Regulation of asialoglycoprotein receptor activity by a novel inactivation/reactivation cycle. Receptor reactivation in permeable rat hepatocytes is mediated by fatty acyl coenzyme A

Impact of asialoglycoprotein receptor deficiency on the development of liver injury

The asialoglycoprotein (ASGP) receptor is a well-characterized hepatic receptor that is recycled via the common cellular process of receptor-mediated endocytosis (RME). The RME process plays an integral part in the proper trafficking and routing of receptors and ligands in the healthy cell. Thus, the mis-sorting or altered transport of proteins during RME is thought to play a role in several diseases associated with hepatocyte and liver dysfunction. Previously, we examined in detail alterations that occur in hepatocellular RME and associated receptor functions as a result of one particular liver injury, alcoholic liver disease (ALD). The studies revealed profound ethanol-mediated impairments to the ASGP receptor and the RME process, indicating the importance of this receptor and the maintenance of proper endocytic events in normal tissue. To further clarify these observations, studies were performed utilizing knockout mice (lacking a functional ASGP receptor) to which were administered several liver toxicants. In addition to alcohol, we examined the effects following administration of anti-Fas (CD95) antibody, carbon tetrachloride (CCl(4)) and lipopolysaccharide (LPS)/galactosamine. The results of these studies demonstrated that the knockout mice sustained enhanced liver injury in response to all of the treatments, as shown by increased indices of liver damage, such as enhancement of serum enzyme levels, histopathological scores, as well as hepatocellular death. Overall, the work completed to date suggests a possible link between hepatic receptors and liver injury. In particular, adequate function and content of the ASGP receptor may provide protection against various toxin-mediated liver diseases.

Nonpalmitoylated human asialoglycoprotein receptors recycle constitutively but are defective in coated pit-mediated endocytosis, dissociation, and delivery of ligand to lysosomes

The hepatic asialoglycoprotein receptor (ASGP-R) internalizes desialylated glycoproteins via the clathrin-coated pit pathway and mediates their delivery to lysosomes for degradation. The human ASGP-R contains two subunits, H1 and H2. Cytoplasmic residues Cys(36) in H1, as well as Cys(54) and Cys(58) in H2 are palmitoylated (Zeng, F.-Y., and Weigel, P. H. (1996) J. Biol. Chem. 271, 32454). In order to study the function(s) of ASGP-R palmitoylation, we mutated these Cys residues to Ser and generated stably transfected SK-Hep-1 cell lines expressing either wild-type or nonpalmitoylated ASGP-Rs. Compared with wild-type ASGP-Rs, palmitoylation-defective ASGP-Rs showed normal ligand binding, intracellular distribution and trafficking patterns, and pH-induced dissociation profiles in vitro. However, continuous ASOR uptake, and the uptake of prebound cell surface ASOR were slower in cells expressing palmitoylation-defective ASGP-Rs than in cells expressing wild-type ASGP-Rs. Unlike native ASGP-Rs in hepatocytes or hepatoma cells, which mediate endocytosis via the clathrin-coated pit pathway and are almost completely inhibited by hypertonic medium, only approximately 40% of the ASOR uptake in SK-Hep-1 cells expressing wild-type ASGP-Rs was inhibited by hyperosmolarity. This result suggests the existence of an alternate nonclathrin-mediated internalization pathway, such as transcytosis, for the entry of ASGP-R.ASOR complexes into these cells. In contrast, ASOR uptake mediated by cells expressing palmitoylation-defective ASGP-Rs showed only a marginal difference under hypertonic conditions, indicating that most of the nonpalmitoylated ASGP-Rs were not internalized and processed normally through the clathrin-coated pit pathway. Furthermore, cells expressing wild-type ASGP-Rs were able to degrade the internalized ASOR, whereas ASOR dissociation was impaired and degradation was barely detectable in cells expressing nonpalmitoylated ASGP-Rs. We conclude that palmitoylation of the ASGP-R is required for its efficient endocytosis of ligand by the clathrin-dependent endocytic pathway and, in particular, for the proper dissociation and delivery of ligand to lysosomes.

Phosphorylation-dependent interaction of the asialoglycoprotein receptor with molecular chaperones

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

Palmitoylation-defective asialoglycoprotein receptors are normal in their cellular distribution and ability to bind ligand, but are defective in ligand uptake and degradation

The hepatic asialoglycoprotein receptor (ASGP-R) is an endocytic recycling receptor that mediates the endocytosis of desialylated glycoproteins. The human ASGP-R is composed of two homologous subunits, H1 and H2, and the cytoplasmic Cys residues in both subunits are palmitoylated. To study the effects of palmitoylation on ASGP-R activity and function, we generated four types of stably transfected cell lines in SK-Hep-1 hepatoma cells, expressing wild-type, or partially or completely palmitoylation-defective ASGP-Rs containing Cys-to-Ser mutations in either one or both subunits. Scatchard analysis showed that all four stable cell lines expressed a similar number of binding sites for asialo-orosomucoid, with comparable dissociation constants of approximately 1-3nM. Immunofluorescence confocal microscopy indicated a normal distribution of the palmitoylation-defective H1 and H2 subunits compared to the wild-type. However, cell lines expressing palmitoylation-defective ASGP-Rs had markedly reduced rates of ligand uptake and degradation compared to cells expressing wild-type ASGP-Rs. We conclude that failure to palmitoylate Cys residues in either or both subunits of human ASGP-Rs results in very inefficient uptake and degradation of ligands.

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.

Effects of inhibitors of the vacuolar proton pump on hepatic heterophagy and autophagy

Bafilomycin A(1) (BAF) and concanamycin A (ConcA) are selective inhibitors of the H(+)-ATPases of the vacuolar system. We have examined the effects of these inhibitors on different steps in endocytic pathways in rat hepatocytes, using [(125)I]tyramine-cellobiose-labeled asialoorosomucoid ([(125)I]TC-AOM) and [(125)I]tyramine-cellobiose-labeled bovine serum albumin ([(125)I]TC-BSA) as probes for respectively receptor-mediated endocytosis and pinocytosis (here defined as fluid phase endocytosis). The effects of BAF and ConcA were in principle identical, although ConcA was more effective than BAF. The main findings were as follows. (1) BAF/ConcA reduced the rate of uptake of both [(125)I]TC-AOM and [(125)I]TC-BSA. The reduced uptake of [(125)I]TC-AOM was partly due to a redistribution of the asialoglycoprotein receptors (ASGPR) such that the number of surface receptors was reduced approximately 40% without a change in the total number of receptors. (2) BAF/ConcA at the same time increased retroendocytosis (recycling) of both probes. The increased recycling of the ligand ([(125)I]TC-AOM) is partly a consequence of the enhanced pH in endosomes, which prevents dissociation of ligand. (3) It was furthermore found that the ligand remained bound to the receptor in presence of BAF/ConcA and that the total amount of ligand molecules internalized in BAF/ConcA-treated cells was only slightly in excess of the total number of receptors. These data indicate that reduced pH in endosomes is the prime cause of receptor inactivation and release of ligand in early endosomes. (4) Subcellular fractionation experiments showed that [(125)I]TC-AOM remained in early endosomes, well separated from lysosomes in sucrose gradients. The fluid phase marker, [(125)I]TC-BSA, on the other hand, seemed to reach a later endosome in the BAF/ConcA-treated cells. This organelle coincided with lysosomes in the gradient, but hypotonic medium was found to selectively release a lysosomal enzyme (beta-acetylglucosaminidase), indicating that even [(125)I]TC-BSA remained in a prelysosomal compartment in the BAF/ConcA-treated cells. (5) Electron microscopy using horseradish peroxidase (HRP) as a fluid phase marker verified that BAF/ConcA inhibited transfer of material from late endosomes ('multivesicular bodies'). (6) BAF/ConcA led to accumulation of lactate dehydrogenase (LDH) in autophagic vacuoles, but although the drugs partly inhibited fusion between autophagosomes and lysosomes a number of autolysosomes was formed in the presence of BAF/ConcA. This observation explains the reduced buoyant density of lysosomes (revealed in sucrose density gradients). In conclusion, BAF/ConcA inhibit transfer of endocytosed material from late endosomes to lysosomes, but do not at the same time prevent fusion between autophagosomes and lysosomes.

A novel casein kinase 2 alpha-subunit regulates membrane protein traffic in the human hepatoma cell line HuH-7

A previously isolated endocytic trafficking mutant (TRF1) isolated from HuH-7 cells is defective in the distribution of subpopulations of cell-surface receptors for asialoorosomucoid (asialoglycoprotein receptor (ASGR)), transferrin, and mannose-terminating glycoproteins. The pleiotropic phenotype of TRF1 also includes an increased sensitivity to Pseudomonas toxin and deficient assembly and function of gap junctions. HuH-7xTRF1 hybrids exhibited a normal subcellular distribution of ASGR, consistent with the TRF1 mutation being recessive. A cDNA expression library derived from HuH-7 mRNA was transfected into TRF1 cells, which were subsequently selected for resistance to Pseudomonas toxin. Sequence analysis of a recovered cDNA revealed a unique isoform of casein kinase 2 (CK2), CK2alpha". Western blot analysis of TRF1 proteins revealed a 60% reduction in total CK2alpha expression. Consistent with this finding, the hybrids HuH-7xHuH-7 and HuH-7xTRF1 expressed equivalent amounts of total CK2alpha. Immunoblots using antibodies against peptides unique to the previously described CK2 isoforms CK2alpha and CK2alpha' and the novel CK2alpha" isoform showed that, although TRF1 and parental HuH-7 cells expressed comparable amounts of CK2alpha and CK2alpha', the mutant did not express CK2alpha". Based on the genomic DNA sequence, RNA transcripts encoding CK2alpha" apparently originate from alternative splicing of a primary transcript. Protein overexpression following transfection of TRF1 cells with cDNAs encoding either CK2alpha or the newly cloned CK2alpha" restored the parental HuH-7 phenotype, including Pseudomonas toxin resistance, cell-surface ASGR binding activity, phosphorylation, and the assembly of gap junctions. This study suggests that HuH-7 cells express at least three CK2alpha isoforms and that the pleiotropic TRF1 phenotype is a consequence of a reduction in total CK2 expression.

Effects of ethanol on receptor-mediated endocytosis in the liver

Ethanol administration impairs multiple aspects in the process of receptor-mediated endocytosis (RME) in the liver. Studies from our laboratory over the last 10 years have carefully examined RME by the hepatocyte-specific asialoglycoprotein receptor (ASGP-R). We have identified a time course for ethanol-induced defects in RME and established that many of the impairments occur initially in the centrilobular region of the liver and as early as one week after ethanol administration. Impaired intravesicular acidification in ethanol-fed animals has been identified, and these defects in acidification could alter multiple protein trafficking pathways including RME. In addition to altered acidification, altered receptor function (including receptor inactivation) could also contribute to impaired trafficking. Current studies in our laboratory are aimed at an examination of posttranslational modifications in the receptor (acylation and phosphorylation) that are known to affect its function. A role for the ASGP-R in the process of alcoholic apoptosis is also being examined because proper functioning of the ASGP-R is thought to be important in clearance of apoptotic cells.

Ethanol-impaired hepatic protein trafficking: concepts from the asialoglycoprotein receptor system

OBJECTIVES

Alcohol abuse with its resulting liver injury is a major health problem worldwide. Recent studies have shown that the process of receptor-mediated endocytosis (RME) is especially susceptible to the deleterious effects of ethanol.

DESIGN AND METHODS

In our laboratory, we have shown that after as early as 1 week of ethanol administration, binding, internalization and degradation of asialoorosomucoid, a ligand for the asialoglycoprotein receptor (ASGP-R), is significantly impaired. We have also demonstrated that ethanol administration impairs ATP-dependent acidification of prelysosomal endosomes.

RESULTS

These impairments are seen using ligands internalized by the non-specific process of fluid phase endocytosis as well as those internalized by coated pit endocytosis. In addition, we have identified ethanol-induced alterations in post-translational modifications of the receptor including phosphorylation and fatty acid modification (palmitoylation).

CONCLUSIONS

Impaired function of this receptor could lead to alterations of membrane internalization events after ethanol administration and contribute to ethanol-induced alterations in protein trafficking and signaling in the liver.

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

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

Decreased binding and autophosphorylation of the epidermal growth factor receptor in ethanol-fed rats

We have shown previously that binding and processing of epidermal growth factor are impaired in livers of ethanol-fed rats. In the current study, we examined these ethanol-induced alterations in greater detail by studying both high and low affinity epidermal growth factor binding as well as the ability of added ligand to stimulate receptor autophosphorylation. We also measured the binding of anti-receptor antibody to intact and permeabilized cells in order to determine more accurately the levels of receptor protein. Hepatocytes were isolated from ethanol-fed and pair-fed control rats. Ligand binding, antibody binding, and ligand-induced receptor autrophosphorylation were measured in the respective cell populations. In ethanol-fed animals, binding to both high and low affinity states of the hepatic epidermal growth factor receptor was decreased by 40-50% (P < 0.01). This ethanol-induced decrease in ligand binding was accompanied by a reduced ability of epidermal growth factor to stimulate receptor autophosphorylation (32% decrease, P < 0.01). In contrast, binding of anti-receptor antibody was not altered in ethanol-fed animals. In conclusion, chronic ethanol feeding decreased epidermal growth factor binding with a concomitant decrease in the ability of the receptor tyrosine kinase to phosphorylate tyrosine residues. These changes were not accompanied by an actual decrease in receptor protein content. These findings could be relevant to modified responses to this growth factor in the livers of chronic ethanol-fed animals.

Fatty acylation of the rat and human asialoglycoprotein receptors. A conserved cytoplasmic cysteine residue is acylated in all receptor subunits

Functional rat or human asialoglycoprotein receptors (ASGP-Rs) are hetero-oligomeric integral membrane glycoproteins. Rat ASGP-R contains three subunits, designated rat hepatic lectins (RHL) 1, 2, and 3; human ASGP-R contains two subunits, HHL1 and HHL2. Both receptors are covalently modified by fatty acylation (Zeng, F.-Y., Kaphalia, B. S., Ansari, G. A. S., and Weigel, P. H. (1995) J. Biol. Chem. 270, 21382-21387; Zeng, F.-Y., Oka, J. A., and Weigel, P. H. (1996) Biochem. Biophys. Res. Commun. 218, 325-330). We report here that the single Cys residue in the cytoplasmic domain of each RHL or HHL subunit is fatty acylated. The degree of acylation is >/=90% per subunit. Deacylation of affinity-purified ASGP-Rs with hydroxylamine results in the spontaneous formation of dimers through reversible disulfide bonds, indicating that deacylation concomitantly generates free thiol groups. Reaction of hydroxylamine-treated ASGP-R with [14C]iodoacetamide resulted in the specific incorporation of radioactivity into all RHL and HHL subunits, verifying that fatty acids are attached via thioester linkages. To identify the Cys residue involved in the thioester linkages, 14C-carboxyamidomethylated RHL subunits were separated by SDS-polyacrylamide gel electrophoresis and digested in-gel with trypsin, and the resulting peptides were separated by reverse-phase high performance liquid chromatography. Amino acid sequence of radioactive peptides revealed that Cys35 in RHL1 and Cys54 in RHL2 and RHL3 were radiolabeled and, therefore, are fatty acylation sites. Fatty acylation of HHL subunits was analyzed by site-directed mutagenesis. Metabolic labeling of Cos7 cells transfected with wild type HHL1 cDNA resulted in substantial incorporation of [3H]palmitate into purified HHL1. Incorporation of [3H]palmitate into a C36S mutant of HHL1 was negligible ( approximately 1%) compared with wild type. This result also shows that Cys57 within the transmembrane domain of HHL1 is not normally palmitoylated. We conclude that Cys35 in RHL1, Cys54 in RHL2 and RHL3, and Cys36 in HHL1 are fatty acylated. Cys57 in HHL1 and probably Cys56 in RHL1 are not palmitoylated.

Hydroxylamine treatment differentially inactivates purified rat hepatic asialoglycoprotein receptors and distinguishes two receptor populations

We previously showed that two subpopulations of asialoglycoprotein receptors (ASGP-Rs), designated State 1 and State 2 ASGP-Rs, are present in intact cells and that State 2 ASGP-Rs can be inactivated in permeable rat hepatocytes in a temperature- and ATP-dependent manner. These inactivated ASGP-Rs can be quantitatively reactivated by the addition of palmitoyl-CoA (Weigel, P. H., and Oka, J. A. (1993) J. Biol. Chem. 268, 27186-27190). Here we show that approximately 50% of purified rat ASGP-Rs are inactivated by treatment with hydroxylamine under mild conditions. The activity of affinity-purified ASGP-Rs was assessed by measuring the specific binding of 125I-asialo-orosomucoid (ASOR) in a dot-blot assay after immobilization onto nitrocellulose. Treatment of ASGP-Rs in solution with 0.0125-1.0 M NH2OH, pH 7.4, at 4 degrees C for 4 h resulted in a progressive loss of ASOR binding activity. ASGP-R inactivation with NH2OH occurred more readily at basic pH or at room temperature. Similar treatment with Tris had no effect on ASGP-R activity. The kinetics of ASGP-R activity loss and the dose-response for this inactivation were both biphasic, indicating the presence of two equal populations of ASGP-Rs with different sensitivities to NH2OH. The more sensitive population of ASGP-Rs (approximately 50%) was inactivated by treatment with 0.2 M NH2OH (4 degrees C, 4 h) or with 1.0 M NH2OH (4 degrees C, 1 h) without detectable peptide cleavage as assessed by SDS-polyacrylamide gel electrophoresis. State 1 ASGP-Rs, purified from chloroquine- or monensin-treated hepatocytes, showed significantly less sensitivity to NH2OH treatment (both in kinetics and dose dependence). Furthermore, under mild conditions NH2OH caused dissociation and inactivation of approximately 50% of the total ASGP-Rs (State 1 and State 2) that were prebound to ASOR-Sepharose, whereas the same treatment caused dissociation of only < 20% of State 1 ASGP-Rs from such preformed complexes. As shown in the accompanying paper (Zeng, F. Y., Kaphalia, B. S., Ansari, G. A. S., and Weigel, P. H. (1995) J. Biol. Chem. 270, 21382-21387) all three RHL subunits of active ASGP-Rs, in fact, contain covalently attached palmitate and stearate. In cultured cells, [3H]palmitic acid is metabolically incorporated into all three subunits. These radiolabeled fatty acids are completely released from purified ASGP-Rs by mild NH2OH treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Fatty acylation of the rat asialoglycoprotein receptor. The three subunits from active receptors contain covalently bound palmitate and stearate

Rat hepatic asialoglycoprotein receptors (ASGP-Rs) are hetero-oligomers composed of three homologous glycoprotein subunits, designated rat hepatic lectins (RHL) 1, 2, and 3. ASGP-Rs mediate the endocytosis and degradation of circulating glycoconjugates containing terminal N-acetylgalactosamine or galactose, including desialylated plasma glycoproteins. We have shown in permeable rat hepatocytes that the ligand binding activity of one subpopulation of receptors (designated State 2 ASGP-Rs) can be decreased or increased, respectively, by ATP and palmitoyl-CoA (Weigel, P. H., and Oka, J. A. (1993) J. Biol. Chem. 268, 27186-27190). We proposed that a reversible and cyclic acylation/deacylation process may regulate ASGP-R activity during endocytosis, receptor-ligand dissociation, and receptor recycling. In the accompanying paper (Zeng, F-Y., and Weigel, P. H. (1995) J. Biol. Chem. 270, 21388-21395), we show that the ligand binding activity of affinity-purified State 2 ASGP-Rs is decreased by treatment with hydroxylamine under mild conditions consistent with these ASGP-Rs being fatty acylated in vivo. In this study, we used a chemical method to determine the presence of covalently-bound fatty acids in individual ASGP-R subunits. The affinity-purified ASGP-R preparations were separated by SDS-polyacrylamide gel electrophoresis under nonreducing conditions, and the gel slices containing individual RHL subunits were treated with alkali to release covalently bound fatty acids, which were subsequently analyzed by gas chromatography and confirmed by gas chromatography-mass spectrometry. Both stearic and palmitic acids were detected in all three receptor subunits. Pretreatment of ASGP-Rs with hydroxylamine before SDS-polyacrylamide gel electrophoresis reduced the content of both fatty acids by 66-80%, indicating that most of these fatty acids are attached to cysteine residues via thioester linkages. Furthermore, when freshly isolated hepatocytes were cultured in the presence of [3H]palmitate, all three RHL subunits in affinity-purified ASGP-Rs were metabolically labeled. We conclude that RHL1, RHL2, and RHL3 are modified by fatty acylation in intact cells.

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.