Recycling of the asialoglycoprotein receptor in isolated rat hepatocytes. Dissociation of internalized ligand from receptor occurs in two kinetically and thermally distinguishable compartments

Alcoholic vs non-alcoholic fatty liver in rats: distinct differences in endocytosis and vesicle trafficking despite similar pathology

Background

Non-alcoholic and alcoholic fatty liver disease (NAFLD and AFLD, respectively) are major health problems, as patients with either condition can progress to hepatitis, fibrosis, and cirrhosis. Although histologically similar, key differences likely exist in these two models. For example, altered content of several vesicle trafficking proteins have been identified in AFLD, but their content in NAFLD is unknown. In this study, we compared select parameters in NAFLD and AFLD in a rat model.

Methods

We fed either Lieber- DeCarli liquid control or alcohol-containing (35 % as calories) diet (AFLD model) or lean or high-fat (12 or 60 % derived from fat, respectively) pellets (NAFLD model) for 8–10 weeks, n = 8 in each model. Serum, hepatocytes and liver tissue were analyzed. Liver injury markers were measured in serum, triglyceride content and endocytosis (binding and internalization of ¹²⁵I- asialoorosomucoid) was measured in isolated hepatocytes, and content of selected trafficking proteins (Rab3D, Rab7 and Rab18) were determined in whole liver tissue.

Results

Although liver injury markers and triglyceride content were similar in both models, binding and internalization of ¹²⁵I- asialoorosomucoid was significantly impaired in the hepatocytes from AFLD, but not NAFLD, animals. In addition, protein content of the asialoglycoprotein receptor (ASGPR) and three trafficking proteins, Rab3D, Rab7and Rab18, were significantly decreased after alcohol, but not high-fat feeding. Levels of protein carbonylation, amount of glutathione stores, and lipid peroxidation were similar irrespective of the insult to the livers that resulted in fatty liver.

Conclusion

Impairments in protein trafficking in AFLD are likely a direct result of alcohol administration, and not a function of fatty liver.

A hyaluronan receptor for endocytosis (HARE) link domain N-glycan is required for extracellular signal-regulated kinase (ERK) and nuclear factor-κB (NF-κB) signaling in response to the uptake of hyaluronan but not heparin, dermatan sulfate, or acetylated low density lipoprotein (LDL)

The human hyaluronan (HA) receptor for endocytosis (HARE; the 190-kDa C terminus of Stab2) is a major clearance receptor for multiple circulating ligands including HA, heparin (Hep), acetylated LDL (AcLDL), dermatan sulfate (DS), apoptotic debris, and chondroitin sulfate types A, C, D, and E. We previously found that HARE contains an N-glycan in the HA binding Link domain (at Asn(2280)), and cells expressing membrane-bound HARE(N2280A) bind and endocytose HA normally (Harris, E. N., Parry, S., Sutton-Smith, M., Pandey, M. S., Panico, M., Morris, H. R., Haslam, S. M., Dell, A., and Weigel, P. H. (2010) Glycobiology 20, 991-1001). Also, NF-κB-mediated signaling is activated by HARE-mediated endocytosis of HA, Hep, AcLDL, or DS but not by chondroitin sulfates (Pandey, M. S., and Weigel, P. H. (2014) J. Biol. Chem. 289, 1756-1767). Here we investigated the role of Link N-glycans in ligand uptake and NF-κB and ERK1/2 signaling. HA·HARE-mediated ERK1/2 activation was HA size- dependent, as found for NF-κB activation. HARE(N2280A) cells internalized HA, Hep, AcLDL, and DS normally. No ERK1/2 activation occurred during HA endocytosis by HARE(N2280A) cells, but activation did occur with Hep. Dual-luciferase recorder assays showed that NF-κB-mediated gene expression occurred normally in HARE(N2280A) cells endocytosing Hep, AcLDL, or DS but did not occur with HA. Activation of NF-κB by endogenous degradation of IκB-α was observed for HARE(N2280A) cells endocytosing Hep, AcLDL, or DS but not HA. We conclude that a Link domain complex N-glycan is required specifically for HARE·HA-mediated activation of ERK1/2 and NF-κB-mediated gene expression and that this initial activation mechanism is different from and independent of the initial mechanisms for HARE-mediated signaling in response to Hep, AcLDL, or DS uptake.

NMR study of ligand release from asialoglycoprotein receptor under solution conditions in early endosomes

Asialoglycoprotein receptor (ASGP-R) is an endocytic C-type lectin receptor in hepatocytes that clears plasma glycoconjugates containing a terminal galactose or N-acetylgalactosamine. The carbohydrate recognition domain (CRD) of ASGP-R has three Ca(2+) binding sites (sites 1, 2 and 3), with Ca(2+) at site 2 being directly involved in ligand binding. Following endocytosis, the ligands are released from ASGP-R in endosomes to allow receptor recycling to the cell membrane. Although dissociation of the receptor-ligand complex is mediated by the acidic environment within the mature endosomes, many of these complexes also dissociate in the early time of endocytosis, where pH is approximately neutral. To investigate the mechanism of ligand release from ASGP-R in early endosomes, we examined the binding mode of Ca(2+) and ligands to ASGP-R CRD by NMR. We demonstrate that sites 1 and 2 of ASGP-R are high affinity Ca(2+) binding sites, site 3 is low affinity, and that Ca(2+) ions bind to sites 1 and 2 cooperatively. The pH and Ca(2+) concentration dependences of Ca(2+) binding states indicated that early endosome conditions favor apo-ASGP-R CRD, allowing ligand release. Our results elucidated that the cooperative binding mode of Ca(2+) makes it possible for ASGP-R to be more sensitive to Ca(2+) concentrations in early endosomes, and plays an important role in the efficient release of ligand from ASGP-R. In our proposed mechanism, ASGP-R can rapidly release Ca(2+) and its ligand even at nearly neutral pH. Sequence comparisons of endocytic C-type lectin receptors suggest that this mechanism is common in their family.

Single vesicle analysis of endocytic fission on microtubules in vitro

Following endocytosis, internalized molecules are found within intracellular vesicles and tubules that move along the cytoskeleton and undergo fission, as demonstrated here using primary cultured rat hepatocytes. Although the use of depolymerizing drugs has shown that the cytoskeleton is not required to segregate endocytic protein, many studies suggest that the cytoskeleton is involved in the segregation of protein in normal cells. To investigate whether cytoskeletal-based movement results in the segregation of protein, we tracked the contents of vesicles during in vitro microscopy assays. These studies showed that the addition of ATP causes fission of endocytic contents along microtubules, resulting in the segregation of proteins that are targeted for different cellular compartments. The plasma membrane proteins, sodium (Na+) taurocholate cotransporting polypeptide (ntcp) and transferrin receptor, segregated from asialoorosomucoid (ASOR), an endocytic ligand that is targeted for degradation. Epidermal growth factor receptor, which is degraded, and the asialoglycoprotein receptor, which remains partially bound to ASOR, segregated less efficiently from ASOR. Vesicles containing ntcp and transferrin receptor had reduced fission in the absence of ASOR, suggesting that fission is regulated to allow proteins to segregate. A single round of fission resulted in 6.5-fold purification of ntcp from ASOR, and 25% of the resulting vesicles were completely depleted of the endocytic ligand.

Human uridine-cytidine kinase phosphorylation of ribavirin: a convenient method for activation of ribavirin for conjugation to proteins

Ribavirin is a synthetic nucleoside analog that is used for the treatment of hepatitis C virus (HCV) infection. Its primary toxicity is hemolytic anemia, which sometimes necessitates dose reduction or discontinuation of therapy. Selective delivery of ribavirin into liver cells would be desirable to enhance its antiviral activity and avoid systemic side effects. One approach to liver-specific targeting is conjugation of the ribavirin with asialoglycoprotein that is taken up specifically by liver cells. Human uridine-cytidine kinase-1 (UCK-1) was used for ribavirin phosphorylation to its monophosphate form. 1-Ethyl-3-diisopropylaminocarbodiimide (EDC) was used as a coupling agent. The best results were obtained using direct conjugation protocol with a molar ratio of 6.5 ribavirin monophosphate (RMP) molecules per one asialoorosomucoid (AsOR) molecule. Our findings show that ribavirin is a potential substrate of UCK-1, and RMP formed could be chemically coupled to AsOR to form a conjugate for liver specific targeting.

Involvement of disulfide bonds and histidine 172 in a unique beta-sheet to alpha-helix transition of alpha 1-acid glycoprotein at the biomembrane interface

Human alpha(1)-acid glycoprotein (AGP), which is comprised of 183 amino acid residues and 5 carbohydrate chains, is a major plasma protein that binds to basic and neutral drugs as well as to steroid hormones. It has a beta-sheet-rich structure in aqueous solution. Our previous findings suggest that AGP forms an alpha-helix structure through an interaction with biomembranes. We report herein on a study of the mechanism of alpha-helix formation in AGP using various modified AGPs. The disulfide reduced AGP (R-AGP) was extensively unfolded, whereas asialylated AGP (A-AGP) maintained the native structure. Intriguingly, reduced and asialylated AGP (RA-AGP) increased the alpha-helix content as observed in the presence of biomembrane models, and showed a significant decrease in ligand binding capacity. This suggests that AGP has an innate tendency to form an alpha-helix structure, and disulfide bonds are a key factor in the conformational transition between the beta-sheet and alpha-helix structures. However, RA-AGP with all histidine residues chemically modified (HRA-AGP) was found to lose the intrinsic ability to form an alpha-helix structure. Furthermore, disulfide reduction of the H172A mutant expressed in Pichia pastoris also caused a similar loss of folding ability. The present results indicate that disulfide bonds and the C-terminal region, including H172 of AGP, play important roles in alpha-helix formation in the interaction of the protein with biomembranes.

Endocytic function, glycosaminoglycan specificity, and antibody sensitivity of the recombinant human 190-kDa hyaluronan receptor for endocytosis (HARE)

The human hyaluronan receptor for endocytosis (hHARE) mediates the endocytic clearance of hyaluronan (HA) and chondroitin sulfate from lymph fluid and blood. Two hHARE isoforms (190 and 315 kDa) are present in sinusoidal endothelial cells of liver, spleen, and lymph nodes (Zhou, B., McGary, C. T., Weigel, J. A., Saxena, A., and Weigel, P. H. (2003) Glycobiology 13, 339-349). Here we report the specificity and function of the 190-kDa HARE, expressed without the larger isoform, in Flp-In 293 cell lines (190hHARE cells). Like the native protein, recombinant hHARE contains approximately 25 kDa of N-linked oligosaccharides, binds HA in a ligand blot assay, cross-reacts with three anti-rat HARE monoclonal antibodies, and is inactivated by reduction. The 190hHARE cell lines mediated rapid, continuous (125)I-HA endocytosis and degradation for >1 day. About 30-50% of the total cellular receptors were on the cell surface, and their recycling time for reutilization was approximately 8.5 min. The average K(d) for the binding of HA to the 190-kDa hHARE at 4 degrees C was 7 nm with 118,000 total HA binding sites per cell. Competition studies at 37 degrees C indicated that the 190-kDa hHARE binds HA and chondroitin better than dermatan sulfate and chondroitin sulfates A, C, D, and E, but it does not bind to heparin, heparan sulfate, or keratan sulfate. Although competition was observed at 37 degrees C, none of the glycosaminoglycans tested, except HA, competed for (125)I-HA binding by 190hHARE cells at 4 degrees C. Anti-HARE monoclonal antibodies #30 and #154, which do not inhibit (125)I-HA uptake mediated by the 175-kDa rat HARE, partially blocked HA endocytosis by the 190-kDa hHARE. We conclude that the 190-kDa hHARE can function independently of other hHARE isoforms to mediate the endocytosis of multiple glycosaminoglycans. Furthermore, the rat and human small HARE isoforms have different glycosaminoglycan specificities and sensitivities to inhibition by cross-reacting antibodies.

Effects of lipid rafts on dynamics of retroviral entry and trafficking: Quantitative analysis

The association of cell surface receptors with sterol-sphingolipid-enriched microdomains of the plasma membrane, so-called lipid rafts, may affect the receptor-mediated entry and trafficking dynamics of viruses. A model retrovirus, subgroup A avian sarcoma and leukosis virus (ASLV-A), can initiate infection by binding to either of two forms of the tumor virus subgroup A (TVA) receptor, a lipid-raft-associated glycosylphosphatidylinositol (GPI)-anchored receptor (TVA800) or a transmembrane receptor (TVA950). Narayan et al. previously found that virus particles bound to TVA950 were more rapidly internalized than virions bound to TVA800, and the internalization via TVA950 exhibited biphasic kinetics. To explore potential molecular mechanisms for these results we developed a mathematical model that accounts for internalization of viruses through cellular pits, trafficking to an endosomal compartment where fusion occurs, and viral DNA synthesis. By fitting the model to experimental data we found that viruses bound to TVA950 were internalized up to 2.6-fold more rapidly than viruses bound to TVA800. Two- to threefold greater lateral diffusivities of transmembrane proteins, relative to GPI-anchored proteins, observed in other systems, suggest that the internalization rate of ASLV-A is diffusion-limited. Furthermore, by allowing for recycling of internalized TVA950-bound viruses back to the cell surface, we can account for the observed biphasic internalization kinetics. This mechanism is also consistent with the observed slower rate of DNA synthesis for viruses that enter via TVA950. Overall, the model provides a means to generate new experimentally testable hypotheses and sets a foundation for building a quantitative and integrated understanding of viral entry, trafficking, and intracellular dynamics.

Receptor-mediated endocytosis by the asialoglycoprotein receptor: effect of ethanol administration on endosomal distribution of receptor and ligand

Using the asialoglycoprotein receptor (ASGP-R) and a representative ligand, asialoorosomucoid (ASOR), we have previously shown ethanol-induced impairment of endosomal acidification, receptor recycling and ligand binding, internalization, and degradation. In the current study, we further investigated ethanol-induced alterations in receptor/ligand trafficking by labeling endosomes in vivo with either Texas-Red-ASOR or 125I-ASOR, and then assessing the receptor/ligand content of endosomes. We assessed two fractions after both 5 and 25 min of labeling: 'early endosomes' (EEs; endosomes from the cell periphery) and 'late endosomes' (LEs; endosomes farther into the cell interior). At both time points, significantly more ligand was found in EE fractions isolated from chow- and pair-fed controls (3:1, EE to LE, respectively). However, endosomes isolated from ethanol-fed animals showed a shift over time toward a more equal ligand distribution between endosome fractions (P < or = 0.05). Analysis of the ASGP-R content revealed a distribution pattern between the endosome fractions similar to that observed for ligand distribution. Impairment of receptor-ligand dissociation was assessed in endosome fractions by determining bound/free ligand ratios. Analysis showed that most of the ligand present in both endosome fractions was free (56-99%), although more was bound to receptor in EE vs LE of both control and ethanol animals (P < or = 0.05). At 5 min, more ligand remained bound in endosomes from ethanol-fed animals compared with control endosomes (P < or = 0.05), and the same pattern was observed at the latter time point. These results suggest that delayed dissociation may cause the receptor ligand complexes to travel farther into the cell interior, which may impair proper trafficking of the ligand to lysosomes and alter the receptor recycling.

Impaired receptor-mediated endocytosis by the asialoglycoprotein receptor in ethanol-fed mice: implications for studying the role of this receptor in alcoholic apoptosis

During receptor-mediated endocytosis (RME), extracellular molecules are internalized after being recognized and bound to specific cell surface receptors. In previous studies of the asialoglycoprotein receptor (ASGPR) in rats, we showed that ethanol impairs RME at multiple ASGPR sites. Ethanol administration has been shown to increase apoptosis, and we demonstrated increased sensitization to apoptotic induction in hepatocytes from ethanol-fed rats. Although a physiological role for the ASGPR has not been identified, investigators have shown its involvement in the uptake/clearance of apoptotic cells in vitro. This suggests a potential role for the ASGPR in the removal of apoptotic cells, and the recent availability of an ASGPR-deficient mouse strain provides an excellent opportunity to examine the role of the ASGPR during ethanol impairment. In this study, we examined ethanol-impaired RME in mice and began the characterization of ASGPR-deficient mice for use in ethanol studies. Similar to our findings with rats, ligand binding, internalization, and degradation were decreased 45-50% in hepatocytes from ethanol-fed wild-type mice. In ASGPR-deficient mice, these parameters did not vary among the chow-fed, pair-fed control, or ethanol groups and were negligible compared with those of wild-type mice. TUNEL analysis of liver sections showed an ethanol-induced increase in apoptotic bodies in all mouse strains with a significant difference in the receptor-deficient mice. Further, the livers of ASGPR-deficient mice had three times more apoptotic bodies, in all feeding groups, compared with wild-type mice. These results support the use of the ASGPR-deficient mouse model for studying ethanol-induced liver injury, specifically ethanol-induced apoptosis.

The minor subunit splice variants, H2b and H2c, of the human asialoglycoprotein receptor are present with the major subunit H1 in different hetero-oligomeric receptor complexes

The hepatic asialoglycoprotein receptor (ASGP-R) is an endocytic receptor that mediates the internalization of desialylated glycoproteins and their delivery to lysosomes. The human ASGP-R is a hetero-oligomeric complex composed of H1 and H2 subunits. There are three naturally occurring H2 splice variants, designated H2a, H2b, and H2c, although the expression of the H2c protein had not been reported. Following deglycosylation of purified ASGP-R, we detected the H2b and H2c proteins in HepG2 and HuH-7 hepatoma cells, using an antibody directed against a COOH-terminal peptide common to all H2 isoforms (anti-H2-COOH) and another antibody against a 19-amino acid cytoplasmic insert found only in H2b (anti-H2-Cyto19). H1 and both H2b and H2c were co-purified by affinity chromatography, using asialo-orosomucoid (ASOR)-, anti-H1-, or anti-H2-COOH-Sepharose, whereas only H1 and H2b were immunoprecipitated with anti-H2-Cyto19. These results indicate that H2b and H2c are not present in the same ASGP-R complexes with H1. Similar to the H2b isoform, H2c was also palmitoylated, indicating that the 19-residue cytoplasmic insert does not regulate palmitoylation. Stably transfected SK-Hep-1 cell lines expressing ASGP-R complexes containing H1 and either H2b or H2c had similar binding affinities for ASOR and endocytosed and degraded ASOR at similar rates. The pH dissociation profiles of ASOR.ASGP-R complexes were also identical for complexes containing either H2b or H2c. We conclude that the H2b and H2c isoforms are both functional but are not present with H1 in the same hetero-oligomeric ASGP-R complexes. This structural difference between two functional subpopulations of ASGP-Rs may provide a molecular basis for the existence of two different pathways, designated State 1 and State 2, by which several types of recycling receptors mediate endocytosis.

Ligand-induced downregulation of receptor-mediated clearance of hepatocyte growth factor in rats

The change in tissue uptake clearance of 125I-labeled hepatocyte growth factor (HGF) after an intravenous injection of an excess (120 micrograms/kg) of unlabeled HGF was examined in rats. The heparin-washable component of the hepatic uptake clearance of 125I-HGF was only slightly changed, whereas the heparin-resistant component was significantly reduced 30 min after injection of excess HGF, followed by gradual recovery with a half-life of 3.2 h. Because the former clearance mainly represents 125I-HGF association with heparan sulfate proteoglycan on the cell surface and/or extracellular matrix, whereas the latter includes relatively specific clearance, such as receptor-mediated endocytosis, this result suggests that injection of excess HGF selectively causes downregulation of receptor-mediated HGF clearance in the liver. Downregulation could also be observed for HGF receptor density in isolated liver plasma membrane, assessed by Western blot analysis by means of anti-receptor antibody, 30 min after injection of excess unlabeled HGF, supporting the hypothesis that the overall elimination of HGF from the systemic circulation can be affected by a change in HGF receptor density on the liver cell surface.

Impairment of the asialoglycoprotein receptor by ethanol oxidation

It is well established that ethanol exposure impairs the process of receptor-mediated endocytosis in hepatic cells, although the molecular mechanism(s) and the physiological consequence(s) of this impairment are unclear. Because addressing these mechanistic questions is difficult in vivo, we have developed a recombinant cell line of hepatic origin capable of metabolizing ethanol. In this study, we have used these recombinant cells, designated HAD cells, to investigate the ethanol-induced impairment to the receptor-mediated endocytosis of the hepatic asialoglycoprotein receptor. Comparing the binding of the ligand asialoorosomucoid in both the parental Hep G2 cells and the recombinant HAD cells, maintained in the presence and absence of ethanol, revealed decreased ligand binding in the HAD cells. This impairment was accentuated by prolonging the ethanol exposure, reaching approximately 40% in both surface and total receptor populations by 7 days. Addition of the alcohol dehydrogenase inhibitor pyrazole to the ethanol-containing medium abolished this impairment, indicating that the decreased binding was a result of the alcohol dehydrogenase-mediated oxidation of ethanol. Furthermore, using antibody specific to the asialoglycoprotein receptor, it was demonstrated that the ethanol-induced impairment in ligand binding was a consequence of decreased ligand binding and not a result of diminished receptor numbers. These results indicated that ethanol oxidation was required for the ethanol-induced impairment in ligand binding, and that the reduced ligand binding was a result of a decrease in the ability of the ligand to bind to the receptor.

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

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

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 hyperosmolarity on both receptor-mediated and fluid-phase endocytosis in ethanol-fed animals

We have shown previously that chronic ethanol administration impairs hepatic receptor-mediated endocytosis (RME) of asialoorosomucoid (ASOR), epidermal growth factor and insulin, whereas early uptake by fluid-phase endocytosis (FPE) of a fluorescent dye, Lucifer Yellow (LY), is not altered. Results of these studies suggested that ethanol-induced injury was primarily affecting endocytosis in coated pit areas of the plasma membrane while internalization in noncoated membrane areas was unaffected. In the present study, we investigated the effects of blocking clathrin-coated pit mediated endocytosis by hyperosmolarity on FPE of LY and on RME of ASOR. We also examined the effects of hyperosmolarity on the binding and internalization of insulin, a ligand endocytosed by both RME and FPE. Uptake of LY by noncoated regions of the membrane was not altered in control animals, whereas in hepatocytes from ethanol-fed animals uptake of LY was decreased by 35-40% in the presence of 0.12 M sucrose (P < 0.05). These hyperosmolar conditions almost completely inhibited (> 85%) the endocytosis of 125I-ASOR by RME in both ethanol and control cells. Results with insulin showed slight effects (20-30% impairment) on uptake of the ligand in the presence of sucrose. These results are consistent with previous reports that in normal cells the coated pit pathway is impaired by hyperosmolarity, whereas endocytosis in noncoated regions is unaltered. It appears, however, that both FPE and RME in hepatocytes from ethanol-fed animals are susceptible to perturbation by hyperosmolarity. These results indicate that the noncoated pit pathway may be sensitive to stressful conditions such as hyperosmolarity after ethanol treatment.

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.

Zonal differences in ethanol-induced impairments in hepatic receptor binding

We have previously shown ethanol-induced defects in receptor-mediated endocytosis of asialoorosomucoid (ASOR), epidermal growth factor (EGF), and insulin in isolated rat hepatocytes. The present study was undertaken to compare the binding of these three ligands in both Zone 1 (periportal [PP] region) and Zone 3 (perivenule [PV] region) of rat liver. Cells from the PV region of ethanol-fed animals bound significantly less EGF (40% decrease) than did cells from the same area in control rats. EGF binding was decreased to a lesser extent (15-25%) in PP cells from ethanol-fed animals compared to controls. When binding of ASOR was examined, ethanol feeding significantly impaired binding in both PP cells (30-35% decrease) and PV cells (50-55% decrease), again showing a greater ethanol-induced impairment in the PV region. Insulin binding in ethanol animals was decreased by 20-25% in both regions compared to controls. In addition, we found that ASOR receptor recycling was impaired to a greater extent in the PV than in the PP region of liver after ethanol feeding, indicating selective impairment of receptor function in the centrilobular region of the liver.

The effects of chronic ethanol administration on the rates of internalization of various ligands during hepatic endocytosis

The purpose of the present study was to further characterize the ethanol-induced impairments in hepatic endocytosis. Specifically, we examined the effects of ethanol treatment on receptor-ligand internalization via the coated and noncoated pit pathways. Insulin, epidermal growth factor (EGF) and asialoorosomucoid (ASOR) were used as model ligands to study internalization by isolated hepatocytes. ASOR and EGF are thought to be internalized strictly in coated pit regions of the cell membrane, while insulin may be internalized in both coated and uncoated membrane regions. Ethanol administration for 5-7 weeks decreased internalization of ASOR and EGF while internalization of insulin was unchanged during a single round of endocytosis of surface-bound ligand. Similarly, a more quantitative measure of endocytosis, the endocytic rate constant, was decreased for EGF and ASOR but not for insulin in livers of experimental rats. When endocytosis of Lucifer yellow, a fluorescent dye known to be internalized in the cell by fluid-phase endocytosis was examined, the initial rates of dye uptake were not significantly altered by alcohol administration. These results indicate that ethanol may selectively impair internalization occurring by coated pits while it has a minimal effect on initial uptake of molecules which are internalized by noncoated membrane regions.

Mechanisms of metal transport across liver cell plasma membranes

The liver's pivotal role in the homeostasis of essential trace metals and detoxification of exogenous metals is attributed to its ability to efficiently extract metals from plasma, metabolize, store, and redistribute them in various forms either into bile or back into the bloodstream. Bidirectional transport across the sinusoidal plasma membrane allows the liver to control plasma concentrations and therefore availability to other tissues. In contrast, transport across the canalicular membrane is largely, but not exclusively, unidirectional and is a major excretory pathway. Although each metal has relatively distinct hepatic transport characteristics, some generalizations can be made. First, movement of metals from plasma to bile follows primarily a transcellular route. The roles of the paracellular pathway and of ductular secretion appear minimal. Second, intracellular binding proteins and in particular metallothionein play only indirect roles in transmembrane flux. The amounts of metallothionein normally secreted into plasma and bile are quite small and cannot account for total metal efflux. Third, metals traverse liver cell plasma membranes largely by facilitated diffusion, and by fluid-phase, adsorptive, and receptor-mediated endocytosis/exocytosis. There is currently no evidence for primary active transport. Because of the high rate of hepatocellular membrane turnover, metal transport via endocytic vesicles probably makes a larger contribution than previously recognized. Finally, there is significant overlap in substrate specificity on the putative membrane carriers for the essential trace metals. For example, zinc and copper share many transport characteristics and apparently compete for at least one common transport pathway. Similarly, canalicular transport of five of the metals discussed in this overview (Cu, Zn, Cd, Hg, and Pb) is linked to biliary GSH excretion. These metals may be transported as GSH complexes by the canalicular glutathione transport system(s). Unfortunately, none of the putative membrane carrier proteins have been studied at the subcellular or molecular level. Our knowledge of their biochemical properties is rudimentary and rests almost entirely on indirect evidence obtained in vivo or in intact cell systems. The challenge for the future is to isolate and characterize these putative metal carriers, and to determine how they are functionally regulated.

Effect of chronic ethanol administration on the uptake and degradation of asialoglycoproteins by the perfused rat liver

We have shown previously reduced binding, internalization, degradation and receptor-ligand dissociation during receptor-mediated endocytosis (RME) of 125I-asialoorosomucoid (ASOR) by hepatocytes isolated from rats fed ethanol for 4-6 weeks. In the present study, we investigated the effect of ethanol feeding on RME by using the intact perfused liver as a model. Male, Sprague-Dawley rats were fed a liquid diet containing either ethanol (36% of calories) or isocaloric carbohydrate. Receptor-mediated endocytosis of 125I-ASOR was then examined over a time course of perfusion. In all cases, clearance of the labeled glycoprotein was followed by a slower but steady appearance of acid-soluble products in the medium. Ethanol-fed animals had a significantly (P less than 0.01) slower rate of clearance of the labeled ligand from the circulating perfusate than did control animals. Impairment of ASOR surface binding and degradation in ethanol-fed animals was also demonstrated in this model. When we examined the subcellular distribution of labeled ligand after various times of perfusion, we found that in control livers, a shift of radiolabeled ligand from the subcellular fractions containing endosomes and plasma membranes to fractions containing lysosomes occurred, while significantly less ligand was shifted to the lysosomes of ethanol-treated rats. These results show that ethanol administration inhibits RME of ASOR in the isolated perfused liver model, thus confirming our earlier reported defects in isolated hepatocytes. In addition, transport of ligand along the intracellular RME pathway was also shown to be altered by ethanol treatment as indicated by the impaired movement of ASOR from the endosomal to the lysosomal compartment.

Mouse zygotes express endogenous lectins

Cell interactions during mouse development have been shown to involve carbohydrate-containing macromolecules (glycoconjugates). We have therefore used a series of fluorescein-labelled synthetic glycoproteins to determine if mouse oocytes and zygotes also express sugar binding molecules (endogenous lectins) which might participate in such interactions. Unfertilized secondary oocytes did not express endogenous lectins at 4 degrees C but a low level of expression of fucose, mannose, and galactose-binding activity could be detected at 37 degrees C. In contrast, the zygote clearly expressed three classes of endogenous lectins, with preferential binding for i) fucose or mannose, ii) glucose or galactose, and iii) lactose. The expression of these lectins was much reduced at 4 degrees C and maximal binding at 37 degrees C was achieved only after 2 h incubation. We therefore conclude that a low level of endogenous lectin expression in the mouse oocyte is greatly enhanced after fertilisation and that, at both stages, expression, or the detection of expression, is markedly temperature dependent.

Targeted protection of hepatocytes from galactosamine toxicity in vivo

We present an in vivo model for specific protection of normal hepatocytes from damage by the highly specific hepatotoxin galactosamine. The idea is based on the fact that normal, unlike malignant, hepatocytes possess unique cell-surface receptors that can bind and internalize galactose terminal (asialo)glycoproteins by receptor-mediated endocytosis. A targetable carrier-antagonist conjugate was formed by coupling asialofetuin to the galactosamine antagonist uridine monophosphate. Intravenous injection of the antagonist conjugate resulted in specific uptake by the liver. Rats treated with carrier-antagonist conjugate together with a toxic dose of galactosamine developed significantly less hepatotoxicity than did controls. We conclude that a galactosamine antagonist can be targeted to liver, resulting in specific protection of hepatocytes from galactosamine toxicity in vivo. Because hepatoma cells lack asialoglycoprotein receptor activity, this "targeted rescue" may be of value in the differential protection of normal cells in the treatment of hepatocellular carcinoma.

Effect of chronic ethanol administration on total asialoglycoprotein receptor content and intracellular processing of asialoorosomucoid in isolated rat hepatocytes

Chronic ethanol administration markedly impairs the process of receptor-mediated endocytosis (RME) of a representative asialoglycoprotein, asialoorosomucoid (ASOR), by the liver (Casey et al. (1987) J. Biol. Chem. 262, 2704-2710). Decreased surface binding was the major defect reported in our initial study, along with impaired internalization and degradation of 125I-ASOR in chronically-fed ethanol animals. In this study, we further characterized these impairments by examining the content of intracellular receptors and by investigating ligand processing directed by these intracellular receptors. Ethanol administration for 5-7 weeks decreased intracellular ASOR receptor content by 40%, a result which was confirmed by using both a ligand-binding assay and an antibody-binding assay. In addition to a decreased number of intracellular receptors, an impairment in intracellular processing of receptor-ligand complexes was identified. In ethanol-fed animals, dissociation of receptor-ligand complexes was decreased during steady-state conditions of endocytosis at 37 degrees C. Impaired receptor-ligand dissociation did not alter the fate of the ligand which was to undergo diacytosis (ligand recycling), but did appear to impair degradation of intracellular ligand. These results indicate that chronic ethanol administration decreases ligand binding due to a decreased number of receptors and impairs intracellular processing of ASOR in hepatocytes.

Ethanol-induced impairments in receptor-mediated endocytosis of asialoorosomucoid in isolated rat hepatocytes: time course of impairments and recovery after ethanol withdrawal

Chronic ethanol administration markedly impairs the process of receptor-mediated endocytosis (RME) of a representative asialoglycoprotein, asialoorosomucoid (ASOR), by the liver. In this study, we further characterized these impairments by identifying the time of onset for ethanol-induced changes in RME as well as establishing the time course for recovery to normal endocytotic values after ethanol withdrawal. Ethanol administration for 3 days did not alter any aspect of endocytosis examined in this study. After feeding ethanol to rats for 7 days, however, significant decreases in amounts of ligand bound, internalized, and degraded were apparent. These impairments persisted throughout the 5-week feeding study although the effects were somewhat attenuated with more prolonged ethanol feeding. In addition, an accumulation of intracellular receptors was observed in ethanol-fed animals relative to controls after 7 days of ethanol feeding. In all cases, recovery of endocytotic values to control levels was partially completed after 2 to 3 days of refeeding control diet and was fully completed after 7 days of refeeding. These results indicate that ethanol feeding for as little as 7 days profoundly impairs the process of RME by the liver. These impairments can be reversed after refeeding control diet for 7 days.

Effects of temperature on the degradation and biliary secretion of asialoorosomucoid by the perfused rat liver: evidence for two intracellular pathways

We have utilized the in situ perfused rat liver under nonrecirculating conditions to examine the effect of temperature on the metabolism and biliary secretion of [125I]-asialoorosomucid (ASOR). In this manner we were able to follow the fate of a single round of internalized ligand. In control livers perfused at 37 degrees C, approximately 50% of [125I]-ASOR injected into the portal vein was extracted on first pass. Five minutes after the injection, radioactivity, which had been extracted initially, began to appear in the hepatic venous effluent. Within 25 min, 50% of the initially extracted radioactivity was released into the perfusion medium; the bulk of this radioactivity (greater than 95%) was soluble in trichloroacetic acid. In livers perfused at temperatures slightly less than 37 degrees C (30-35 degrees C), first-pass extraction of [125I]-ASOR was similar to that observed at 37 degrees C. However, a severalfold decrease in the rate of release of radioactivity from the liver into the perfusion medium was noted at the lower perfusion temperatures; whereas greater than 50% of the initially extracted radioactivity was released within 30 min from livers perfused at 37 degrees C, only 5% was released at 30 degrees C. At the lower perfusion temperature, a larger proportion of the released radioactivity was acid precipitable (24% vs. 5%). Some radioactivity also was recovered in the bile; of the total amount of radioactivity released from the liver in 30 min at 37 degrees C, approximately 5% was directed into the bile. At lower temperatures of perfusion, a greater fraction of the radioactivity that was released from the liver was directed into the bile (20% at 30 degrees C vs. 5% at 37 degrees C). The data imply that the endosomal pathway to the lysosome is highly sensitive to slight reductions in temperature while the transcytotic route into bile is less sensitive. Lower temperatures might prolong the residence time of ASOR in the prelysosomal endosomal compartments, and thereby increase the likelihood that undegraded ligand will be returned to the blood or be missorted into bile.

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

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

Effects of 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.

Kinetics and temperature dependence of exposure of endocytosed material to proteolytic enzymes and low pH: evidence for a maturation model for the formation of lysosomes

The temperature dependence of acidification of internalized dextran by Swiss 3T3 cells was determined using dual fluorescence flow cytometry. Essentially no acidification was observed at 11 degrees C; acidification was limited to pH 6-6.5 at temperatures between 13 degrees C and 17 degrees C. In contrast, a rapid drop to pH 6-6.5 followed by acidification to pH 5-5.5 was observed at temperatures above 19 degrees C. These results confirm the biphasic nature of the acidification process (J. Cell Biol. (1984) 98: 1757-1762). The timing of exposure of material internalized by fluid-phase endocytosis to lysosomal enzymes was determined for Swiss 3T3 cells by using a fluorogenic substrate specific for Cathepsin B. Hydrolysis of the substrate, as measured by both fluorometry and flow cytometry, began within minutes of its addition to cells at 37 degrees C, and was inhibited by coincubation with leupeptin, a competitive inhibitor of the enzyme, or by weak bases, which raise the pH of acidic compartments. At temperatures between 13 degrees (and 21 degrees C, the rate of hydrolysis was reduced to 31-44% of that at 37 degrees C. Thus, in contrast to previous reports, exposure of endocytosed material to at least one lysosomal enzyme is not inhibited below 20 degrees C; the reduction in hydrolysis rate may be explained by the temperature effects on the efficiency of the enzyme. The results for acidification and proteolysis are consistent with, but do not prove, a maturation model for the formation of lysosomes. We suggest that at lower temperatures, part of the maturation involving recycling and/or concentration of the contents of the endosome is inhibited. This causes the endosome to remain as a mildly acidic, low-density organelle containing lysosomal enzymes.

Kinetics of epidermal growth factor binding and processing in isolated intact rat hepatocytes. Dynamic externalization of receptors during ligand internalization

The kinetics of binding and processing of epidermal growth factor (EGF) was studied in freshly isolated rat hepatocytes. After isolation the hepatocytes had a nonhomogeneous population of surface EGF receptors consisting of approximately 9000 high-affinity sites (Kd 21 pM) and 165,000 low-affinity sites (Kd 0.62 nM). Incubation at 37 degrees C (45 min) increased the number of surface receptors per cell to about 260,000. This increase was selective for the low-affinity receptors and was cycloheximide-sensitive. During 5 h of incubation at 37 degrees C the hepatocytes internalized 6-7-times more EGF molecules than the number of cell surface receptors, based on clearance measurements. The uptake was unaffected by cycloheximide. Concomitant estimation, using acid/salt elution, of surface-bound EGF and internalized EGF showed that the number of internalized EGF molecules exceeded the decrease in surface-binding 6 times. The ratio between internalized EGF and the decrease in surface binding was temperature-dependent, being reduced to a one-to-one stoichiometry at 10 degrees C. After down-regulation (approximately equal to 75%) induced by 5 nM unlabeled EGF the surface EGF receptors did not recover during subsequent incubation (2 h) at 37 degrees C. However, the remaining surface receptors internalized EGF in ninefold excess of their number. The large discrepancy between internalization capacity and cell surface binding capacity was also found in the presence of cycloheximide. The results support the idea that internalized EGF receptors are partly replaced by externalization of preformed intracellular receptors during EGF uptake in isolated hepatocytes, involving recycling of a small population of EGF receptors and/or recruitment of unexposed, pre-existing receptors.