Monensin inhibits receptor-mediated endocytosis of asialoglycoproteins in rat hepatocytes

Hair cell BK channels interact with RACK1, and PKC increases its expression on the cell surface by indirect phosphorylation

Large conductance (BK) calcium activated potassium channels (Slo) are ubiquitous and implicated in a number of human diseases including hypertension and epilepsy. BK channels consist of a pore forming α-subunit (Slo) and a number of accessory subunits. In hair cells of nonmammalian vertebrates these channels play a critical role in electrical resonance, a mechanism of frequency selectivity. Hair cell BK channel clusters on the surface and currents increase along the tonotopic axis and contribute significantly to the responsiveness of these hair cells to sounds of high frequency. In contrast, messenger RNA levels encoding the Slo gene show an opposite decrease in high frequency hair cells. To understand the molecular events underlying this paradox, we used a yeast two-hybrid screen to isolate binding partners of Slo. We identified Rack1 as a Slo binding partner and demonstrate that PKC activation increases Slo surface expression. We also establish that increased Slo recycling of endocytosed Slo is at least partially responsible for the increased surface expression of Slo. Moreover, analysis of several PKC phosphorylation site mutants confirms that the effects of PKC on Slo surface expression are likely indirect. Finally, we show that Slo clusters on the surface of hair cells are also increased by increased PKC activity and may contribute to the increasing amounts of channel clusters on the surface of high-frequency hair cells.

Fc receptor-mediated, antibody-dependent enhancement of bacteriophage lambda-mediated gene transfer in mammalian cells

Lambda phage vectors mediate gene transfer in cultured mammalian cells and in live mice, and in vivo phage-mediated gene expression is increased when mice are pre-immunized with bacteriophage lambda. We now show that, like eukaryotic viruses, bacteriophage vectors are subject to Fc receptor-mediated, antibody-dependent enhancement of infection in mammalian cells. Antibody-dependent enhancement of phage gene transfer required FcgammaRI, but not its associated gamma-chain, and was not supported by other FcgammaR family members (FcgammaRIIA, FcgammaRIIB, and FcgammaRIII). Studies using chlorpromazine and latrunculin A revealed an important role for clathrin-mediated endocytosis (chlorpromazine) and actin filaments (latrunculin A) in antibody-enhanced phage gene transfer. This was confirmed by experiments using inhibitors of endosomal acidification (bafilomycin A1, monensin) and by immunocytochemical colocalization of internalized phage particles with early endosome-associated protein-1 (EAA1). In contrast, microtubule-targeting agents (nocodazole, taxol) increased the efficiency of antibody-enhanced phage gene transfer. These results reveal an unexpected antibody-dependent, FcgammaRI-mediated enhancement of phage transduction in mammalian cells, and suggest new approaches to improve bacteriophage-mediated gene transfer.

Gene transfer into hepatoma cells mediated by galactose-modified alpha-helical peptides

To develop a receptor-mediated gene delivery system into hepatoma cells using the cationic alpha-helical peptide as the gene carrier molecule, we modified an alpha-helical peptide, which is known to have transfection abilities into cells, with a multi-antennary ligand containing several galactose residues that provide efficient binding to the asialoglycoprotein receptor. The galactose-modified peptides formed complexes with a plasmid DNA and showed gene transfer abilities into HuH-7 cells, a human hepatoma cell line. The transfection efficiency of the peptide was increased by increasing the number of modified galactose residues on the peptide. Furthermore, considerable inhibition of the transfection efficiency by the addition of asialofetuin, which is a ligand for the asialoglycoprotein receptor, was observed in all galactose-modified peptides. Based on this result, we could confirm that the internalization of the galactose-modified peptides occurred by the receptor-mediated endocytosis pathway. In addition, to understand the transport route of the peptide-DNA complex in the cell, the effects on the transfection efficiencies with several endocytosis inhibitors were examined. As a result, it was suggested that the translocation of the peptide-DNA complex from the endocytic compartments to the cytosol mainly occurred during an early endosome step.

Agonist-dependent AT(4) receptor internalization in bovine aortic endothelial cells

Recent studies have characterized a specific binding site for the C-terminal 3-8 fragment of angiotensin II (Ang IV). In the present study we looked at the internalization process of this receptor on bovine aortic endothelial cells (BAEC). Under normal culture conditions, BAEC efficiently internalized (125)I-Ang IV as assessed by acid-resistant binding. Internalization of (125)I-Ang IV was considerably decreased after pretreatment of cells with hyperosmolar sucrose or after pretreatment of BAEC with inhibitors of endosomal acidification such as monensin or NH(4)Cl. About 50% of internalized (125)I-Ang IV recycled back to the extracellular medium during a 2 h incubation at 37 degrees C. (125)I-Ang IV remained mostly intact during the whole process of internalization and recycling as assessed by thin layer chromatography. As expected, internalization of (125)I-Ang IV was completely abolished by divalinal-Ang IV, a known AT(4) receptor antagonist. Interestingly, (125)I-divalinal-Ang IV did not internalize into BAEC. These results suggest that AT(4) receptor undergoes an agonist-dependent internalization and recycling process commonly observed upon activation of functional receptors.

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

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

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

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

Physiological functions of endosomal proteolysis

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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.

Agonist-induced muscarinic cholinergic receptor internalization, recycling and degradation in cultured neuronal cells. Cellular mechanisms and role in desensitization

Short-term incubation of intact neuronal cells with muscarinic cholinergic agonists resulted in a rapid decrease of the specific binding of [3H]methylscopolamine to cell surface receptors indicative of receptor internalization. The agonists induced the internalization of both the muscarinic receptor subtypes coupled to adenylyl cyclase and those coupled to phosphoinositide turnover. Receptor internalization, which was inhibited at 0-4 degrees and by depletion of intracellular K+, is thought to occur through coated pits formation and was rapidly reversible. Receptor recycling did not imply protein synthesis. Down-regulation of muscarinic receptors occurred slowly in the presence of agonists, needed intact cytoskeleton (demonstrated by the inhibitory effect of colchicine) and involved lysosomal activity. Both receptor internalization and down-regulation were prevented by muscarinic receptor antagonists. Receptor internalization and down-regulation are agonist-induced cellular mechanisms that with receptor phosphorylation and uncoupling, may induce desensitization. These processes may contribute to complex intracellular regulatory processes and may be involved in some of the long-term effects of neurotransmitters (mainly neuropeptides and growth hormones) or drugs.

Hepatic uptake and intracellular processing of LDL in rainbow trout

The process of receptor-mediated endocytosis in poikilothermic vertebrates such as salmonid fish have not been subjected to much research, compared to the detailed studies done in mammalian systems. We have investigated the hepatic uptake and intracellular processing of low-density lipoprotein (LDL) in rainbow trout liver. After intravenous injection of the [125I]tyramine-cellobiose ([125I]TC) -labelled lipoprotein, the liver was perfused and cells isolated or fractionated by differential centrifugation and isopycnic centrifugation in Nycodenz gradients. We found that LDL was mainly endocytosed by parenchymal cells of the liver. Cell fractionation experiments showed that LDL was localized sequentially in three groups of organelles of increasing density. Initially, LDL was localized in small, slowly sedimenting endosomes before being transferred to denser endosomes (prelysosomes) and finally to dense lysosomes. The lysosomes were identified by three lysosomal marker enzymes. Degradation products formed from [125I]TC-labelled LDL could also be detected in prelysosomal vesicles. In vitro experiments with cultured trout hepatocytes demonstrated that intracellular processing of [125I]TC-LDL in these cells could be suppressed by endocytic and lysosomal inhibitors. The catabolism of LDL in rainbow trout therefore follows the endocytic-lysosomal pathway described for many macromolecules in mammalian cells.

Adhesion of CHO cells to fibronectin is mediated by functionally and structurally distinct adhesion plaques

We have investigated the dynamics between free fibronectin receptors and clusters of them organized into adhesion plaques on CHO cells using the ability of these free integrins to be endocytosed and recycled to the plasma membrane. Indirect inhibition of the endocytic cycle by monensin resulted in the subsequent internalization of free receptors, which we followed by indirect immunostaining and confocal microscopy. Consequently, all the adhesive structures that were in equilibrium with free integrins became progressively disorganized. The cellular morphological changes were analyzed and correlated with the distribution of cell-substratum contacts viewed by confocal images obtained after immunostaining with antibodies raised against the fibronectin receptor, talin, vinculin and actin. After cell adhesion to fibronectin, blockage of the endocytic cycle induced disruption of the adhesion plaques that were mainly localized at the cell periphery, and disappearance of the stress fibers. However, the cells remained firmly attached to the substratum through focal contacts localized in the central part of the cell. These central focal contacts, but not the peripheral adhesion plaques, could form when the vesicular traffic was blocked prior to adhesion and they allowed the cells to attach and flatten onto the substratum. Whereas both adhesive structures contained the same receptors linked to talin and vinculin, the central adhesive structures were attached to a short stretch of actin but never permitted the organization of stress fibers.

Studies of binding and internalization of human recombinant monocyte chemotactic and activating factor (MCAF) by monocytic cells

Recombinant human monocyte chemotactic and activating factor (MCAF) was iodinated and specific binding sites for this cytokine were detected on human peripheral blood monocytes, the monocytic leukemia cell line THP-1, and on PMA-differentiated HL60 and U937 cell lines. The binding sites were specific for MCAF since other polypeptide cytokines and the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP) failed to compete for 125I-rhMCAF binding. Steady-state binding experiments at 4 degrees C revealed the presence of 13,000 and 18,000 receptor sites/cell on monocytes and THP-1 cells with Kd values of 22.5 nM and 25.7 nM, respectively. Compared to a human natural MCAF, rhMCAF was less potent in inducing maximal monocyte migration. Human natural MCAF similarly competed more efficiently for 125I-rhMCAF binding than unlabelled rhMCAF. The ligand-receptor association was highly temperature-dependent, with maximal ligand uptake at 37 degrees C accompanied by internalization of the ligand-receptor complexes. The internalized 125I-MCAF was progressively degraded and released into the culture medium starting at 30 min. Lysosomotropic ammonium chloride could inhibit the degradation of this ligand suggesting the involvement of lysosomal enzymes in the proteolytic digestion. Incubation with cycloheximide did not block the rapid reappearance of MCAF receptors within 20 min on the cell surface indicative of receptor recycling rather than new protein synthesis. These data indicate that monocytic cells express specific receptors for rhMCAF which can be dynamically regulated by MCAF.

Interaction of low density lipoproteins with liver cells in rainbow trout

Liver is the main catabolic tissue for low density lipoprotein in rainbow trout (Gjøen and Berg 1992). We have investigated the interaction of LDL with isolated trout liver cells and liver membranes. (125)I-TC labelled trout LDL bound to isolated trout liver cells in a time dependent and saturable manner with an apparant Kd of 20.1 μg/ml, suggesting the existence of a specific binding site on the surface of these cells. The binding was Ca(2+) dependent assessed by the 50% reduction obtained by 5 mM EDTA. Saturable binding to isolated trout liver membranes could also be demonstrated, but with lower affinity as compared to intact cells. Degradation of (125)I-TC-LDL in hepatocytes was also saturable as degradation could be inhibited about 60% by a 100 fold surplus of unlabelled LDL. The rate of degradation increased with temperature up to 20°C. Both cell association (binding + uptake) and degradation were reduced down to 20% of control in the presence of microtubular and lysosomal inhibitors. Hepatic catabolism of trout LDL therefore seems to depend on receptormediated endocytosis, followed by lysosomal degradation.

Pathways of internalization of the hCG/LH receptor: immunoelectron microscopic studies in Leydig cells and transfected L-cells

Monoclonal anti-receptor antibodies were used to study the cellular traffic of the hCG/LH receptor by immunoelectron microscopy. The LHR38 antibody was shown to bind to the extracellular domain of the receptor but not to interfere with hormone binding, adenylate cyclase activation or with the rate of internalization of the receptor. Pig Leydig cells and a permanent L-cell line expressing the LH receptor were used for the study. Incubation with LHR38-gold complexes showed the LH receptors to be randomly distributed over the cell surface including the clathrin coated pits. The LH receptors were internalized via a route including coated pits, coated vesicles and multivesicular bodies to lysosomes. This route is different from that observed for beta-adrenergic, muscarinic, and yeast mating factor receptors and considered previously as possibly general for G-protein-coupled receptors. The use of [125I]LHR38 allowed precise measurement of the rate of internalization, showing the existence of a constitutive pathway which was increased 11-fold by hormone administration. Double labeling experiments suggested that the hormone (hCG-Au15nm) and the receptor (labeled with LHR38-Au5nm) have similar routes of endocytosis, both of them being degraded in lysosomes. Studies of the reappearance of LHR38-Au5nm on the surface of the cells and the use of monensin indicated that only a very small proportion of the receptor molecules were recycled to the cell surface. The distribution and the intracellular pathways of LH receptors are very similar in Leydig cells and transfected L-cells. This opens the possibility of using the latter to study, by in vitro mutagenesis, the molecular mechanisms involved in the cellular traffic of LH receptors.

Effect of monensin on the neuronal ultrastructure and endocytic pathway of macromolecules in cultured brain neurons

1. The endocytic pathway of horseradish peroxidase (HRP) was investigated in the perikarya of cultured neurons by electron microscopy and enzyme cytochemistry. The tracer was observed in endocytic pits and vesicles, endosomes, multivesicular bodies, and lysosomes. It took approximate 15 min for the transfer of HRP from the exterior of the cell to the lysosomes. 2. Monensin induced distension of the Golgi apparatus and formation of intracellular vacuoles. When neurons were incubated with both monensin and HRP for 30 to 120 min, the number of HRP-labeled endosomes was greater than that in the monensin-free group, whereas the reverse was seen for HRP-positive lysosomes. The formation of HRP-positive lysosomes in monensin-treated cells was blocked by 47 to 79%. 3. These results indicate that the intracellular transport of the endocytosed macromolecule is pH dependent. It is also possible that the export of lysosomal enzymes is inhibited by monensin, resulting in an accumulation of the endosomes and a reduction of the lysosomes.

Characterization of the interaction both in vitro and in vivo of tissue-type plasminogen activator (t-PA) with rat liver cells. Effects of monoclonal antibodies to t-PA

The interaction of 125I-labelled tissue-type plasminogen activator (125I-t-PA) with freshly isolated rat parenchymal and endothelial liver cells was studied. Binding experiments at 4 degrees C with parenchymal cells and endothelial liver cells indicated the presence of 68,000 and 44,000 high-affinity t-PA-binding sites, with an apparent Kd of 3.5 and 4 nM respectively. Association of 125I-t-PA with parenchymal cells was Ca(2+)-dependent and was not influenced by asialofetuin, a known ligand for the galactose receptor. Association of 125I-t-PA with liver endothelial cells was Ca(2+)-dependent and mannose-specific, since ovalbumin (a mannose-terminated glycoprotein) inhibited the cell association of t-PA. Association of 125I-t-PA with liver endothelial cells was inhibited by anti-(human mannose receptor) antiserum. Anti-(galactose receptor) IgG had no effect on 125I-t-PA association with either cell type. Degradation of 125I-t-PA at 37 degrees C by both cell types was inhibited by chloroquine or NH4Cl, indicating that t-PA is degraded lysosomally. in vitro experiments with three monoclonal antibodies (MAbs) demonstrated that anti-t-PA MAb 1-3-1 specifically decreased association of 125I-t-PA with the endothelial cells, and anti-t-PA Mab 7-8-4 inhibited association with the parenchymal cells. Results of competition experiments in rats in vivo with these antibodies were in agreement with findings in vitro. Both antibodies decreased the liver uptake of 125I-t-PA, while a combination of the two antibodies was even more effective in reducing the liver association of 125I-t-PA and increasing its plasma half-life. We conclude from these data that clearance of t-PA by the liver is regulated by at least two pathways, one on parenchymal cells (not galactose/mannose-mediated) and another on liver endothelial cells (mediated by a mannose receptor). Results with the MAbs imply that two distinct sites on the t-PA molecule are involved in binding to parenchymal cells and liver endothelial cells.

Kinetics of binding, endocytosis, and recycling of EGF receptor mutants

This report describes analysis of factors which regulate the binding of EGF to EGF receptor, receptor internalization, and receptor recycling. Three different methods were used to inhibit high-affinity EGF binding as measured at equilibrium: treatment of cells with an active phorbol ester (PMA), binding of a mAb directed against the EGF receptor (mAb108), and truncation of most of the cytoplasmic domain of the receptor. These treatments reduced the rate at which low concentrations of EGF bound to cells, but did not affect the rate of EGF dissociation. We conclude that high-affinity EGF binding on living cells results from a difference in the apparent on rate of EGF binding. We then used these conditions and cell lines to test for the rate of EGF internalization at different concentrations of EGF. We demonstrate that internalization of the EGF receptor is stimulated roughly 50-fold at saturating concentrations of EGF, but is stimulated an additional two- to threefold at low concentrations (less than 1 nM). Four treatments reduce the rate of internalization of low concentrations of EGF to the rate seen at saturating EGF concentrations. Phorbol ester treatment and mAb108 binding to "wild type" receptor reduce this rate (and reduce high-affinity binding). Point mutation at Lys721 (kinase negative EGF receptor) and point mutation at Thr654 (removing a major site of protein kinase C phosphorylation) reduce the internalization rate, without affecting high-affinity binding. We suggest that while EGF stimulates endocytosis for all receptors, high-affinity receptors bind and are internalized more quickly than low-affinity receptors. Tyrosine kinase activity and the Thr654 region appear necessary for this response.

Effects of vasopressin on receptor-mediated endocytosis of asialoglycoprotein by hepatocytes from normal and diabetic rats

The hepatic asialoglycoprotein receptor is a membrane glycoprotein used as a model to study receptor-mediated endocytosis. In order to examine the ability of second messengers to modulate intracellular trafficking, we performed a comparative study on normal and diabetic rat hepatocytes exploring the effects of an in vivo modulation, streptozotocin-diabetes, and an in vitro modulator, vasopressin, which transduces signals via the phosphoinositide pathway. We studied three main experimental aspects: (1) constitutive endocytosis, (2) continuous ligand flux, and (3) a synchronous wave of ligand. In normal cells, vasopressin decreased ligand-binding capacity by 20%, without altering the mechanism of internalization, and decreased the level of degradation, without affecting the distribution of degradation products. Diabetic cells were characterized by a 50% decrease in cell-surface and intracellular receptor ligand-binding capacity, slowed internalization of a synchronous wave of ligand, and markedly reduced degradation with an altered distribution of degraded products. Vasopressin had no additive effect on the modification induced by diabetes. These results suggest that second messengers generated by hormones play a role in the regulation of receptor-mediated endocytosis. They also confirm that receptors are subdivided into those susceptible to modulation of any kind and those insensitive to modulation, although the boundary between the two subsets is variable.

Endocytosis and intracellular processing of tissue-type plasminogen activator by rat liver cells in vivo

Endocytosis of tissue-type plasminogen activator (t-PA) by different types of rat liver cells was studied in immunocytochemically labelled cryosections as well as in biochemical experiments. For morphological localization of the ligand in different endocytic compartments involved in its catabolism, rat livers were fixed at various times (1-24 min) after injection of t-PA. Late-endosomal and lysosomal compartments were identified by double-labelling the sections with antibodies to the lysosomal proteins glycoprotein Igp 120 and cathepsin D. In liver t-PA was localized in sinusoidal endothelial cells (EC), parenchymal cells (PC) and to some extent in Kupffer cells (KC), indicating that it is internalized and degraded in all three cell types. In specimens fixed 6 min after injection PC, EC and KC were found to contribute to 69, 24 and 7% respectively of total t-PA endocytosed. The transfer from late endosomes to lysosomes was found to be faster in EC than in PC. The morphological findings were supported by studies of the endocytic mechanisms employing isolated perfused livers and primary hepatocytes. The presence of monensin, an inhibitor of lysosomal protein degradation, reduced the amount of t-PA degraded to about 50% of the control values. The catalytic site seems not to be required for the catabolism of t-PA in hepatic cells. The inhibition of t-PA by D-phenylalanyl-L-prolylarginyl-chloromethane did not influence receptor recognition and catabolic processing, as determined in morphological studies using labelled cryosections, in binding studies employing liver cell membranes and primary hepatocytes, as well as in liver-perfusion experiments.

Characterization of ligand binding and processing by gastrin-releasing peptide receptors in a small-cell lung cancer cell line

The ligand-binding properties of the gastrin-releasing peptide (GRP) receptor and the cellular processing of GRP have been studied in the small-cell lung cancer (SCLC) cell line COR-L42. Scatchard analysis of GRP receptor expression indicated a single class of high-affinity receptors (Kd 1.5 nM) and approx. 6700 receptors/cell. GRP bound to its receptor with a Ki of 2.4 nM. The bombesin-related peptides neuromedin B (NMB) and phyllolitorin also bound to GRP receptors with Ki values of 22.7 and 59.1 nM respectively. Binding of 125I-GRP to COR-L42 cells increased rapidly at 37 degrees, achieved a maximum at 10 min and declined rapidly thereafter. At 4 degrees C, maximum binding was achieved at 30 min and the subsequent decline in cell-associated radioactivity was slower than that seen at 37 degrees C. Acid/salt extraction, to separate surface-bound ligand from internalized GRP, indicated that after receptor binding 125I-GRP was rapidly internalized. To determine the pathway of 125I-GRP degradation, binding studies were carried out with the lysosomotropic agent chloroquine (5 mM), and with phosphoramidon (10 microM), an inhibitor of the membrane-bound enzyme (EC 3.4.24.11). Both agents markedly inhibited the degradation of GRP, indicating that this process involves a lysosomal pathway and a phosphoramidon-sensitive pathway, possibly involving the EC 3.4.24.11 enzyme. GRP receptor down-regulation was observed following a 10 min exposure to 100 nM-GRP. With longer pretreatment times the number of binding sites recovered to 80% of control values. Treatment with 5 mM-chloroquine plus GRP or cycloheximide (10 micrograms/ml) plus GRP demonstrated that the majority of GRP receptors are recycled. NMB and phyllolitorin pretreatment did not influence the subsequent binding of 125I-GRP, suggesting that these peptides do not down-regulate GRP receptors.

Comparative effects of diabetes and monensin on the lectin asialoglycoprotein receptor: biosynthesis, structure and function in rats

The hepatic asialoglycoprotein receptor is the first studied mammalian lectin. Modulations in vivo by diabetes and in vitro by the carboxylic ionophore monensin gave rise to similar apparent alterations on its biosynthesis, structure and ligand binding capacity. In normal rats, the receptor (whether purified by ligand or antibody-affinity chromatography) presented a similar pattern in SDS-PAGE analysis, with a major 42-kDa band and two minor ones (49 and 52-54 kDa). In diabetic rats, a new 38-kDa band appeared, but only after antibody-affinity purification. In vitro biosynthesis of the receptor by normal hepatocytes in the presence of 35S-methionine showed that this 38-kDa band was present at the end of a 30-min pulse but decreased during a 180-min chase, in association with an increase in the major 42-kDa band. In diabetic cells, this evolution was retarded. Using a 30-min pulse followed by a 120-min chase in the presence of 100 microM monensin, we showed that this carboxylic ionophore had similar effects on diabetes, leading to a delay in the maturation process of the 42-kDa band and the persistent emergence of the 38-kDa species. Allowing incubation in the presence of 25 or 100 microM monensin, we observed a decrease in the number of ligand binding sites both at the surface (40%) and within the cell (28%). In hepatocytes from diabetic rats, monensin showed no additional effect on the partial diabetes-induced inactivation.

An inhibitory effect of a protein kinase inhibitor, staurosporine, on delivery of endocytosed asialoglycoprotein to lysosome in monolayer culture of rat hepatocytes

An inhibitor of protein kinases, staurosporine (ssp), was found to affect the endocytic pathway of asialoglycoproteins subsequent to endocytosis in monolayer cultures of rat hepatocytes. The effect of 5 or 10 microM staurosporine on the internalization of a synthetic ligand (galBSA-HRP: bovine serum albumin exposing galactose, horseradish peroxidase conjugates) prebound to the cell surface was minimal. The presence of 5, 7, or 10 microM ssp during a 1-h chase period resulted in the ligand remaining in a low density (1.04-1.05 g/ml), nonlysosomal subcellular fraction in a Percoll gradient. The ligand, arrested by 7 microM ssp, was further processed to the lysosome during subsequent incubation in the absence of ssp. Cells maintained the ability to internalize ligand at 37 degrees C for 1 h in the presence of these concentrations of ssp. During a 1-h continuous uptake of 0-50 micrograms/ml nonlabeled ligand, the presence of 7 microM ssp did not cause any decrease in the amount of asialoglycoprotein receptor at the cell surface, which indicates receptor recycling occurred normally. These results suggest a possible involvement of protein kinase(s), which can be inhibited by ssp, in the delivery of endocytosed ligand to the lysosome, but not in ligand endocytosis and receptor recycling.

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.

Role of monocytes in anti-CD3-induced T-cell DNA synthesis: effect of chloroquine and monensin on anti-CD3-induced human T-cell activation

Anti-CD3 monoclonal antibody (MoAb) induces proliferation of freshly isolated peripheral blood T cells only in the presence of monocytes/macrophages and requires binding of the Fc portion of antibody to monocytes/macrophages. In this investigation, we examined whether monocytes process anti-CD3 similar to any soluble antigen and present to T cells in context with HLA-DR to induce maximal DNA synthesis. Adherent monocytes were pulsed with anti-CD3 MoAb in the presence or absence of the lysozomotropic agents chloroquine and monensin, which are known to inhibit processing of soluble antigens, washed extensively, and then incubated with autologous T cells in the absence of soluble anti-CD3, and 3H-thymidine incorporation and CD25 expression were measured. Both monensin and chloroquine inhibited anti-CD3-pulsed monocyte-induced T-cell DNA synthesis and CD25 expression in a dose-dependent manner. This inhibitory effect was not due to any loss in cell viability or the effect on the expression of HLA-DR on monocytes. Paraformaldehyde-fixed monocytes pulsed with anti-CD3 MoAb induced significantly less DNA synthesis, HLA-DR expression, and CD25 antigen expression on autologous T cells as compared to responses induced by unfixed anti-CD3-pulsed monocytes. The treatment of anti-CD3-pulsed monocytes with framework-specific anti-HLA-DR MoAb inhibited their capacity to induce T-cell DNA synthesis. These data suggest that monocytes, in addition to serving as the matrix for cross-linking, also process anti-CD3 MoAb and present to the T cells in the context of HLA-DR antigens to induce optimal DNA synthesis.

Kinase activity controls the sorting of the epidermal growth factor receptor within the multivesicular body

We compared the internalization and intracellular sorting of epidermal growth factor receptor (EGF-R) and point mutant kinase-negative EGF-R separately expressed in NIH 3T3 cells lacking endogenous receptor. Both EGF-Rs internalized rapidly, but kinase-negative receptor was surface down-regulated only with monensin or at 20 degrees C. Furthermore, EGF internalized by mutant receptor alone was, in significant proportion, returned to the cell surface undegraded. Hence unlike wild-type receptor, kinase-negative EGF-R recycles. By electron microscopy the early pathways of endocytosis for the two receptors were identical; however, after 10-20 min the pathways diverged at the multivesicular body (MVB). Wild-type EGF-R, destined for degradation, localized to internal vesicles, while kinase-negative EGF-R, destined for recycling, localized to surface membranes of the MVBs and moved to small tubulovesicles. We conclude that sorting of internalized receptor for degradation or recycling can occur through spatial segregation within the MVB, and sorting of EGF-R is controlled by tyrosine kinase activity.

Internalization of the fibronectin receptor is a constitutive process

Using a monoclonal antibody specific for the hamster fibronectin receptor (FnR), we have demonstrated that a portion of the CHO cell FnR population is constitutively endocytosed. Three independent techniques were used to study the internalization: 1) after saturation binding of an anti-FnR antibody (PB1) to cells at 4 degrees C, internalization was initiated by warming to 37 degrees C, and then acid/salt elution of membrane-bound ligand was used to quantitate the internalized 125I-PB1; 2) cell vesicular traffic was pharmacologically disrupted with monensin or chloroquine, and the subsequent reduction of the cell surface pool of FnR was monitored; and 3) selective immunoprecipitation was used to separate surface and internalized 125I-labeled FnR. These experiments indicate that about 30% of the cell surface FnR is endocytosed with a t1/2 of 7 min and that this internalization occurs regardless of the ligation state of the receptor. Other observations indicate that the larger fraction of the cell surface FnR pool (70-75%) is apparently shed from the cell upon ligation with antibody at 37 degrees C. This process occurs much more slowly than receptor internalization and leads to an overall reduction in the amount of cell surface FnR. Our results suggest physically or chemically distinct populations of FnR, one of which is unavailable for internalization and recycling.

Chloroquine alters the processing of secretin in isolated rat pancreatic acinar cells

In this study we considered the effect of chloroquine on the processing and intracellular distribution of internalized secretin radioligand in acinar cells. Chloroquine (100 microM) had no effect on the total amount of 125I-secretin bound but had marked effects on the processing of this radioligand in acinar cells. After an initial 60 min of radioligand binding in the presence and absence of chloroquine, cells were washed free of unbound radioligand, resuspended and then processed for different times at 37 degrees C. During 60, 120 and 180 min of processing, the amount of internalized radioligand in the presence of 100 microM chloroquine was increased by 116, 194 and 273%, respectively, compared to untreated control samples. Chloroquine also increased the amount of intact 125I-secretin radioligand within the cell as measured by rebinding to pancreatic plasma membranes. After 120 and 180 min of processing, intact peptide within the acinar cell was 25 and 66% greater in the presence of this agent than in control samples (P less than or equal to 0.01). To determine if chloroquine affected intracellular localization of the secretin radioligand, we measured the amount of radioactivity in soluble and particulate fractions of cell homogenates. Chloroquine decreased radioactivity entering particulate fractions of the cell by greater than 35% after 120 and 180 min of processing (P less than or equal to 0.01). This study demonstrates that (1) chloroquine inhibits the intracellular degradation of secretin in acinar cells and (2) chloroquine alters intracellular localization of this peptide during processing.

Loss of surface galactosyl receptor activity on isolated rat hepatocytes induced by monensin or chloroquine requires receptor internalization via a clathrin coated pit pathway

We studied the effect of hyperosmotic inhibition of the clathrin coated pit cycle on the monensin- and chloroquine-dependent loss of surface galactosyl (Gal) receptor activity on isolated rat hepatocytes. Cells treated for 60 min without ligand at 37 degrees C with 25 microM monensin or 300 microM chloroquine in normal medium (osmolality congruent to 275 mmol/kg) bound 40-60% less 125I-asialo-orosomucoid (ASOR) at 4 degrees C than untreated cells. Cells exposed to monensin or chloroquine retained progressively more surface Gal receptor activity, however, when the osmolality of the medium was increased above 400 mmol/kg (using sucrose as osmolite) 10 min prior to and during drug treatment. Cells pretreated for 10 min with hyperosmolal media (600 mmol/kg) alone internalized less than or equal to 10% of surface-bound 125I-ASOR. Thus, the ligand-independent loss of surface Gal receptor activity on monensin- and chloroquine-treated hepatocytes requires internalization of constitutively recycling receptors via a coated pit pathway.

The low density lipoprotein receptor

The study of familial hypercholesterolemia at the molecular level has led to its advancement from a clinical syndrome to a fascinating experimental system. FH was first described 50 years ago by Carl Müller who concluded that the disease produces high plasma cholesterol levels and myocardial infarctions in young people, and is transmitted as an autosomal dominant trait determined by a single gene. The existence of two forms of FH, namely heterozygous and homozygous, was recognized by Khachadurian and Fredrickson and Levy much later. The value of FH as an experimental model system lies in the availability of homozygotes, because mutant genes can be studied without interference from the normal gene. The first and most important breakthrough was the realization that the defect underlying FH could be studied in cultured skin fibroblasts. Rapidly, the LDL receptor pathway was conceptualized and its dysfunction in cells from FH homozygotes was demonstrates. Isolation of the normal LDL receptor protein and studies on the biosynthesis and structure of abnormal receptors in mutant cell lines provided essential groundwork for elucidation of defects at the DNA level. The power of the experimental system, FH, became nowhere more obvious than in work that correlated structural information at the protein level with the elucidation of defined defects in the LDL receptor gene. In addition to revealing important structure-function relationships in the LDL receptor polypeptide and delineating mutational events, studies of FH have established several more general concepts. First, the tight coupling of LDL binding to its internalization suggested that endocytosis was not a non-specific process as suggested from early observations. The key finding was that LDL receptors clustered in coated pits, structures that had been described by Roth and Porter 10 years earlier. These investigators had demonstrated, in electron microscopic studies on the uptake of yolk proteins by mosquito oocytes, that coated pits pinch off from the cell surface and form coated vesicles that transport extracellular fluid into the cell. Studies on the LDL receptor system showed directly that receptor clustering in coated pits is the essential event in this kind of endocytosis, and thus established receptor-mediated endocytosis as a distinct mechanism for the transport of macromolecules across the plasma membrane. Subsequently, many additional systems of receptor-mediated endocytosis have been defined, and variations of the overall pathway have been described.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of monensin on fluid phase and receptor mediated endocytosis by rat hepatocyte monolayers

We have investigated the effect of monensin on the uptake of sucrose and sucrose-asialofetuin by rat hepatocyte monolayers and on the intracellular traffic of these molecules. Endocytosis of sucrose is not affected by monensin while that of sucrose-asialofetuin is markedly inhibited. Monensin causes a decrease of galactosyltransferase, a Golgi membrane enzyme without affecting dipeptidylpeptidase IV a plasma membrane enzyme and three lysosomal hydrolases. The inhibition of sucrose-asialofetuin uptake is proportional to the reduction of galactosyltransferase activity. Differential and isopycnic centrifugation results indicate that monensin at a concentration (10 mumol/l) that markedly decreases sucrose-asialofetuin uptake does not prevent the transfer of sucrose and sucrose-asialofetuin from endosomes to lysosomes.

High sensitivity towards monensin of receptor-mediated endocytosis of formaldehyde treated albumin by liver endothelial cells

Endocytosis of formaldehyde-treated serum albumin (f-albumin) in isolated liver sinusoidal endothelial cells was studied. Uptake occurs via the scavenger receptor and was found to be very sensitive to the ionophore monensin. Binding at 4 degrees C of f-albumin was reduced to 50% of control values by preincubation for 2 min with 2 microM monensin. Both uptake and degradation of f-albumin were more sensitive to monensin. No lag-phase in the inhibitory effect on uptake and degradation was detected. A concentration of 0.1 microM monensin reduced uptake of f-albumin by 50%. Degradation of internalized f-albumin was reduced by 50% in the presence of 0.2 microM monensin. Since uptake and degradation of f-albumin were very sensitive to monensin, the effect of introducing the drug during endocytosis of the ligand was tested. All processing of f-albumin stopped instantly upon addition of monensin; hence, there seems to be no step in the endocytic process beyond which monensin is ineffective. The data suggest that the scavenger receptor of liver endothelial cells is internalized and recycled very rapidly.

Effect of monensin on receptor recycling during continuous endocytosis of asialoorosomucoid

The binding of asialoglycoproteins to their liver cell receptor results in internalization of the ligand-receptor complex. These complexes rapidly appear in intracellular compartments termed endosomes whose acidification results in ligand-receptor dissociation. Ligand and receptor subsequently segregate: ligand is transported to lysosomes and is degraded while receptor recycles to the cell surface. The proton ionophore monensin prevents acidification of endosomes and reversibly inhibits this acid-dependent dissociation of ligand from receptor. The present study determined the effect of monensin treatment of short-term cultured rat hepatocytes on cell-surface-receptor content, determined both by their binding activity and immunologically, following continuous endocytosis of asialoorosomucoid. Inclusion of 5 microM monensin in the incubation medium reduced the number of immunologically detectable cell-surface receptors by 20% in the absence of ligand. During continuous endocytosis of asialoorosomucoid, inclusion of monensin resulted in a 30-40% reduction of cell-surface receptor detectable either by ligand binding or immunologically. These results suggest that the reduced liver-cell-surface content of receptor in monensin is due to intracellular trapping of ligand-receptor complexes. The reduction of surface receptor during monensin incubation in the absence of ligand suggests that "constitutive recycling" of plasma membrane components also requires intracellular acidification.

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.

Inhibition of very-low-density lipoprotein secretion by chloroquine, verapamil and monensin takes place in the Golgi complex

The effects of chloroquine, verapamil and monensin on secretion of very-low-density lipoproteins (VLDLs) were studied in cultured rat hepatocytes. Maximum inhibition of VLDL-triacylglycerol secretion by 50-90% of control was reached at 200 microM chloroquine, 200 microM verapamil and 5 microM monensin, whereas no effect on cellular triacylglycerol synthesis was observed. The inhibition could be seen within 15 min and was reversible after washout of the drugs. Chloroquine and verapamil inhibited both cellular protein synthesis and protein secretion, whereas monensin reduced protein secretion without any effect on protein synthesis. Control experiments with cycloheximide revealed that intact protein synthesis was not necessary for secretion of VLDL-triacylglycerol during 2 h. Electron micrographs of cells treated with chloroquine, verapamil or monensin showed swollen Golgi cisternae containing VLDL-like particles. By morphometry, a more than 2-fold increase in volume fractions and size indices of Golgi complexes and secondary lysosomes was observed, except that monensin had no significant effect on these parameters of secondary lysosomes. These results suggest that the inhibition of VLDL secretion by chloroquine, verapamil and monensin which takes place in the Golgi complex might be due to disruption of trans-membrane proton gradients. An increase in pH of acidic Golgi vesicles may cause swelling and disturb sorting and membrane flow through this organelle.

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.

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.

Receptor mediated endocytosis of formaldehyde treated albumin, yeast invertase and chondroitin sulfate in suspensions of rat liver endothelial cells

Isolated rat liver endothelial cells take up and degrade formaldehyde serum albumin (FSA), invertase and chondroitin sulfate (CS) efficiently. Degradation products start to appear in the medium after 5-30 min. Calcium was necessary for binding of invertase to the cells, but not for the two other ligands. Ammonia and monensin inhibited uptake as well as degradation of all three ligands, whereas leupeptin only inhibited the degradation of FSA and invertase. Uptake of CS was strongly inhibited in the presence of 1 microM FSA. The possibility that these two ligands bind to a common receptor is discussed.

Monensin-dependent and -independent mechanisms of cell-matrix adhesion

Attachment and spreading of human FL cells on a subcellular matrix (SCM) preparation made by treating confluent cell monolayers with deoxycholate are insensitive to the presence of monensin. However, if the cell suspension is surface-iodinated prior to adhesion using the LPO/H2O2 system, cell spreading on SCM is inhibited by 1 microM monensin. The suggested interpretation is that cell surface components required for cell spreading on SCM are inactivated by iodination and need replacement from intracellular reserves by a monensin-sensitive pathway. This pathway is not required in the absence of iodination when sufficient surface components (or a monensin-independent pathway of surface expression) are available. Support for this interpretation is obtained by means of double-iodination experiments in which surface-labelled cells adhere and spread, are detached and labelled a second time and then allowed to adhere again to SCM. Cell spreading in the second case is inhibited by approximately 80%, suggesting that both previously expressed and newly recruited receptors are inactivated.

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.

Receptor-mediated endocytosis

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