimers of H1 are formed. In contrast, when expressed in 3T3 cells without H1, H2 is synthesized as its 43-kDa precursor, bearing high-mannose oligosaccharides, but is rapidly degraded. When H1 and H2 are coexpressed in the same cell, the H1 polypeptide "rescues" the H2 polypeptide; H2 is processed to its characteristic 50-kDa mature form and is transported to the surface. We conclude that the human ASGP-R is a multichain heterooligomer, probably a trimer of H1 molecules in noncovalent association with one, two, or three H2 molecules, and that the two polypeptides normally interact early in biosynthesis.

Heterogeneous acinar localization of the asialoglycoprotein internalization system in rat hepatocytes

Desialylated glycoprotein is rapidly cleared from plasma by a receptor-mediated endocytic mechanism located on hepatocytes. We studied the hepatic acinar distribution of this asialoglycoprotein transport system with the ligand 125I-asialoorosomucoid using rat liver perfused in either antegrade or retrograde direction in combination with quantitative light microscopic autoradiography. Grain distribution along the acinus appeared dependent on the perfusion direction. A rather shallow zone 1 to zone 3 gradient was observed if livers were perfused in the normal direction. However, a statistically significantly steeper zone 3 to zone 1 gradient was detected in retrograde perfusions. Kinetic analysis of perfusate clearance profiles yielded a hepatic clearance of 21.6 +/- 1.3 ml per min in antegradely perfused liver. Hepatic extraction was calculated to be 60.1 +/- 7.4%. Biliary secretion of radioactivity amounted to 1.89 +/- 0.18% of the dose within 1 hr after injection and consisted of intact material (1.39 +/- 0.25%) and radioactive low-molecular-weight degradation products (0.52 +/- 0.08%), of which more than 90% could be accounted for by 125I-. Apart from a minor difference regarding biliary secretion of an unidentified glycopeptide (less than 0.1% of the injected dose), transport data for the retrogradely perfused livers were identical to those obtained with livers perfused in antegrade direction, emphasizing the functional equivalence of both groups of livers. The autoradiographic data indicate that zone 3 hepatocytes take up 125I-asialoorosomucoid more avidly than zone 1 cells. The kinetic and biochemical data indicate that further processing in the hepatocytes is virtually similar in the two zones.(ABSTRACT TRUNCATED AT 250 WORDS)

Lactosaminated Fab fragments specific for low density lipoproteins/hepatocyte targeting and hypolipoproteinemic activity

We have previously reported that Fab fragments of IgGs modified by lactosamination (lac-Fab) can direct macromolecules, including low density lipoproteins (LDL), to the liver. In the present paper we demonstrate that lac-Fab that is specific for LDL is an effective and selective hypolipoprotein agent. A plasma pool of about 60 mg/dl of apoprotein B (apo B) was induced in rats by bolus injection of human LDL (hLDL), which increased the cholesterol value to about 150 mg/dl. Three hours after injection of the highest dose of lac-Fab, the total cholesterol decreased to 80 mg/dl, compared to 120 mg/dl in control animals. Studies conducted with 131I-tyramine-cellobiose-labeled LDL indicated that the liver was the only organ in which lac-Fab increased LDL uptake and degradation. The effect of lac-Fab was dose-dependent. With amounts of lac-Fab between 13 to 42 mg/kg body weight, the amount of hLDL cleared through the lac-Fab mechanism ranged from 30% to 70% of the initial pool. Analysis of the plasma lipoprotein subfractions revealed that high density lipoprotein levels were not affected. Histologic examination of liver sections after sequential injection of fluorescently labeled hLDL and lac-Fab indicated specific uptake in the hepatocytes when compared to control sections obtained from animals injected with Dil-LDL alone. The uptake of fluorescent LDL induced by lac-Fab was completely prevented by a co-injection of an excess of asialofetuin. We conclude that lac-Fab that is specific for LDL is a selective hypolipoproteinemic agent and a specific carrier to the hepatocytes.

Effects of hyperosmolarity on ligand processing and receptor recycling in the hepatic galactosyl receptor system

Binding, endocytosis, and degradation of asialo-orosomucoid (ASOR) mediated by the galactosyl (Gal) receptor were examined in isolated rat hepatocytes in complete media supplemented with an osmolite. The specific binding of 125I-ASOR to cells at 4 degrees C was unaffected by up to 0.4 M sucrose or NaCl. Unlike sucrose or NaCl, mannitol stimulated 125I-ASOR binding at low concentrations but inhibited binding at higher concentrations. Continuous internalization at 37 degrees C, which requires receptor recycling, was completely blocked at 0.2 M sucrose or 0.15 M NaCl, corresponding in each case to a total osmolality of about 550 mmol/kg. This effect was reversed and endocytic function was restored by washing the cells, indicating that cell viability was unaffected. The rate of degradation of internalized 125I-ASOR was also inhibited by increasing sucrose concentrations. This inhibition is due to a block in the delivery of ligand to lysosomes and not an effect on degradation per se. In the presence of 0.2 M sucrose, the rate and extent of endocytosis of surface-bound 125I-ASOR were, respectively, 33.0 +/- 8.1% and 69.4 +/- 10.5% (n = 8) of the control without sucrose. Under these conditions, the dissociation of internalized receptor-ASOR complexes was completely inhibited. When sucrose was added, the effect on the endocytosis of surface-bound 125I-ASOR was virtually immediate. Previous studies showed that about 40% of the surface-bound 125I-ASOR which is internalized can return to the cell surface still bound to receptor (Weigel and Oka: J Biol Chem 259:1150, 1984). If 0.2 M sucrose was added after endocytosis occurred, 125I-ASOR still returned to the cell surface, although the rate and extent of return were inhibited by more than 50%. Interestingly, hyperosmolarity is the only treatment we have found which can reversibly inhibit, although only partially, the endocytosis of surface-bound 125I-ASOR.

Intracellular transport and processing of secretory component in cultured rat hepatocytes

Membrane secretory component (mSC) mediates the transcellular transport of polymeric immunoglobulin A from the sinusoidal surface of rat hepatocytes to the bile, where it is released as a proteolytic fragment, fSC. We have examined the biosynthesis, posttranslational processing, transport, and cleavage of secretory component in cultured rat hepatocytes. Membrane secretory component is detected at the cell surface beginning 1.0-1.5 h after synthesis, whereas fSC is not found in the medium until 2.5-3 h after synthesis. Approximately 16% of metabolically labeled mSC is accessible at the cell surface at 4 degrees C. Surface accessible mSC labeled with 125I at 4 degrees C is internalized with a half-time of less than 5 min after warming to 37 degrees C and begins to be released as fSC after 20 min at 37 degrees C. Posttranslational processing and cleavage of mSC by cultured hepatocytes yields products that appear to be identical to those produced in vivo. Although the kinetics of some of these events are significantly slower than those observed in vivo, the major fraction of mSC accessible at the surface of cultured hepatocytes is internalized before cleavage to fSC, as occurs with mSC present on the sinusoidal domain of hepatocytes in vivo. Cultured hepatocytes provide a suitable model system for the examination of mSC transport and cleavage to fSC.

Receptor-mediated endocytosis of enterokinase by rat liver. Preliminary characterisation of low-density endosomes

The endocytosis of enterokinase by rat hepatocytes has been studied both in a perfused liver system and in the intact, anaesthetised animal. 10 min after administration of the enzyme, only 70% of the activity was cleared by the perfused liver, whereas clearance was total in the intact animal. In both cases, about 85% of the internalised enzyme co-purified with the smooth microsomes and virtually all (more than 90%) of the catalytic activity was latent and could only be detected in the presence of detergent. After 10 min, 22.5% of the activity remained with the sinusoidal plasma membrane in the case of the perfused liver, while for the intact animal this figure was only 10%, confirming the more efficient clearance of enterokinase in the intact animal. Further subcellular fractionation showed that in the anaesthetised animal 8% of the internalised enzyme was associated with a low-density Golgi-like endosomal compartment (prepared from the mitochondrial pellet), whereas the corresponding value for the perfused liver was only 2.5%. Enterokinase specific activity was also up to 50-times greater in the low-density endosomes prepared from the intact animal. A second low-density Golgi-like compartment (purified from the smooth microsomes) also contained latent enterokinase, which together with the endosomes derived from the mitochondria accounted for 20% of the total enterokinase internalised by the liver 10 min after its administration to the intact animal. The passage of enterokinase through these two low-density compartments was shown not to be synchronous with its passage through the peripheral (sinusoidal membrane) and internal endosomes (smooth microsomes). There were qualitative differences in marker enzymes and polypeptide composition between the mitochondria and microsome-derived low-density endosomes. The sub-fractionation of low-density fractions on shallow sucrose gradients revealed a complex enzyme and polypeptide heterogeneity both between and within fractions. There was an apparent density-dependent separation of enterokinase from galactosyltransferase and the asialoglycoprotein receptor which was coincident with marked changes in the polypeptide composition of the endosomal membranes, particularly in the 30-45 kDa range.

Lactosylation of albumin reduces uptake rate of dibromosulfophthalein in perfused rat liver and dissociation rate from albumin in vitro

Two types of models have recently been proposed to describe hepatic uptake kinetics of protein bound drugs: a model in which dissociation from plasma protein is rate limiting the process, and a model in which an interaction between protein and hepatocyte surface is thought to promote dissociation and uptake of the drug. This study was designed to investigate several aspects of both models, using lactosylated albumin as a binding protein that can interact with the Ashwell receptor abundantly present on the hepatocyte. Dibromosulfophthalein clearance was studied in rat liver in the presence of 150 microM (1%) albumin or 150 microM lactosylated albumin. Initial disappearance rate from perfusate in the presence of lactosylated albumin indicated a 2-fold decrease in hepatic uptake rate compared with native albumin. This was confirmed by compartmental analysis, showing a similar decrease in hepatic uptake rate constant. Protein binding of dibromosulfophthalein to lactosylated albumin was only marginally different from normal albumin. Consequently, modification of the protein retarded uptake of the organic anion at an essentially unchanged unbound concentration. Fluorescence spectroscopy of lactosylated albumin showed a blue-shifted tryptophan emission spectrum compared with albumin, indicating increased hydrophobicity of the neoglycoprotein. We therefore considered a change in off-and-on rate for binding of dibromosulfophthalein in lactosylated albumin. Rapid filtration experiments indicated that the dissociation rate constant of dibromosulfophthalein from lactosylated albumin was half that of controls. We conclude that the decreased off-rate from lactosylated albumin can explain the retarding influence on hepatic uptake rate of dibromosulfophthalein. This observation argues for the concept of dissociation-limited uptake in the hepatic clearance of the organic anion.

Ligand- and weak base-induced redistribution of asialoglycoprotein receptors in hepatoma cells

The receptor for asialoglycoproteins (ASGPR) was localized in human hepatoma Hep G2 cells by means of quantitative immunoelectron microscopy. Without ligand added to the culture medium, we found 34% of the total cellular receptors on the plasma membrane, 37% in compartment of uncoupling receptor and ligand (CURL), and 21% in a trans-Golgi reticulum (TGR) that was defined by the presence of albumin after immuno-double labeling. A small percent of the ASGPR was associated with coated pits, the Golgi stacks, and lysosomes. After incubation of the cells with saturating concentrations of the ligand asialo-orosomucoid (ASOR), the number of cell surface receptors decreased to 20% of total cellular receptors, whereas the receptor content of CURL increased by a corresponding amount to 50%. The ASGPR content of TGR remained constant. In contrast, after treatment of the cells with 300 microM of the weak base primaquine (PMQ), cell surface ASGPR had decreased dramatically to only 4% of total cellular receptors whereas label in the TGR had increased to 42%. ASGPR labeling of CURL increased only to 47%. The labeling of other organelles remained unchanged. This affect of PMQ was independent of the presence of additional ASOR. Implications for the intracellular pathway of the ASGPR are discussed.

Sorting of endocytosed transferrin and asialoglycoprotein occurs immediately after internalization in HepG2 cells

After receptor-mediated uptake, asialoglycoproteins are routed to lysosomes, while transferrin is returned to the medium as apotransferrin. This sorting process was analyzed using 3,3'-diaminobenzidine (DAB) cytochemistry, followed by Percoll density gradient cell fractionation. A conjugate of asialoorosomucoid (ASOR) and horseradish peroxidase (HRP) was used as a ligand for the asialoglycoprotein receptor. Cells were incubated at 0 degree C in the presence of both 131I-transferrin and 125I-ASOR/HRP. Endocytosis of prebound 125I-ASOR/HRP and 131I-transferrin was monitored by cell fractionation on Percoll density gradients. Incubation of the cell homogenate in the presence of DAB and H2O2 before cell fractionation gave rise to a density shift of 125I-ASOR/HRP-containing vesicles due to HRP-catalyzed DAB polymerization. An identical change in density for 125I-transferrin and 125I-ASOR/HRP, induced by DAB cytochemistry, is taken as evidence for the concomitant presence of both ligands in the same compartment. At 37 degrees C, sorting of the two ligands occurred with a half-time of approximately 2 min, and was nearly completed within 10 min. The 125I-ASOR/HRP-induced shift of 131I-transferrin was completely dependent on the receptor-mediated uptake of 125I-ASOR/HRP in the same compartment. In the presence of a weak base (0.3 mM primaquine), the recycling of transferrin receptors was blocked. The cell surface transferrin receptor population was decreased within 6 min to 15% of its original size. DAB cytochemistry showed that sorting between endocytosed 131I-transferrin and 125I-ASOR/HRP was also blocked in the presence of primaquine. These results indicate that transferrin and asialoglycoprotein are taken up via the same compartments and that segregation of the transferrin-receptor complex and asialoglycoprotein occurs very efficiently soon after uptake.

Drug targeting to the liver with lactosylated albumins: does the glycoprotein target the drug or is the drug targeting the glycoprotein?

The isolated perfused rat liver preparation was employed to study hepatic disposition of the model drug-carrier conjugate fluorescein-lactosylated albumin (F-LnHSA) with special reference to the influence of the organic anion fluorescein on liver cell specificity of the endocytosed neoglycoprotein. Hepatic clearance of fluoresceinated neoglycoproteins was significantly faster than clearance of radioiodinated neoglycoproteins. Perfusate clearance of F-L7HSA and F-L25HSA could not completely be inhibited by a dose of 10 mg asialoorosomucoid that saturates the hepatocyte receptor-mediated endocytic process. From these data, we inferred an additional hepatic uptake mechanism, competing with the Ashwell-receptor-mediated internalization of galactose-terminated glycoproteins. Clearance experiments with fluoresceinated 125I-human serum albumin in the presence of the polyanionic probe dextran sulfate revealed a nearly complete (approximately 90%) inhibition of hepatic uptake, while also a pronounced effect was obtained with colloidal carbon. These data point to nonparenchymal cell uptake of fluoresceinated protein via interaction with scavenger receptors. In wash-out studies, it was shown that about 25% of ligand sequestrated by sinusoidal liver cells escaped degradation and recycled to the perfusion medium. Our results show that care should be taken in the use of neoglycoproteins as drug carriers to hepatocytes, since a load of only 2 to 3 moles fluorescein per mole neoglycoprotein considerably affects intrahepatic distribution. The relative contribution of nonparenchymal cell uptake by coupling of acidic drugs to the neoglycoproteins is very probably inversely related to the number of exposing galactose groups per molecule neoglycoprotein. This phenomenon of "inversed targeting" could therapeutically both be useful or detrimental, dependent on the spectrum of cell types that should be reached by the drug.

In vivo quantification of removal of asialo-orosomucoid from the circulation in anaesthetized streptozotocin-diabetic rats

The in vivo kinetic of removal of 3H asialo-orosomucoid from plasma was investigated in control and streptozotocin-diabetic rats after intravenous injection of 1 mg of asialo-orosomucoid/100 g body wt. Michaelis-Menten kinetics of disappearance were observed. In diabetic rats the maximal rate (Vmax) of disappearance of 3H asialo-orosomucoid was decreased by 30% with no modification of Michaelis constant. Since no accumulation of desialylated orosomucoid in the circulation was observed, the slower rate of removal of 3H asialo-orosomucoid was attributed to a decrease in the number of hepatic asialoglycoprotein receptors which are largely involved in the catabolism of asialoglycoproteins. Our estimate on in vivo maximal rates was 10- to 20-fold greater than our previous in vitro estimate of the maximal rate of endocytosis. In contrast, the values of the Michaelis constant obtained in vivo and in vitro were very similar.

A reassessment of the assay for the asialoglycoprotein receptor and its use in the quantification of receptor distribution in hepatocytes

The assay for the fucose-binding protein described by Lehrman & Hill [(1983) Methods Enzymol. 98, 309-319] was adapted for the measurement of the asialoglycoprotein receptor in rat liver. The amount of ligand bound to the plasma-membrane-associated or affinity-purified receptor was acutely sensitive to the concentrations of Triton X-100 and NaCl in the assay: 0.02% (v/v) Triton X-100 increased ligand binding to the two preparations by 100% and 40% respectively. Higher concentrations of detergent progressively decreased binding, and in 0.32% Triton X-100 it was about 30% of the value obtained in detergent-free buffer. The addition of increasing concentrations of NaCl to the assay progressively inhibited ligand binding to the membrane-associated receptor, whereas there was a 60% increase in binding to the pure receptor in the presence of 0.1-0.2 M-NaCl. These effects could not be identified in the original assay procedure described by Hudgin, Pricer, Ashwell, Stockert & Morell [(1974) J. Biol. Chem. 249, 5536-5543]. Using optimal assay conditions the binding of 125I-beta-D-galactosyl-bovine serum albumin to both the membrane-associated and purified receptor was inhibited by 50% by 1 nM-beta-D-galactosyl-bovine serum albumin and -asialoorosomucoid and by approx. 100 microM-beta-L-fucosyl-bovine serum albumin, whereas beta-D-galactose, lactose and beta-L-fucose had no effect on ligand binding up to concentrations of 1 mM, 500 microM and 5 mM respectively. KD values of 0.94 and 1.25 nM and Bmax. values of 40 and 1660 pmol of D-galactosyl-bovine serum albumin bound/mg of receptor were obtained for the membrane-bound and purified receptor respectively. Hill-plot analysis of the same data gave slopes of 0.96 and 1.01. Scatchard analysis of saturation-binding studies with other subcellular fractions indicated that the receptor was distributed in the proportions 72:23:2.5:2.5 between total microsomal fractions, plasma membrane, Golgi and canalicular membrane respectively. The receptor was about 1% of the total protein in each compartment and was estimated to be about 0.3% of the total liver protein.

An internal signal sequence: the asialoglycoprotein receptor membrane anchor

The human asialoglycoprotein receptor H1 is anchored in the membrane by a single stretch of 20 hydrophobic amino acids; the hydrophilic amino terminus faces the cytoplasm, and the carboxyl terminus is exoplasmic. We show here that glycosylation and insertion of the asialoglycoprotein receptor into the endoplasmic reticulum membrane is cotranslational and SRP-dependent and occurs without proteolytic cleavage. The membrane-anchor domain is necessary for membrane insertion, since a receptor with the segment deleted is neither inserted nor glycosylated. The segment is also sufficient for membrane insertion, since it will initiate translocation of a carboxy-terminal domain of rat alpha-tubulin across the membrane. We propose that a helical hairpin mechanism of membrane insertion is used both by cleaved amino-terminal and uncleaved internal signal sequences.

Golgi and secreted galactosyltransferase

Galactosyltransferase (GT) belongs to the glycosyltransferases. In several tissues and cell lines, the enzyme is localized by immunocytochemistry to the two to three trans cisternae of the Golgi complex and may thus be considered a specific membrane component of this type of endomembrane. As a consequence, it is the most common Golgi "marker" enzyme in cell fractionation studies. Study of its biosynthesis, membrane orientation, and turnover in several tissues and cultured cell lines has broadened our knowledge about Golgi function itself. The enzyme is oriented towards the lumen of the cisternal space. In this orientation, it catalyzes the transfer of galactose to glycoprotein-bound acetylglucosamine and, in the presence of alpha-lactalbumin, to glucose, as shown in the Golgi complex of mammary gland epithelial cells. The enzymatic properties of GT are well known. The metabolism of GT has been extensively studied in HeLa and human hepatoma cells. The enzyme is synthesized in the rough endoplasmic reticulum (RER) and provided with one N-linked oligosaccharide and palmitate residues. In the Golgi complex, terminal sugars are attached to the N-linked oligosaccharide and extensive O-glycosylation takes place. The half-life of the enzyme is about 20 hr, after which a soluble form appears in the culture medium. Release of GT into the medium is observed in all cell lines studied. This phenomenon is in accordance with the presence of soluble GT in body fluids such as serum, ascites, milk, and saliva. In patients suffering from ovarian and breast cancer, increased levels of GT enzyme activity have been reported. Whether extracellular GT is of biological significance is still a point of discussion.

Presence of asialoglycoprotein receptors in the Golgi complex in the absence of protein synthesis

In rat hepatocytes the Golgi complex contains a considerable amount of receptors for asialoglycoproteins (ASGP-R). To establish whether the presence of ASGP-R in the Golgi complex originate from de novo synthesis isolated rat hepatocytes were incubated with 100 micrograms/ml cycloheximide to stop protein synthesis. Provided that protein synthesis was completely inhibited by cycloheximide, uptake and degradation of ligand (asialo-orosomucoid) were unaffected. Also intracellular transport of newly synthesized proteins, as determined by monitoring biosynthesis and intracellular transport of albumin and ASGP-R, was not affected. After culturing the cells for 3.5 h in the presence of cycloheximide, no more albumin could be detected in the Golgi complex with immunofluorescence microscopy. However, immunocytochemical assessment showed that the ASGP-R was still in the Golgi complex. These results suggest that the Golgi complex contains a pool of ASGP-R which is independent of neosynthesis for several hours.

Intracellular transport of asialoglycoproteins in rat hepatocytes. Evidence for two subpopulations of lysosomes

The intracellular transport and degradation of asialoorosomucoid (AOM) in isolated rat hepatocytes was studied by means of subcellular fractionation in Nycodenz gradients. The asialoglycoprotein was labelled by covalent attachment of a radioiodinated tyramine-cellobiose adduct ( [125I]TC) which leads to labelled degradation products being trapped intracellularly and thus serving as markers for the degradative organelles. The ligand was initially (1 min) in a slowly sedimenting (small) vesicle and subsequently in larger endosomes. Acid-soluble, radioactive degradation products were first found in a relatively light lysosome whose distribution coincided in the gradient with that of the larger endosome. Later (30 min) degradation products were found in denser lysosomes which banded in the same region of the gradient as the lysosomal enzyme, beta-acetylglucosaminidase. Colchicine, monensin and leupeptin all inhibited degradation of [125I]tyramine-cellobiose asialoorosomucoid ( [125I]TC-AOM) and reduced the formation of degradation products in both the light and the dense lysosomes. In presence of monensin and colchicine no undegraded ligand was seen in the dense lysosome, suggesting that uptake in these vesicles was inhibited. Leupeptin allowed accumulation of undegraded ligand in the dense lysosome. Therefore, transfer from light to dense lysosomes is not dependent on degradation as such. In the presence of monensin two peaks of undegraded ligand were found in the gradients. It seems possible that in the monensin-sensitive endosomes, dissociation of the ligand-receptor complex is inhibited, allowing ligand to recycle with the receptors in small vesicles.

Subfractionation of hepatic endosomes in Nycodenz gradients and by free-flow electrophoresis. Separation of ligand-transporting and receptor-enriched membranes

The complexity of rat liver endosome fractions containing internalized radioiodinated asialotransferrin, asialo-(alkaline phosphatase), insulin and prolactin was investigated by using free-flow electrophoresis and isopycnic centrifugation in Nycodenz gradients. Two subfractions were separated by free-flow electrophoresis. Both subfractions contained receptors for asialoglycoprotein and insulin. Glycosyltransferase activities were associated with the more electronegative vesicles, whereas 5'-nucleotidase and alkaline phosphodiesterase activities were associated with the less electronegative vesicles. Three subfractions were separated on Nycodenz gradients. Two subfractions, previously shown to become acidified in vitro, contained the ligands. At short intervals after uptake (1-2 min), ligands were mainly in subfraction DN-2 (density 1.115 g/cm3), but movement into subfraction DN-1 (density 1.090 g/cm3) had occurred 10-15 min after internalization. Low amounts of glycosyltransferase activities were associated with subfraction DN-2, and 5'-nucleotidase and alkaline phosphodiesterase activities were mainly located in subfraction DN-1. The binding sites for asialoglycoproteins and insulin were distributed towards the higher density range in the Nycodenz gradients, thus indicating a segregation of receptor-enriched vesicles and those vesicles containing the various ligands 10-15 min after internalization. Electron microscopy of the subfractions separated on Nycodenz gradients indicated that whereas the ligand-transporting fractions consisted mainly of empty vesicles (average diameter 100-150 nm), the receptor-enriched component was more granular and smaller (average diameter 70-95 nm). The properties of the endosome subfraction are used to assign their origin to the regions of the endocytic compartment where ligand-receptor dissociation and separation occur.

Sequence of a second human asialoglycoprotein receptor: conservation of two receptor genes during evolution

The asialoglycoprotein (ASGP) receptor isolated from human liver and from the human hepatoma cell line HepG2 migrates on NaDodSO4 gel electrophoresis as a single species of 45,000 daltons. Recently, we isolated a cDNA clone encoding this receptor (H1) from a HepG2 lambda gt11 library. From the same library, we have isolated and sequenced a clone encoding a second ASGP receptor, H2, with a protein sequence homology of 58% to H1. There are two subspecies of H2 that differ only by the presence of a five-amino acid insertion in the COOH-terminal extracytoplasmic domain. Comparison with the available sequences of the two rat ASGP receptors R1 and R2 indicates that H1 is more homologous to R1 than to H2, and H2 is more similar to R2 than to H1. Thus, the two receptor genes evolved before the separation of rat and man. As judged by RNA blot hybridization of HepG2 RNA using RNA transcribed in vitro from cDNA clones of the human receptors as standards, H1 and H2 mRNA are present in equimolar amounts, each 0.005-0.01% of the total mRNA. This finding raises the question of whether the three ASGP receptor proteins are functional as heterodimers or whether they might serve different functions in the cell.

Cell-free synthesis and co-translational processing of the human asialoglycoprotein receptor

The human asialoglycoprotein receptor is a 46-kDa membrane glycoprotein. It is initially synthesized as a 40-kDa precursor species possessing two N-linked high-mannose oligosaccharides which is subsequently converted to the 46-kDa mature product upon modification of its oligosaccharides of the complex form [Schwartz, A. L. & Rup, D. (1983) J. Biol. Chem. 258, 11 249-11 255]. To investigate further the biosynthesis of the human asialoglycoprotein receptor, we have utilized a cell-free wheat germ translation system supplemented with dog pancreatic microsomal membranes and programmed with HepG2 and human liver RNA. The primary translation product of the human receptor is a single 34-kDa species and this species is expressed throughout human fetal and adult development. The primary translation product possesses no cleavable signal peptide and is cotranslationally glycosylated to form the 40-kDa precursor species. In addition, the human asialoglycoprotein receptor is co-translationally inserted into microsomal membranes such that a 4-kDa cytoplasmic tail is susceptible to trypsin digestion.

An alternative route of infection for viruses: entry by means of the asialoglycoprotein receptor of a Sendai virus mutant lacking its attachment protein

During the first stage of infection, the paramyxovirus Sendai virus attaches to host cells by recognizing specific receptors on the cell surface. Productive virus-cell interactions result in membrane fusion between the viral envelope and the cell surface membrane. It has recently been shown that the ganglioside GD1a and its more complex homologs GT1b and GQ1b are cell surface receptors for Sendai virus. We report in this paper that the temperature-sensitive mutant ts271 of the Enders strain of Sendai virus lacks the viral attachment protein HN and the biological activities of hemagglutination and sialidase activity associated with it when the virus is grown at 38 degrees C. This HN- virus was unable to infect or agglutinate conventional host cells that contained receptor gangliosides and were readily infected by the parental wild-type virus. The HN- virus did, however, attach to and infect Hep G2 cells, a line of hepatoma cells that retains the asialoglycoprotein receptor (ASGP-R) upon continuous culture. This receptor is a mammalian lectin that recognizes galactose- or N-acetylgalactosamine-terminated proteins. In accordance with the known properties of this receptor, infection by the HN- virus was abolished by treatment of Hep G2 cells with sialidase, by the presence of Ca2+ chelators, and by competition with N-acetylgalactosamine, asialoorosomucoid, and antibody to the receptor. F, the only glycoprotein on the HN- virus, was shown to compete with the galactose-terminated protein asialoorosomucoid for the ASGP-R. The ability of the HN- virus to cause cell-cell fusion of Hep G2 cells indicated that attachment of this virus to the ASGP-R still permitted viral entry by its usual mode--i.e., membrane fusion at the cell surface. These results open up the possibility that enveloped viruses, which contain glycosylated proteins or lipids, may make use of naturally occurring lectins in addition to their normal receptors as a means of attachment to host cells.

Cell biology of the asialoglycoprotein receptor system: a model of receptor-mediated endocytosis

Substantial information about the ASGP-R has accumulated in the 10 years following the initial studies of this receptor by Ashwell and Morell. Many of its biochemical properties, its structure, and its orientation within the plasma membrane are now known. The pathways of ASGP ligand and receptor, with the CURL organelle being a central component, are summarized in Fig. 18. The major pathway of the ligand through the cell, beginning with binding at the cell surface and ending with degradation in lysosomes, has been investigated in detail. Recently, alternate routes of the ligand such as the ligand recycling pathway have been observed. With regard to the itinerary of the receptor, there is now biochemical, kinetic, and morphological evidence to support receptor recycling. The new concept of CURL as an important intracellular organelle has originated from studies of ASGP-R recycling. Its importance in the dissociation and segregation of ligand and receptor as well as in receptor recycling is now evident. In addition, there has been a concurrent investigation of other receptor systems that participate in receptor-mediated endocytosis, providing parallels and contrasts to the ASGP-R of hepatocytes. Many critical issues still exist in the cell biology of the ASGP-R. What are the structural requirements of the receptor for ligand binding and subsequent endocytosis of the receptor-ligand complex? Very little is known about the interactions between the receptor and the lipid bilayer in which it resides. How does the receptor move laterally in the plasma membrane? Are there proteins or glycolipids closely associated with the ASGP-R and, if so, what is their function? What is the mechanism that causes receptor clustering into coated pits? Although the existence of a pathway for ligand recycling has been demonstrated, there are still many issues to be addressed. What signals a particular ligand molecule for recycling? Is it a stochastic process? What is the function of this route of ligand movement? How are the various ligand pathways coordinated and regulated? In addition, there are many unanswered questions regarding the receptor pathway. How does CURL mediate the sorting of ASGP-R from ligand? How are receptors with different destinations (e.g., ASGP-R and IgA receptor) sorted in CURL? What is the mechanism of ASGP-R degradation and how is it regulated? Finally, how does the Golgi function in the ASGP system and what is the relationship between the Golgi and CURL? Future investigation of these issues will require further observations with existing techniques as well as new approaches.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphorylation of the human asialoglycoprotein receptor

The human asialoglycoprotein receptor was isolated via immune precipitation from hepatoma Hep G2 cells following incubation with [32P]Pi. Analysis on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed incorporation of 32P into both the 46 000 Da mature form of the receptor as well as the 40 000 Da precursor. The incorporated 32P was associated with phosphoserine. The degree of 32P incorporation was not substantially altered in cells endocytosing asialoglycoprotein ligand at maximal rates nor in cells in which receptor recycling was abolished by incubation with primaquine. That endocytosis and phosphorylation can be dissociated is supported by the observation that 32P is incorporated from [gamma-32P]ATP into the asialoglycoprotein receptor in isolated plasma membranes of Hep G2 cells.