Recycling of the hepatic asialoglycoprotein receptor in isolated rat hepatocytes. Receptor-ligand complexes in an intracellular slowly dissociating pool return to the cell surface prior to dissociation

Phosphorylation-dependent interaction of the asialoglycoprotein receptor with molecular chaperones

A membrane protein trafficking mutant (Trf1) of HuH-7 alters the asialoglycoprotein (ASGPR) and transferrin receptor subcellular distribution. Expression cloning of a cDNA complementing the trf1 mutation led to the discovery of a novel casein Kinase 2 catalytic subunit (CK2alpha"). To purify potential CK2alpha" phosphorylation-dependent sorting proteins from cytosol, the ASGPR cytoplasmic domain was expressed as a GST fusion protein and immobilized on glutathione-agarose. In the absence of phosphorylation, only trace amounts of cytosol protein were bound and eluted. When the fusion protein was phosphorylated, a heterocomplex of potential sorting proteins was recovered. Mass spectrometer and immunoblot analysis identified five of these proteins as gp96, HSP70, HSP90, cyclophilin-A, and FKBP18. Treatment of HuH-7 with rapamycin to disrupt the heterocomplex reduced surface ASGPR binding activity by 65 +/- 5.7%. In Trf1 cells, surface-binding activity was 48 +/- 7% of that in HuH-7 and was not further reduced by rapamycin treatment. Immunoanalysis showed significantly fewer surface receptors on rapamycin-treated HuH7 cells than on nontreated cells, with no affect on the level of surface receptors in Trf1 cells. The data presented provide evidence that phosphorylation of the ASGPR cytoplasmic domain is required for the binding of specific molecular chaperones with the potential to regulate receptor trafficking.

Deficient assembly and function of gap junctions in Trf1, a trafficking mutant of the human liver-derived cell line HuH-7

The Trf1 cell line, selected from the human hepatoma cell line HuH-7, manifests altered trafficking of various plasma membrane proteins. In particular, there is a striking loss of State 2 asialoglycoprotein receptors. This cell line is shown here to also manifest defects in function and assembly of gap junctions comprising connexin43 (Cx43). No alteration of Cx43 expression or phosphorylation was apparent. Nevertheless, immunostaining of Cx43 revealed that fewer and smaller gap junctions were present at appositional membrane areas in Trf1 cells as compared with parental HuH-7. This correlated with a significant attenuation in gap junction-mediated communication between Trf1 cells as demonstrated by markedly decreased dye transfer and their reduced ability to propagate mechanically evoked Ca(2+) waves. Isoelectric focusing (IEF) of Cx43 in HuH-7 cells indicated that the pIs of this protein were significantly lower than that predicted from its amino acid sequence; no differences in pI were evident in Cx43 from Trf1 cells and the HuH-7 cell line. The effects of the Trf1 mutation on assembly and function of gap junctions indicate that this mutation influences trafficking of Cx43. Connexins differ in several respects from other membrane proteins thus far analyzed in Trf1 mutants: gap junctions localize exclusively to the lateral cell surface; they are not glycoproteins; and they do not play a role in endocytic pathways. The disruption of trafficking of Cx43 by this mutation suggests that the Trf1 phenotype is a defect at a common point along the trafficking pathway of cell-surface proteins, irrespective of their ultimate destination on the cell surface or their glycosylation profile.

Depletion of divalent cations within the secretory pathway inhibits the terminal glycosylation of complex carbohydrates of thyroglobulin

Newly synthesized thyroglobulin transiting the secretory pathway is posttranslationally modified by addition of oligosaccharides to asparagine N-linked residues. The effect of divalent cation depletion on oligosaccharide processing of Tg was studied in FRTL-5 cells. Treatment with an ionophore, A23187, or thapsigargin, an inhibitor of the sarcoplasmic/endoplasmic reticulum ATPases delayed Tg secretion. These effects were accompanied by a normal distribution of the marker of the endoplasmic reticulum protein disulfide isomerase. Analysis of the thyroglobulin oligosaccharides by Bio-gel P4 chromatography showed that in the presence of A23187 and thapsigargin the addition of peripheral sialic acid and possibly galactose is inhibited. These findings were strengthened by experiments of exoglycosidase digestion and SDS-PAGE analysis of the resulting products. These results reveal a cellular mechanism of production of thyroglobulin with incompletely processed complex chains, i.e., the ligand of the recently described GlcNAc and asialoglycoprotein receptors of the thyroid. Since A23187 and thapsigargin inhibit biosynthetically the addition of peripheral sugars on N-linked oligosaccharides chains, the thyroglobulin molecules secreted in the presence of A23187 and thapsigargin should greatly facilitate studies on the function of the GlcNAc and asialoglycoprotein receptors of the thyroid.

Hepatocyte culture on carbohydrate-modified star polyethylene oxide hydrogels

We describe the synthesis and in vitro biological characterization of a new class of carbohydrate-modified hydrogels based on radiation-cross-linked star polyethylene oxide (PEO). Hydrogels were synthesized from either of two types of PEO star molecules in order to vary the terminal hydroxyl content of the gels while keeping other gel properties such as molecular weight between cross-links and water content constant. The resulting gels were covalently modified with monosaccharide ligands and the behaviour of primary rat hepatocytes on the modified gels was evaluated under culture conditions. Hepatocytes exhibited a sugar-specific adhesion to the modified gels, adhering to gels bearing galactose but not glucose. Cell spreading was observed on both types of galactose-modified PEO star gels; moreover, the gels supported long-term (6 d) culture and differentiated function of primary hepatocytes. Further, on comparing the cell spreading behaviour observed on the PEO star gels with that reported previously for galactose-modified polyacrylamide, we find that our gels elicit spreading at ligand concentrations lower by an order of magnitude. A simple mechanistic analysis indicates that this enhanced ability of PEO star gels to support spreading of primary hepatocytes on low concentrations of immobilized galactose derives from freedom of the immobilized ligands to come within sufficiently close proximity to mimic a high-affinity branched oligosaccharide.

Human hepatoma cell mutant defective in cell surface protein trafficking

To isolate a mutant liver cell defective in the endocytic pathway, a selection strategy using toxic ligands for two distinct membrane receptors was devised. Ovalbumin-gelonin and asialoorosomucoid (ASOR)-gelonin were incubated with mutagenized HuH-7 cells, and a rare survivor termed trafficking mutant 1 (Trf1) was isolated. Trf1 cells were stably 3-fold more resistant than the parental HuH-7 to both toxic conjugates. The anterograde steps of intracellular endocytic processing of ASOR, including internalization, endosomal acidification, and ligand degradation, were unaltered in Trf1 cells. In contrast, retrograde diacytosis of asialoglycoprotein receptor (ASGR).ASOR complex back to the cell surface was enhanced by about 250%. Selective labeling revealed an approximately 46% reduction in cell surface-associated ASGR in Trf1 cells, although their total cellular ASGR content was essentially equivalent to that in HuH-7. Similar results were obtained with the transferrin receptor. Binding of 125I-ASOR and 125I-transferrin was reduced in Trf1 cells to 49 +/- 2.5% and 30 +/- 2%, respectively, of HuH-7 cells. The methionine transporter was also reduced in Trf1 cells, as revealed by a 2-fold reduction in Vmax with no change in apparent Km. Pretreatment with monensin, sodium azide, or colchicine reduced surface binding of 125I-ASOR in HuH-7 cells by 50% but had no effect on binding to Trf1 cells. This result is predicted for a cell that expresses only State 1 ASGRs, which are resistant to modulation by metabolic and cytoskeletal inhibitors in contrast to State 2, which are responsive to these agents (Weigel, P. H., and Oka, J. A. (1984) J. Biol. Chem. 259, 1150-1154). The Trf1 mutant, having lost the ability to express State 2 receptors, provides genetic evidence for the existence of these two receptor subpopulations and an approach to identifying the biochemical mechanism by which they are generated.

Effect of rate of intracellular transport and diacytosis on cytotoxicity of hybrid toxins. Study with hybrids using hepatic asialoglycoprotein receptor-mediated endocytosis

The effects of diacytosis and intracellular transport rate on cytotoxicity of hybrid toxins were studied with conjugates of diphtheria toxin fragment A (DTA) to asialoorosomucoid (ASOR) and its reduced and carboxymethylated cyanogen bromide fragment I (RC-ASCNBr-I) in cultured rat hepatocytes. In the hepatocytes the kinetics of uptake of the conjugate of asialoorosomucoid (DTA-ASOR) and that of the conjugate of the cyanogen bromide fragment (DTA-RC-ASCNBr-I) were quite similar, but the rate of accumulation of DTA moiety into the lysosomes, as determined by Percoll density gradient centrifugation, was found to be greater for the latter than the former. However, after internalization, DTA-RC-ASCNBr-I was diacytosed to a lesser extent than that of DTA-ASOR, particularly when colchicine was present during internalization. Analysis of the subunits of DTA-ASOR internalized by the hepatocytes indicated that they were accumulated disproportionately in a time-dependent manner so that the glycoprotein moiety was accumulated progressively more than the toxin moiety. Cytotoxicity of DTA-ASOR toward the hepatocytes was 2-times as much as that of DTA-RC-ASCNBr-I. Colchicine enhanced the toxicity of DTA-RC-ASCNBr-I (33-fold) to a greater extent than that of DTA-ASOR (12-fold). The difference in enhancement by colchicine was also observed in the rate of cell intoxication by the conjugates. Both conjugates were more toxic to the hepatocytes after incubation with the cells at 18 degrees C than at 37 degrees C. In the presence of vanadate (0.2 mM), which enhanced diacytosis, toxicity of DTA-ASOR decreased by 5-fold. After incubation with the hepatocytes, a partial dissociation of DTA-ASOR was found to occur independently of the receptor-mediated endocytosis. Taken together, these results indicate that diacytosis, subunit dissociation and rapid transport of conjugate toward lysosomes affect kinetically the rate of accumulation of the conjugate into a yet unidentified compartment of toxin translocation.

Cationic lipid enhances in vitro receptor-mediated transfection

The efficiency of cell-specific transfection by receptor-mediated uptake is improved by the use of cationic lipids. Asialoglycoprotein (AP) was conjugated to poly-L-lysine (PL) and complexed with the plasmid pCMVL that contains a luciferase reporter gene. The asialoglycoprotein-poly-L-lysine:pCMVL (AP-PL:pCMVL) complexes then were mixed with the cationic lipid dioctadecylamidoglycylspermine (DOGS). This complex was taken up by the hepatocyte-like cell line, Hep G2, via the asialoglycoprotein receptor. The expression of luciferase in cells transfected with the DOGS/AP-PL: pCMVL complexes were significantly increased compared with AP-PL:pCMVL complexes without DOGS. The ratio of AP-PL to DOGS is an important determinant for both transfection efficiency and for maintaining receptor specificity. Therefore, cationic lipids significantly increased the efficiency of asialoglycoprotein receptor mediated transfection in the hepatoblastoma cell line, Hep G2. The use of cationic lipids with receptor-mediated gene delivery systems could potentially increase transfection efficiency yet maintain cell-target specificity.

Expression of haptoglobin receptors in human hepatoma cells

The uptake of radio-labeled hemoglobin-haptoglobin complex (Hb-Hp) by human hepatoma PLC/PRF/5 and HepG2 cells was investigated in an attempt to characterize the uptake process and intracellular transport. Human hepatoma cells took up Hb-Hp in a receptor-mediated manner. Scatchard analysis of binding revealed that PLC/PRF/5 and HepG2 cells exhibited about 21,000 and 63,000 haptoglobin receptors/cell, with a dissociation constant (Kd) of 8.0 and 17 nM, respectively. Human hepatocytes in primary culture also expressed about 84,000 receptors/cells, with a Kd of 7.4 nM. The hemoglobin-haptoglobin complex was internalized and subsequently the internalized Hb-Hp was slowly degraded in the cells. Preincubation of the cells with Hb-Hp resulted in a decrease in binding of the radioactive Hb-Hp to the cell surface, and was accompanied with an accumulation of intracellular receptors. The uptake of Hb-Hp by the cells was not inhibited by 100 microM chloroquine or by 10 mM methylamine, but was inhibited by 50 microM monodansylcadaverine. Hemoglobin-heme taken up by the cells induced microsomal heme oxygenase. Thus, human hepatoma PLC/PRF/5 and HepG2 cells can take up Hb-Hp by haptoglobin receptor-mediated endocytosis and Hb-Hp probably causes translocation of the haptoglobin receptors from the cell surface to the cell interior where they can be degraded. The internalized heme-moiety of hemoglobin can regulate the expression of heme oxygenase.

Late endosomes derive from early endosomes by maturation

Endocytosed proteins destined for degradation in lysosomes are targeted mainly to early endosomes following uptake. Late endosomes are the major site for entry of newly synthesized lysosomal hydrolases via the cation-independent mannose 6-phosphate receptor into the degradative pathway. No consensus exists as to the mechanism of transport from early to late endosomes. We used asialoorosomucoid and transferrin to label selected parts of the degradative and receptor-recycling pathways, respectively, in the human hepatoma cell line HepG2. Intracellular mixing of sequentially endocytosed 125I- and HRP-labeled ligands was monitored by using 3,3'-diaminobenzidine-mediated density perturbation. The entire endocytic pathway of asialoorosomucoid, except for the lysosomes, remained fully accessible to subsequently endocytosed transferrin conjugated to HRP with unchanged kinetics. These results together with immunoelectron microscopic data support a model in which early endosomes gradually mature into late endosomes.

Total cellular activity and distribution of a subpopulation of galactosyl receptors in isolated rat hepatocytes are differentially affected by microtubule drugs, monensin, low temperature, and chloroquine

We studied the effects of low temperature (20-37 degrees C), monensin, chloroquine, and microtubule drugs on the cellular distribution and activity of galactosyl (Gal) receptors in isolated rat hepatocytes. After equilibration at 37 degrees C, hepatocytes were incubated at 37 degrees C, 31 degrees C, 25 degrees C, or 20 degrees C or treated with or without inhibitors at 37 degrees C in the absence of ligand. The cells were then assayed at 4 degrees C for 125I-asialo-orosomucoid binding, to measure receptor activity, or 125I-anti-Gal receptor IgG binding, to measure receptor protein. Surface or total (surface and intracellular) Gal receptor activity and protein were measured on intact or digitonin-permeabilized cells, respectively. These inhibitors fell into two categories. Type I inhibitors (sub-37 degrees C temperatures or colchicine) induced receptor redistribution but not inactivation. Treated cells lost up to 40% of surface Gal receptor activity and protein. Lost surface receptors were recovered intracellularly with no loss of receptor activity. Type II inhibitors (monensin or chloroquine) induced receptor inactivation but not redistribution. Treated cells lost 50-65% of their surface Gal receptor activity but only less than or equal to 15% of their surface receptor protein. These cells lost up to 60% of total cellular Gal receptor activity with no loss of total receptor protein. Of the total inactive Gal receptors, up to 50% and 75%, respectively, were present intracellularly in monensin- and chloroquine-treated cells. Loss of ligand binding to permeable treated cells was not due to changes in receptor affinity. A third category, Type III inhibitors (metabolic energy poisons that deplete ATP) induce both Gal receptor redistribution and inactivation (Biochemistry 27:2061, 1988). We conclude that only one of the two previously characterized subpopulations of Gal receptors on hepatocytes, termed State 2 receptors (J Biol Chem 265:629, 1990), recycles constitutively. The activity and distribution of State 2 but not State 1 Gal receptors are differentially affected by these specific drugs or treatments.

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.

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.

Alterations in the functional expression of receptors on cirrhotic rat hepatocytes

Reduced hepatic uptake and clearance of macromolecules in liver cirrhosis is due to two major factors: increased diffusional barriers, resulting primarily from the deposition of excessive connective tissue in the space of Disse, and hepatocellular dysfunction, manifested by receptor and/or postreceptor defects. To probe the mechanisms underlying hepatocellular dysfunction in liver cirrhosis, we have investigated receptor-ligand interactions for asialoorosomucoid, insulin and epidermal growth factor in hepatocytes isolated from the livers of rats chronically exposed to phenobarbital and carbon tetrachloride for up to 12 weeks. Viable cells were allowed to attach at 37 degrees C and the high-affinity cell surface binding sites for each ligand were assessed at 4 degrees C in the presence of [125I]-ligand. In parallel incubations, digitonin (0.055%) was added to the binding medium to assess total cellular binding sites. Results demonstrated that chronic treatment of rats with phenobarbital increased hepatocyte asialoorosomucoid surface receptor affinity (p less than 0.05) but had no affect on the number of asialoglycoprotein binding sites. Treatment with CCl4 and phenobarbital significantly reduced the number of surface binding sites for asialoorosomucoid (p less than 0.05) and epidermal growth factor (p less than 0.02), although this treatment had no effect on either the binding affinity or the number of binding sites for insulin. The decrease in cell surface binding sites for asialoorosomucoid and epidermal growth factor was not due to a redistribution of the surface sites to intracellular locations, since the total number of cellular binding sites also was reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Monensin inhibits ligand dissociation only transiently and partially and distinguishes two galactosyl receptor pathways in isolated rat hepatocytes

Monensin has been shown to inhibit the dissociation of internalized asialoorosomucoid (ASOR) from galactosyl (Gal) receptors in hepatocytes (Harford et al., J. Cell. Biol., 96:1824, 1983). Examination of the long-term kinetics of dissociation of a single round of surface-bound 125I-ASOR in the presence of monensin revealed, however, that dissociation resumed after a lag of 30-40 min. Dissociation proceeded slowly with apparent first order kinetics (k = 0.006-0.022 min-1) and reached a plateau after 4 h, both in freshly isolated cells in suspension and in cells cultured for 24 h. Only a portion of the ligand bound to surface Gal receptors was capable of dissociating. The degree of dissociation was correlated with the expression of a subpopulation of receptors we have recently designated as state 1 Gal receptors (Weigel et al., Biochem. Biophys. Res. Commun. 140:43, 1986). The recovery and dissociation of a portion of 125I-ASOR-receptor complexes after the lag period is not due to a depletion of monensin, since a second addition of the drug has no affect once dissociation resumes. Furthermore, as assessed by the accumulation of the fluorescent dye acridine orange, cells have not recovered the ability to acidify intracellular compartments during the time that dissociation occurs. The results support a model for the hepatic Gal receptor system, in which there are two functionally different receptor populations, recycling pathways, and ligand processing pathways. Monensin blocks dissociation of 125I-ASOR from receptors in the major pathway completely. In the minor pathway dissociation proceeds to completion only after a lag. In this minor pathway monensin appears to temporarily delay a maturation or translocation process that must occur prior to dissociation. We conclude that the observed dissociation in the presence of monensin cannot be mediated by low pH, or by pH or pNa gradients.

The hepatic galactosyl receptor system: two different ligand dissociation pathways are mediated by distinct receptor populations

After internalization of 125I-asialo-orosomucoid (ASOR) by isolated rat hepatocytes, ligand dissociates by two kinetically distinct pathways (Oka and Weigel, J. Biol. Chem. 257, 10,253, 1983). These slow and fast dissociation pathways correspond to two functionally different subpopulations of cell surface galactosyl receptors designated, respectively, State 1 and State 2 receptors. Freshly isolated cells or cells equilibrated below 24 degrees C express only State 1 receptors. Cells equilibrated at 37 degrees C express both State 1 and State 2 receptors. Ligand dissociation after internalization of surface-bound 125I-ASOR was measured using the permeabilizing detergent, digitonin. The slow dissociation pathway was mediated by State 1 receptors and was the only pathway expressed by cells which were freshly isolated or had been equilibrated at 24 degrees C. State 2 receptors are expressed at temperatures above about 20 degrees C, and both the fast and slow dissociation pathways occurred in cells equilibrated at 37 degrees C. State 2 receptors therefore mediate the rapid dissociation pathway. Dissociation and subsequent degradation of specifically bound ligand routed in either pathway were complete, respectively, within 3 and 6 hrs.

Receptor-mediated uptake of asialoglycoprotein by the primate liver initiates both lysosomal and transcellular pathways

The degradation of asialoglycoproteins in hepatocytes has been well described in several animal models, but no direct evidence has yet been obtained for asialoglycoprotein processing in the primate liver. A double radiolabeling strategy was employed in the experiments described in this paper to evaluate the fate of asialoorosomucoid in the squirrel monkey. Intravenously injected asialoorosomucoid was taken up by the liver with a half-time of 1 min. Electron microscopic autoradiography showed progression of asialoorosomucoid from the hepatocyte plasma membrane through vesicles to multivesicular bodies and then to secondary lysosomes near the Golgi-rich area of the cell. Over 75% of the grains initially associated with clear endocytic compartments after injection had moved to these later organelles within 20 min. Following degradation of asialoorosomucoid labeled with the Bolton and Hunter reagent, radiocatabolites were secreted into bile, peaking approximately 47 min after injection. We also found that 7 to 8% of the injected protein entered an alternative pathway which led to resecretion of the ligand at the bile canaliculus. This was considerably more than in rats (1 to 3%), but roughly comparable to the amount in guinea pigs (10 to 17%). Intact asialoorosomucoid peaked in monkey bile approximately 27 min after injection and was 3 to 4 times more concentrated than the initial plasma concentration, indicating receptor-mediated transport. Gel filtration chromatography and polyacrylamide gel analysis of the secreted protein indicated that it had arrived in bile unaltered. Since less than 1% of the autoradiographic grains were localized to nonparenchymal cells, the hepatocyte was identified as the cell type simultaneously responsible for both pathways. We propose that missorting of some of the asialoglycoprotein to bile reflects diffusion within intracellular sorting compartments to areas primarily dedicated to the processing of unrelated ligands, such as those newly synthesized for biliary secretion.

Uptake, intracellular transport and release of 125I-poly(vinylpyrrolidone) and [14C]-sucrose-asialofetuin in rat liver parenchymal cells. Effects of ammonia on the intracellular transport

We have studied the intracellular transport of 125I-labeled poly(vinylpyrrolidone) (125I-PVP) and [14C]sucrose-asialofetuin (14C-SAF) in isolated rat hepatocytes. 125I-PVP and 14C-SAF are taken up in the cells by fluid phase and receptor-mediated endocytosis, respectively. The labeled degradation products formed from 14C-SAF are trapped in the lysosomes. They can therefore serve as markers for lysosomes in subcellular fractionation studies. The accumulation of 125I-PVP in the cells was rapid initially and then decreased to a constant value. The diminished rate of accumulation was due to release (exocytosis) of previously endocytosed 125I-PVP. The release of 125I-PVP was studied in cells that had accumulated 125I-PVP for various times and then after washing incubated in new medium at 37 degrees. About 25% of the radioactivity associated with the cells after 1 hr was released to the medium subsequently. No such release was observed in cells that had taken up 14C-SAF. Subcellular distribution of 125I-PVP and 14C-SAF was studied by isopycnic centrifugation in sucrose gradients. Both compounds were sequentially associated with light (1.13 g/ml) and dense (1.19 g/ml) vesicles. Exocytosed 125I-PVP was derived from the light vesicles. The denser organelles were probably lysosomes as their distribution coincided with that of lysosomal enzymes. By measuring radioactivity soluble and precipitable in trichloroacetic acid it could be shown that only degraded 14C-SAF was associated with lysosomes. Undegraded 14C-SAF was associated with vesicles banding at 1.13 g/ml. Degraded 14C-SAF was, however, also seen first in this region of the gradient, suggesting that degradation started in a light lysosome. Both uptake and release of 125I-PVP were temperature dependent; both processes ceased at 10 degrees. Ammonium ions had negligible effects on uptake and release of 125I-PVP. The amine inhibited, however, the transfer of both 125I-PVP and 14C-SAF to the lysosomes.

Effects of monensin on vesicular transport pathways in the perfused rat liver

In the rat hepatocyte, the internalization and degradation of asialoglycoproteins and the secretion of plasma and biliary proteins require specific intracellular sorting of vesicles. To aid in the biochemical characterization of these different vesicular pathways, we examined the effects of the ionophore monensin on the uptake and degradation of 125I-asialoorosomucoid (ASOR) and on the secretion of plasma and biliary proteins by the in situ perfused rat liver. In control livers, 77% of injected 125I-ASOR was extracted on first pass; 93% of the extracted radioactivity was released back into the circulation (totally degraded and some intact ASOR was found); and approximately 2% was recovered in the bile, some of which was intact. Monensin treatment decreased first pass uptake of 125I-ASOR to 57% and abruptly blocked the release of radioactivity into the perfusate and the bile. When hepatic proteins were biosynthetically labeled with 3H-leucine, monensin treatment dramatically reduced and delayed the secretion of newly synthesized proteins into both the perfusate and the bile. In contrast with control livers, in which secretion of protein into the perfusate preceded secretion of protein into the bile, TCA-precipitable 3H-protein appeared in bile about 20 min before TCA-precipitable 3H-protein appeared in the perfusate in monensin-treated livers. Thus, monensin treatment in the perfused liver blocked the degradation of asialoglycoproteins and inhibited the secretion of plasma proteins but had less effect on biliary protein secretion. These data document physiologic effects of monensin in an intact organ and suggest that biochemical distinctions between different vesicular pathways exist in the rat hepatocyte.

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.

Kinetic evidence of a surface membraneous step during the endocytosic process of 3H-labelled asialoorosomucoid and its alteration in diabetic mellitus rats

The apparent internalization rate constant of asialoorosomucoid in normal and diabetic hepatocytes was determined using different experimental processes, either following a synchronous wave of prebound ligand or a continuous flux ligand endocytosis, either alone or simultaneously. In continuous flux conditions, no difference between normal and diabetic hepatocytes appeared (k = 0.15 +/- 0.04 and 0.11 +/- 0.02 min-1, respectively). In contrast, in the one-turn endocytosis of prebound ligand, k was lower for diabetic hepatocytes than for normal ones whether it was measured alone (0.20 +/- 0.03 and 0.59 +/- 0.09 min-1, respectively) or simultaneously with a continuous flux of unlabelled ligand (0.25 +/- 0.03 and 0.70 +/- 0.08 min-1, respectively). These differences are attributed to an impediment or a delay in the preclustering of receptors in coated pits at the cell surface of diabetic cells.

Quantification of endocytosis-derived membrane traffic

The main data covered by this article have been summarized in Table I. A fairly uniform picture is obtained for endocytosis-derived membrane transfer and compartmentation. This may be due to the limited amount of information and the resulting low resolution. Data on mainly three cell types are presented: macrophages, fibroblasts and amoebae. The data vary as much for one cell type as between different cells. Therefore, no possible differences related to cell function emerge. More detailed data, for more cell types, may change the picture. The values for cell surface area, although significantly different in absolute terms (column S in Table I), are rather similar when related to cell diameter, all being about 3-fold in excess of the surface area of the smooth sphere of comparable volume (column xi in Table I). The rate of plasma membrane internalization for macrophages and amoebae both professional phagocytes, is about 2 cell surface area equivalents per h or more. This may be somewhat higher than for fibroblasts (column PM/h in Table I). The average residence time for membrane on the cell surface, therefore, is about 30 min. A most interesting finding seems to be the rather uniform values obtained for the average size (volume weighted) of primary pinosomes, being about 0.3 micron in diameter (column phi-Internalization in Table I). Due to their rapid increase in size as a result of fusion (cf. Fig. 2), it has not been feasible to directly measure the size of primary pinosomes by morphometric means. The values in Table I, give no information on the size distributions of primary pinosomes and on whether these consist of one or more size classes. The steady-state average diameter of pinosomes is noticeably larger than that of primary pinosomes (column phi-pinosomes in Table I; cf. Table II for Acanthamoebae). The corresponding decrease in surface-to-volume ratio can make about 50% of pinosomal membrane available for recycling directly from this membrane compartment. Membrane recycling from the pinosomal compartment occurs after an average residence time of about 3 min for macrophages and 4-6 min for fibroblasts (column tau-pinosomes in Table I). The relative pool size of intracellular membranes participating in shuttling to and from the cell surface is significantly different for animal cells and amoebae (column rho in Table I). For macrophages, fibroblasts, CHO cells, and mast cells, this intracellular membrane pool amounts to about 10-20% the plasma membrane area, compared to 150-200% in the case of amoebae.(ABSTRACT TRUNCATED AT 400 WORDS)

Diacytosis of human asialotransferrin type 3 in the rat liver is due to the sequential engagement of two receptors

The possible role of transferrin receptors in the diacytosis of human asialotransferrin type 3 (HAsTf-3) by the rat liver was studied in vivo. A trace dose of the ligand was allowed to compete for hepatic binding sites against diferric transferrin, the concentration of which was varied between 5 400- and 18 000-fold. Binding of HAsTf-3 was insensitive to the presence of 2Fe-transferrin in this range, and the liver bound the ligand equally efficiently, regardless of whether it was presented in the holo or apo form. In contrast, pretreating the animals with desialylated bovine submaxillary mucin (2 mg/100 g, 2 min before the dose) prevented the asialotransferrin-liver interaction. These findings indicate that endocytosis of HAsTf-3 is mediated by the Gal/GalNAc-specific lectin and not by transferrin receptors. Although 2Fe-transferrin did not affect binding, it did reduce the half-life of the ligand in the liver, thus suggesting that transferrin receptors play an important role in the exocytic leg of the diacytic cycle. Based on our present and earlier data, a model is proposed in which the engagement of lectin and transferrin receptor in the diacytic cycle is envisaged sequentially so that HAsTf-3 switches receptors at an acidified subcellular site.

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)

Retro-endocytosis of low density lipoprotein by cultured human skin fibroblasts

A fraction of the low-density lipoprotein (LDL) internalized by cells via receptor-mediated endocytosis follows a short-circuit pathway, termed "retro-endocytosis," that results in the rapid exocytosis of ligand. Results from the current study suggest that retro-endocytosis of LDL in human fibroblasts is caused by resurfacing of endocytotic vesicles that contain both free and receptor-bound ligand, resulting in discharge of vesicular contents and in spontaneous dissociation of LDL from its receptor. The bulk of the released LDL particles had the same size, density, and immunogenic properties as native LDL, indicating that they were discharged intact. Some of the retro-endocytosed LDL was larger than native LDL, and some exhibited altered sedimentation properties. When fusion of endosomes with lysosomes was inhibited by chilling cells to 18 degrees C, the proportion of intracellular LDL subsequently released was unaffected, suggesting that retro-endocytosis does not require lysosomal participation. Furthermore, the shorter the internalization phase the greater was the proportion of LDL subsequently released, suggesting that LDL was discharged from compartments formed early in endocytosis. Retro-endocytosis of LDL was stimulated by agents that neutralize acid intracellular compartments, such as ionophores (monensin) and weak bases (chloroquine and methylamine). Monensin increased the proportion of intracellular LDL released, suggesting that it had a direct effect on retro-endocytosis. The effect of weak bases appeared to be secondary to their ability to promote cellular accumulation of undegraded LDL. Thus, retro-endocytosis of LDL becomes a major pathway when intracellular compartments fail to maintain a low pH or where the intracellular concentration of LDL reaches abnormal levels.

Diacytosis of 125I-asialoorosomucoid by rat hepatocytes. A non-lysosomal pathway insensitive to inhibition by inhibitors of ligand degradation

Diacytosis of 125I-asialoorosomucoid by rat hepatocytes was studied by preincubating the cells with the labelled ligand at 37 degrees C for 30 min or 18 degrees C for 2 h, washing free of cell surface receptor-bound tracer at 4 degrees C and then reincubating at 37 degrees C. The cells preloaded at 37 degrees C released a maximum of 18% of the total intracellular ligand as undegraded molecules after 1 h of incubation with an apparent first-order rate constant of 0.018 min-1 (t1/2 = 39 min). When the preloaded cells were incubated in the presence of 100 micrograms/ml unlabelled asialoorosomucoid or 5 mM ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, the amount of the released ligand increased to 32 and 37%, respectively, without apparent change in kinetics, indicating that these agents prevented rebinding of the released ligand. In the presence of 5 microM colchicine, 20 microM cytochalasin B, 20 microM chloroquine, 10 mM NH4Cl, 10 microM monensin or 20 microM leupeptin, degradation of the preloaded ligand was inhibited, whereas the release of the ligand was either slightly increased or unchanged. Similar effects of leupeptin, colchicine and asialoorosomucoid were observed with cells preloaded at 18 degrees C. These results indicate that diacytosis of 125I-asialoorosomucoid occurs from a prelysosomal compartment via a route insensitive to inhibition by the inhibitors of ligand degradation.