Two asialoglycoprotein receptor polypeptides in human hepatoma cells

Hepatocyte targeting via the asialoglycoprotein receptor

This review highlights the potential of asialoglycoprotein receptor (ASGPR)-mediated targeting in advancing liver-specific treatments and underscores the ongoing progress in the field. First, we provide a comprehensive examination of the nature of ASGPR ligands, both natural and synthetic. Next, we explore various drug delivery strategies leveraging ASGPR, with a particular emphasis on the delivery of therapeutic nucleic acids such as small interfering RNAs (siRNAs) and antisense oligonucleotides (ASOs). An in-depth analysis of the current status of RNA interference (RNAi) and ASO-based therapeutics is included, detailing approved therapies and those in various stages of clinical development (phases 1 to 3). Afterwards, we give an overview of other ASGPR-targeted conjugates, such as those with peptide nucleic acids or aptamers. Finally, targeted protein degradation of extracellular proteins through ASGPR is briefly discussed.

Beyond conventional treatment: ASGR1 Leading the new era of hypercholesterolemia management

Cardiovascular disease (CVD) remains a leading cause of mortality worldwide, with hypercholesterolemia being a major risk factor. Although various lipid-lowering therapies exist, many patients fail to achieve optimal cholesterol control, highlighting the need for novel therapeutic approaches. ASGR1 (asialoglycoprotein receptor 1), predominantly expressed on hepatocytes, has emerged as a key regulator of cholesterol metabolism and low-density lipoprotein (LDL) clearance. This receptor's ability to regulate lipid homeostasis positions it as a promising target for therapeutic intervention in hypercholesterolemia and related cardiovascular diseases. This review critically examines the biological functions and regulatory mechanisms of ASGR1 in cholesterol metabolism, with a focus on its potential as a therapeutic target for hypercholesterolemia and related cardiovascular diseases. By analyzing recent advances in ASGR1 research, this article explores its role in liver-specific pathways, the implications of ASGR1 variants in CVD risk, and the prospects for developing ASGR1-targeted therapies. This review aims to provide a foundation for future research and clinical applications in hypercholesterolemia management.

Design, synthesis and biological evaluation of the positional isomers of the galactose conjugates able to target hepatocellular carcinoma cells via ASGPR-mediated cellular uptake and cytotoxicity

Galactose as a recognizing motif for asialoglycoprotein receptor (ASGPR) is a widely accepted vector to deliver cytotoxic agents in the therapy of hepatocellular carcinoma (HCC), however, the individual hydroxyl group of galactose (Gal) contributed to recognizing ASGPR is obscure and remains largely unanswered in the design of glycoconjugates. Herein, we designed and synthesized five positional isomers of Gal-anthocyanin Cy5.0 conjugates and three Gal-doxorubicin (Dox) isomers, respectively. The fluorescence intensity of Gal-Cy5.0 conjugates accumulated in cancer cells hinted the optimal modification sites of positions C2 and C6. Comparing to the cytotoxicity of other conjugates, C2-Gal-Dox (11) was the most potent. Moreover, Gal-Dox conjugates significantly the toxicity of Dox. A progressively lower internalization capacity and siRNA technology implied the cellular uptake and cytotoxicity directly related to the ASGPR expression level. Accordingly, position C2 of galactose may be the best substitution site via ASGPR mediation in the design of anti-HCC glycoconjugates.

Glycomimetics for the inhibition and modulation of lectins

Carbohydrates are essential mediators of many processes in health and disease. They regulate self-/non-self- discrimination, are key elements of cellular communication, cancer, infection and inflammation, and determine protein folding, function and life-times. Moreover, they are integral to the cellular envelope for microorganisms and participate in biofilm formation. These diverse functions of carbohydrates are mediated by carbohydrate-binding proteins, lectins, and the more the knowledge about the biology of these proteins is advancing, the more interfering with carbohydrate recognition becomes a viable option for the development of novel therapeutics. In this respect, small molecules mimicking this recognition process become more and more available either as tools for fostering our basic understanding of glycobiology or as therapeutics. In this review, we outline the general design principles of glycomimetic inhibitors (Section 2). This section is then followed by highlighting three approaches to interfere with lectin function, i.e. with carbohydrate-derived glycomimetics (Section 3.1), novel glycomimetic scaffolds (Section 3.2) and allosteric modulators (Section 3.3). We summarize recent advances in design and application of glycomimetics for various classes of lectins of mammalian, viral and bacterial origin. Besides highlighting design principles in general, we showcase defined cases in which glycomimetics have been advanced to clinical trials or marketed. Additionally, emerging applications of glycomimetics for targeted protein degradation and targeted delivery purposes are reviewed in Section 4.

Development of Novel siRNA Therapeutics: A Review with a Focus on Inclisiran for the Treatment of Hypercholesterolemia

Over the past two decades, it was discovered that introducing synthetic small interfering RNAs (siRNAs) into the cytoplasm facilitates effective gene-targeted silencing. This compromises gene expression and regulation by repressing transcription or stimulating sequence-specific RNA degradation. Substantial investments in developing RNA therapeutics for disease prevention and treatment have been made. We discuss the application to proprotein convertase subtilisin/kexin type 9 (PCSK9), which binds to and degrades the low-density lipoprotein cholesterol (LDL-C) receptor, interrupting the process of LDL-C uptake into hepatocytes. PCSK9 loss-of-function modifications show significant clinical importance by causing dominant hypocholesterolemia and lessening the risk of cardiovascular disease (CVD). Monoclonal antibodies and small interfering RNA (siRNA) drugs targeting PCSK9 are a significant new option for managing lipid disorders and improving CVD outcomes. In general, monoclonal antibodies are restricted to binding with cell surface receptors or circulating proteins. Similarly, overcoming the intracellular and extracellular defenses that prevent exogenous RNA from entering cells must be achieved for the clinical application of siRNAs. N-acetylgalactosamine (GalNAc) conjugates are a simple solution to the siRNA delivery problem that is especially suitable for treating a broad spectrum of diseases involving liver-expressed genes. Inclisiran is a GalNAc-conjugated siRNA molecule that inhibits the translation of PCSK9. The administration is only required every 3 to 6 months, which is a significant improvement over monoclonal antibodies for PCSK9. This review provides an overview of siRNA therapeutics with a focus on detailed profiles of inclisiran, mainly its delivery strategies. We discuss the mechanisms of action, its status in clinical trials, and its prospects.

Tissue- and cell-expression of druggable host proteins provide insights into repurposing drugs for COVID-19

Several human host proteins play important roles in the lifecycle of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Many drugs targeting these host proteins have been investigated as potential therapeutics for coronavirus disease 2019 (COVID-19). The tissue-specific expressions of selected host proteins were summarized using proteomics data retrieved from the Human Protein Atlas, ProteomicsDB, Human Proteome Map databases, and a clinical COVID-19 study. Protein expression features in different cell lines were summarized based on recent proteomics studies. The half-maximal effective concentration or half-maximal inhibitory concentration values were collected from in vitro studies. The pharmacokinetic data were mainly from studies in healthy subjects or non-COVID-19 patients. Considerable tissue-specific expression patterns were observed for several host proteins. ACE2 expression in the lungs was significantly lower than in many other tissues (e.g., the kidneys and intestines); TMPRSS2 expression in the lungs was significantly lower than in other tissues (e.g., the prostate and intestines). The expression levels of endocytosis-associated proteins CTSL, CLTC, NPC1, and PIKfyve in the lungs were comparable to or higher than most other tissues. TMPRSS2 expression was markedly different between cell lines, which could be associated with the cell-dependent antiviral activities of several drugs. Drug delivery receptor ICAM1 and CTSB were expressed at a higher level in the lungs than in other tissues. In conclusion, the cell- and tissue-specific proteomics data could help interpret the in vitro antiviral activities of host-directed drugs in various cells and aid the transition of the in vitro findings to clinical research to develop safe and effective therapeutics for COVID-19.

Asialoglycoprotein receptor 1 is a novel PCSK9-independent ligand of liver LDLR cleaved by furin

The hepatic carbohydrate-recognizing asialoglycoprotein receptor (ASGR1) mediates the endocytosis/lysosomal degradation of desialylated glycoproteins following binding to terminal galactose/N-acetylgalactosamine. Human heterozygote carriers of ASGR1 deletions exhibit ∼34% lower risk of coronary artery disease and ∼10% to 14% reduction of non-HDL cholesterol. Since the proprotein convertase PCSK9 is a major degrader of the low-density lipoprotein receptor (LDLR), we investigated the degradation and functionality of LDLR and/or PCSK9 by endogenous/overexpressed ASGR1 using Western blot and immunofluorescence in HepG2-naïve and HepG2-PCSK9-knockout cells. ASGR1, like PCSK9, targets LDLR, and both independently interact with/enhance the degradation of the receptor. This lack of cooperativity between PCSK9 and ASGR1 was confirmed in livers of wildtype (WT) and Pcsk9^−/− mice. ASGR1 knockdown in HepG2-naïve cells significantly increased total (∼1.2-fold) and cell-surface (∼4-fold) LDLR protein. In HepG2-PCSK9-knockout cells, ASGR1 silencing led to ∼2-fold higher levels of LDLR protein and DiI (1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate)-LDL uptake associated with ∼9-fold increased cell-surface LDLR. Overexpression of WT-ASGR1/2 primarily reduced levels of immature non-O-glycosylated LDLR (∼110 kDa), whereas the triple Ala-mutant of Gln240/Trp244/Glu253 (characterized by loss of carbohydrate binding) reduced expression of the mature form of LDLR (∼150 kDa), suggesting that ASGR1 binds the LDLR in both a sugar-dependent and -independent fashion. The protease furin cleaves ASGR1 at the RKMK¹⁰³↓ motif into a secreted form, likely resulting in a loss of function on LDLR. Altogether, we demonstrate that LDLR is the first example of a liver-receptor ligand of ASGR1. We conclude that silencing of ASGR1 and PCSK9 may lead to higher LDL uptake by hepatocytes, thereby providing a novel approach to further reduce LDL cholesterol levels.

Programmed Instability of Ligand Conjugation Manifold for Efficient Hepatocyte Delivery of Therapeutic Oligonucleotides

Ligand-targeted drug delivery (LTDD) has gained more attention in the field of nucleic acid therapeutics. To further elicit the potential of therapeutic oligonucleotides by means of LTDD, we newly developed (R)- and (S)-3-amino-1,2-propanediol (APD) manifold for ligand conjugation. N-acetylgalactosamine (GalNAc)/asialoglycoprotein receptor (ASGPr) system has been shown to be a powerful and robust paradigm of LTDD. Our novel APD-based GalNAc (GalNAc_APD) was shown to have intrinsic chemical instability that could play a role in better manipulation of active drug release. The APD manifold also enables facile production of conjugates through an on-support ligand cluster synthesis. We showed in a series of in vivo studies that while the knockdown activity of antisense oligonucleotides (ASOs) bearing 5'-GalNAc_APD was comparable to the conventional hydroxy-L-prolinol-linked GalNAc (GalNAc_HP), 3'-GalNAc_APD elicited ASO activity by more than twice as much as the conventional 3'-GalNAc_HP. This was ascribed partly to the GalNAc_APD's ideal susceptibility to nucleolytic digestion, which is expected to facilitate cytosolic internalization of ASO drugs. Moreover, an in vivo/ex vivo imaging study visualized the enhancement effect of monoantennary GalNAc_APD on liver localization of ASOs. This versatile manifold with chemical and biological instability would benefit therapeutic oligonucleotides that target both the liver and extrahepatic tissues.

Pharmacokinetics and Clinical Pharmacology Considerations of GalNAc3-Conjugated Antisense Oligonucleotides

Introduction: Triantennary N-acetyl galactosamine (GalNAc₃) - conjugated antisense oligonucleotides (ASOs) have demonstrated improved hepatocyte uptake and pharmacologic activity over their parent unconjugated ASOs in animals and humans. Areas covered: In this review, the ADME (absorption, distribution, metabolism, and excretion) characteristics of GalNAc₃-conjugated ASOs in animals and in humans are summarized, and their clinical relevance is evaluated from the clinical pharmacology perspectives. Expert opinion: ASOs distribute to tissues via receptor-mediated processes, and conjugation to a ligand specific to certain cell types can improve target tissue delivery. GalNAc₃-conjugation represents a good example on this regard and has demonstrated ideal characteristics of a prodrug to target delivery of ASOs to hepatocytes via the asialoglycoprotein receptor (ASGPR). The improved potency and safety margin permit more flexible dosing (e.g. monthly or less frequently if needed) taking full advantage of the long half-life of the parent ASO in humans. However, while still speculative, it should be noted that ASGPR-mediated uptake could become nonlinear with dose and factors that impact ASGPR expression or compete with ASGPR-mediated uptake could potentially affect the uptake of GalNAc₃-conjugated ASOs, further studies are warranted.

Capacity limits of asialoglycoprotein receptor-mediated liver targeting

The abundant cell surface asialoglycoprotein receptor (ASGPR) is a highly selective receptor found on hepatocytes that potentially can be exploited as a selective shuttle for delivery. Various nucleic acid therapeutics that bind ASGPR are already in clinical development, but this receptor-mediated delivery mechanism can be saturated, which will likely result in reduced selectivity for the liver and therefore increase the likelihood for systemic adverse effects. Therefore, when aiming to utilize this mechanism, it is important to optimize both the administration protocol and the molecular properties. We here present a study using a novel ASGPR-targeted antibody to estimate ASGPR expression, turnover and internalization rates in vivo in mice. Using pharmacokinetic data (intravenous and subcutaneous dosing) and an in-silico target-mediated drug disposition (TMDD) model, we estimate an ASGPR expression level of 1.8 million molecules per hepatocyte. The half-life of the degradation of the receptor was found to be equal to 15 hours and the formed ligand-receptor complex is internalized with a half-life of 5 days. A biodistribution study was performed and confirmed the accuracy of the TMDD model predictions. The kinetics of the ASGPR shows that saturation of the shuttle at therapeutic concentrations is possible; however, simulation allows the dosing schedule to be optimized. The developed TMDD model can be used to support the development of therapies that use the ASGPR as a shuttle into hepatocytes.

Specific and Differential Binding of N-Acetylgalactosamine Glycopolymers to the Human Macrophage Galactose Lectin and Asialoglycoprotein Receptor

A range of glycopolymers composed of N-acetylgalactosamine were prepared via sequential Cu(I)-mediated polymerization and alkyne-azide click (CuAAC). The resulting polymers were shown, via multichannel surface plasmon resonance, to interact specifically with human macrophage galactose lectin (MGL; CD301) with high affinity (K_D = 1.11 μM), but they did not bind to the mannose/fucose-selective human lectin dendritic-cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN; CD209). The effect of sugar ligand valency on the binding (so-called "glycoside cluster effect") of poly(N-acetylgalactosamine) to MGL was investigated by varying first the polymer chain length (DP: 100, 64, 40, 23, 12) and then the architecture (4- and 8-arm star glycopolymers). The chain length did not have a significant effect on the binding to MGL (K_D = 0.17-0.52 μM); however, when compared to a hepatic C-type lectin of a similar monosaccharide specificity, the asialoglycoprotein receptor (ASGPR), the binding affinity was more noticeably affected (K_D = 0.37- 6.65 μM). These data suggest that known differences in the specific configuration/orientation of the carbohydrate recognition domains of MGL and ASGPR are responsible for the differences in binding observed between the different polymers of varied chain length and architecture. In the future, this model has the potential to be employed for the development of tissue-selective delivery systems.

Cell Specificity of Macromolecular Assembly of Cholesteryl and Galactoside Groups-Conjugated Pullulan

Galactose or lactose groups were conjugated to cholesterol-bearing pullulan (CHP). The CHP derivatives obtained formed monodisperse nanoparticles upon self-aggregation in water. Nanoparticles of galactoside-conjugated CHP self-aggregates were specifically internalized by rat hepatocytes and HepG2 cells. Galactoside-bearing CHP-coated liposome or oil droplet of O/W-emulsion was also taken up by HepG2 cells. Tissue distribution of the nanoparticle CHP self-aggregates changed dramatically with chemical conjugation of the galactose moiety. Galactoside-bearing nanoparticles were specifically accumulated in the liver.

Downregulation of the small GTPase SAR1A: a key event underlying alcohol-induced Golgi fragmentation in hepatocytes

The hepatic asialoglycoprotein receptor (ASGP-R) is posttranslationally modified in the Golgi en route to the plasma membrane, where it mediates clearance of desialylated serum glycoproteins. It is known that content of plasma membrane-associated ASGP-R is decreased after ethanol exposure, although the mechanisms remain elusive. Previously, we found that formation of compact Golgi requires dimerization of the largest Golgi matrix protein giantin. We hypothesize that ethanol-impaired giantin function may be related to altered trafficking of ASGP-R. Here we report that in HepG2 cells expressing alcohol dehydrogenase and hepatocytes of ethanol-fed rats, ethanol metabolism results in Golgi disorganization. This process is initiated by dysfunction of SAR1A GTPase followed by altered COPII vesicle formation and impaired Golgi delivery of the protein disulfide isomerase A3 (PDIA3), an enzyme that catalyzes giantin dimerization. Additionally, we show that SAR1A gene silencing in hepatocytes mimics the effect of ethanol: dedimerization of giantin, arresting PDIA3 in the endoplasmic reticulum (ER) and large-scale alterations in Golgi architecture. Ethanol-induced Golgi fission has no effect on ER-to-Golgi transportation of ASGP-R, however, it results in its deposition in cis-medial-, but not trans-Golgi. Thus, alcohol-induced deficiency in COPII vesicle formation predetermines Golgi fragmentation which, in turn, compromises the Golgi-to-plasma membrane transportation of ASGP-R.

Cloning expeditions: risky but rewarding

In the 1980s, a good part of my laboratory was using the then-new recombinant DNA techniques to clone and characterize many important cell surface membrane proteins: GLUT1 (the red cell glucose transporter) and then GLUT2 and GLUT4, the red cell anion exchange protein (Band 3), asialoglycoprotein receptor subunits, sucrase-isomaltase, the erythropoietin receptor, and two of the subunits of the transforming growth factor β (TGF-β) receptor. These cloned genes opened many new fields of basic research, including membrane insertion and trafficking of transmembrane proteins, signal transduction by many members of the cytokine and TGF-β families of receptors, and the cellular physiology of glucose and anion transport. They also led to many insights into the molecular biology of several cancers, hematopoietic disorders, and diabetes. This work was done by an exceptional group of postdocs and students who took exceptionally large risks in developing and using novel cloning technologies. Unsurprisingly, all have gone on to become leaders in the fields of molecular cell biology and molecular medicine.

ASGR1 and ASGR2, the Genes that Encode the Asialoglycoprotein Receptor (Ashwell Receptor), Are Expressed in Peripheral Blood Monocytes and Show Interindividual Differences in Transcript Profile

Background. The asialoglycoprotein receptor (ASGPR) is a hepatic receptor that mediates removal of potentially hazardous glycoconjugates from blood in health and disease. The receptor comprises two proteins, asialoglycoprotein receptor 1 and 2 (ASGR1 and ASGR2), encoded by the genes ASGR1 and ASGR2. Design and Methods. Using reverse transcription amplification (RT-PCR), expression of ASGR1 and ASGR2 was investigated in human peripheral blood monocytes. Results. Monocytes were found to express ASGR1 and ASGR2 transcripts. Correctly spliced transcript variants encoding different isoforms of ASGR1 and ASGR2 were present in monocytes. The profile of transcript variants from both ASGR1 and ASGR2 differed among individuals. Transcript expression levels were compared with the hepatocyte cell line HepG2 which produces high levels of ASGPR. Monocyte transcripts were 4 to 6 orders of magnitude less than in HepG2 but nonetheless readily detectable using standard RT-PCR. The monocyte cell line THP1 gave similar results to monocytes harvested from peripheral blood, indicating it may provide a suitable model system for studying ASGPR function in this cell type. Conclusions. Monocytes transcribe and correctly process transcripts encoding the constituent proteins of the ASGPR. Monocytes may therefore represent a mobile pool of the receptor, capable of reaching sites remote from the liver.

Carbohydrate clearance receptors in transfusion medicine

BACKGROUND

Complex carbohydrates play important functions for circulation of proteins and cells. They provide protective shields and refraction from non-specific interactions with negative charges from sialic acids to enhance circulatory half-life. For recombinant protein therapeutics carbohydrates are especially important to enhance size and reduce glomerular filtration loss. Carbohydrates are, however, also ligands for a large number of carbohydrate-binding lectins exposed to the circulatory system that serve as scavenger receptors for the innate immune system, or have more specific roles in targeting of glycoproteins and cells.

SCOPE OF REVIEW

Here we provide an overview of the common lectin receptors that play roles for circulating glycoproteins and cells, and present a discussion of ways to engineer glycosylation of recombinant biologics and cells to improve therapeutic effects.

MAJOR CONCLUSIONS

While the pharmaceutical industry has learned how to exploit carbohydrates to improve pharmacokinetic properties of recombinant therapeutics, our understanding of how to improve cell-based therapies by manipulation of complex carbohydrates is still at its infancy. Progress with the latter has recently been achieved with cold-stored platelets, where exposure of uncapped glycans lead to rapid clearance from circulation by several lectin-mediated pathways.

GENERAL SIGNIFICANCE

Understanding lectin-mediated clearance pathways is essential for progress in development of biological pharmaceuticals.

An internalizing antibody specific for the human asialoglycoprotein receptor

The liver possesses a variety of characteristics that make this organ a very attractive target for gene and drug delivery. The asialoglycoprotein receptor (ASGPR) is a heterodimeric liver-specific C-type lectin that mediates endocytosis and degradation of desialylated glycoproteins and is considered a preferable target for liver-specific drug delivery. Asialoglycoprotein-coupled, galactosylated, or anti-ASGPR antibody-targeted molecules may be used to deliver pharmaceutical agents to the liver. Here we present a new anti-ASGPR single-chain antibody (scFv) that was isolated from the synthetic human "Ronit-1" antibody phage display library. This scFv (B11) was shown to bind the recombinant and native forms of the ASGPR and could also facilitate ASGPR specific internalization of a B11-PE38KDEL immunotoxin and cause cell death. Thus, this newly isolated antibody can serve as a targeting moiety for ASGPR-directed drug delivery.

H2, the minor subunit of the human asialoglycoprotein receptor, trafficks intracellularly and forms homo-oligomers, but does not bind asialo-orosomucoid

The functional human hepatic asialoglycoprotein receptor (ASGP-R) is a hetero-oligomer composed of two subunits, designated H1 and H2, which are highly homologous. Despite their extensive homology, the major H1 subunit is stably expressed by itself, whereas in the absence of H1 most of the H2 subunits are degraded in the ER. In this study, we were able to investigate the capability of the minor ASGP-R subunit, H2, to function independently of H1, because it was apparently stabilized by fusing its NH(2) terminus with an epitope tag. We could thus create stable cell lines in hepatoma-derived SK-Hep-1 cells that expressed the H2 subunit alone. H2 was expressed on the cell surface and was internalized, predominantly through the clathrin-coated pit pathway. Since the internal pool of H2 was also able to traffick to the cell surface, we conclude that H2 recycles between the surface and intracellular compartments, similar to the constitutive recycling of hetero-oligomeric ASGP-R complexes. However, the rate of H2 recycling and internalization was approximately 25-33% that of H1. Similar to H1, the H2 polypeptides were also able to self-associate to form homo-oligomers, including trimers and tetramers. However, unlike H1, which can bind the ligand asialo-orosomucoid (ASOR) when overexpressed in COS-7 cells, H2 failed to bind or endocytose ASOR. In summary, the H2 subunit of the human ASGP-R contains functional, although weak, signal(s) for endocytosis and recycling and has the ability to oligomerize. H2 homo-oligomers, however, do not create binding sites for desialylated glycoproteins, such as ASOR, that contain tri- and tetra-antennary N-linked oligosaccharides. Nonetheless, these results raise the intriguing possibility that naturally occurring H2 homo-oligomers may exist in human hepatocytes and have an as yet undiscovered function.

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.

A 3D model for the human hepatic asialoglycoprotein receptor (ASGP-R)

The human hepatic Asialoglycoprotein Receptor (ASGP-R) consists of two different types of liver specific membrane glycoproteins that bind to terminal galactose and N-acetylgalactosamine residues of serum glycoproteins. The two different polypeptide chains are referred to as two receptor subunits, HH1 and HH2, which are both involved in the activity of the functional receptor. This receptor has served as a model for understanding receptor-mediated endocytosis and carbohydrate mediated recognition phenomena. Here models for the C-terminal extracellular region of both HH1 and HH2 subunit are presented. The standard homology building procedure was modified in order to make it suitable for the modeling problem at hand. The models for the extracellular regions of HH1 and HH2 were initially constructed by exploiting several fragments, belonging to proteins of known 3D structure, and showing high local sequence similarity with respect to the glycoproteins of interest. Putative binding sites were first hypothesized on the basis of the comparison with other complexes of lectins, the crystal structure of which was available in the Protein Data Bank. A model for the complex involving the HH2 subunit and the typical high affinity ligand N-acetylgalactosamine (NacGal) was refined as the first by a suitable combination of MD simulations and Energy Minimization calculations, since it seemed to quickly converge to a plausible structure. An intermediate model for HH1 was then rebuilt on the basis of the refined model for HH2. It was then submitted to a sequence of molecular dynamics simulations with templates which took into account the secondary structure prediction for a final refinement. The structures of small regions of the models, located around the binding sites, were compared with more recent crystallographic data regarding a complex involving the mutant of Mannose Binding Protein QPDWGH (1BCH entry in the Protein Data Bank) and NacGal. This mutant shows high local sequence similarity with HH1 and HH2 at the binding sites. On the basis of the above comparison, different locations of the binding sites were also considered. In addition to other expected interactions, two hydrophobic interactions were observed in the models with Trp residues (positions 243 in HH1 and 181 or 267 in HH2 respectively) and His residues (positions 256 in HHI and 184 in HH2.respectively). The quality of the models was evaluated by the Procheck program and they seemed plausible. This observation together with analogies found between binding sites of the models and IBCH supported the validity of the models. A further validation element arose by comparison between experimental binding data available in the literature about the homologous rat receptor subunits and theoretical interaction energies evaluated, by means of the DOCK 3.5 program, in models for the rat subunits obtained from the corresponding human ones. The new modeling procedure used here appears to be a well-suited method for structural analysis of small regions, located around the ligands, in proteins of unknown 3D structure.

Normal levels of serum glycoproteins maintained in beta-1, 4-galactosyltransferase I-knockout mice

The galactose-mediated clearance of serum glycoproteins from the circulation was evaluated using beta-1,4-galactosyltransferase (beta-1,4-GalT) I-knockout mice. Partial structural study of the oligosaccharides released from mouse serum glycoproteins revealed that 77.4% of the oligosaccharides from beta-1,4-GalT I(+/+) mouse contain galactose, while 7.7% of those from beta-1,4-GalT I(-/-) mouse were galactosylated. Under the conditions, no significant change in serum protein concentrations was observed between the normal and mutant mice. The results indicate that the hepatic asialoglycoprotein receptor-mediated system is not functioning in the clearance of endogenous serum glycoproteins.

APCs Express DCIR, a Novel C-Type Lectin Surface Receptor Containing an Immunoreceptor Tyrosine-Based Inhibitory Motif

We have identified a novel member of the calcium-dependent (C-type) lectin family. This molecule, designated DCIR (for dendritic cell (DC) immunoreceptor), is a type II membrane glycoprotein of 237 aa with a single carbohydrate recognition domain (CRD), closest in homology to those of the macrophage lectin and hepatic asialoglycoprotein receptors. The intracellular domain of DCIR contains a consensus immunoreceptor tyrosine-based inhibitory motif. A mouse cDNA, encoding a homologous protein has been identified. Northern blot analysis showed DCIR mRNA to be predominantly transcribed in hematopoietic tissues. The gene encoding human DCIR was localized to chromosome 12p13, in a region close to the NK gene complex. Unlike members of this complex, DCIR displays a typical lectin CRD rather than an NK cell type extracellular domain, and was expressed on DC, monocytes, macrophages, B lymphocytes, and granulocytes, but not detected on NK and T cells. DCIR was strongly expressed by DC derived from blood monocytes cultured with GM-CSF and IL-4. DCIR was mostly expressed by monocyte-related rather than Langerhans cell related DC obtained from CD34+ progenitor cells. Finally, DCIR expression was down-regulated by signals inducing DC maturation such as CD40 ligand, LPS, or TNF-α. Thus, DCIR is differentially expressed on DC depending on their origin and stage of maturation/activation. DCIR represents a novel surface molecule expressed by Ag presenting cells, and of potential importance in regulation of DC function.

Comparative affinity of synthetic multi-antennary galactosyl derivatives for the Gal/GalNAc receptor of rat hepatocytes and peritoneal macrophages

The binding affinity of synthetic bi- and triantennary galactose ligands (Kichler, A. and Schuber, F. (1995) Glycoconj. Chem., 12, 275 281) has been determined for the Gal/GalNAc receptors of rat hepatocytes and macrophages. The highest affinities were observed with the triantennary structures, in agreement with the clustering effect known to occur with more complex oligosaccharide structures. However, these ligands present very similar affinities for the receptors of both cell types and thus lack the necessary selectivity for specific hepatocyte targeting.

Endoplasmic reticulum quality control of asialoglycoprotein receptor H2a involves a determinant for retention and not retrieval

The human asialoglycoprotein receptor H2a subunit contains a charged pentapeptide, EGHRG, in its ectodomain that is the only sequence absent from the H2b alternatively spliced variant. H2b exits the endoplasmic reticulum (ER) even when singly expressed, whereas H2a gives rise to a cleaved soluble secreted ectodomain fragment; uncleaved membrane-bound H2a molecules are completely retained and degraded in the ER. We have inserted the H2a pentapeptide into the sequence of the H1 subunit (H1i5), which caused complete ER retention but, unexpectedly, no degradation. This suggests that the pentapeptide is a determinant for ER retention not colocalizing in H2a with the determinant for degradation. The state of sugar chain processing and the ER localization of H1i5, which was unchanged at 15 degrees C or after treatment with nocodazole, indicate ER retention and not retrieval from the cis-Golgi or the intermediate compartment. H1i5 folded similarly to H1, and both associated to calnexin. However, whereas H1 dissociated with a half time of 45 min, H1i5 remained bound to the chaperone for prolonged periods. The correct global folding of H2a and H1i5 and of other normal precursors and unassembled proteins and the true ER retention, and not exit and retrieval, suggest a difference in their quality control mechanism compared with that of misfolded proteins, which does involve retrieval. However, both pathways may involve calnexin.

Composition, antigenic properties and hepatocyte surface expression of the woodchuck asialoglycoprotein receptor

We have purified woodchuck hepatic asialoglycoprotein receptor (ASGPR) by ligand affinity chromatography and have identified it as a heterooligomeric complex comprised of two subunits with molecular masses of 40 and 47 kD, designated as woodchuck hepatic lectin 1 and 2 (WHL1 and WHL2), respectively. With the help of antisera generated against the soluble, bioactive woodchuck and rabbit ASGPRs and anti-subunit monospecific antibodies, distinct antigenic specificity of each of the ASGPR polypeptide subunits and interspecies immunologic cross-reactivity of the receptor polypeptides displaying comparable molecular masses were documented. In contrast to the purified woodchuck receptor, WHL2 antigenic reactivity was not identifiable in woodchuck hepatocyte plasma membranes unless the intact membranes were exposed to an asialylated ligand or a soluble membrane fraction was incubated with anti-receptor antibody. These findings imply that both WHL1 and WHL2 are expressed on the hepatocyte surface and contribute to ligand binding, since antibody specific to either subunit blocks ligand attachment. Our results also indicate that ligand binding modifies antigenic properties of the membrane expressed ASGPR.

Membrane-bound versus secreted forms of human asialoglycoprotein receptor subunits. Role of a juxtamembrane pentapeptide

The H2a alternatively spliced variant of the human asialoglycoprotein receptor H2 subunit differs from the H2b variant by an extra pentapeptide, EGHRG, present in the ectodomain next to the membrane-span. This difference causes retention and degradation in the endoplasmic reticulum (ER) of H2a when expressed without the H1 subunit in 3T3 cells. In contrast, a significant portion of singly expressed H2b is Golgi-processed and reaches the cell surface. Using a new specific anti-H2a antibody, we found that in HepG2 cells, H2a is rapidly cleaved to a 35-kDa fragment, comprising the entire ectodomain, most of which is secreted into the medium. The cleavage site for the secreted fragment was located at the lumenal end of the membrane span. No membrane-bound H2a exits the ER, indicating that the pentapeptide is a signal for ER retention and degradation of the membrane form but does not hinder secretion of the cleaved soluble form. H2a does not form a membrane receptor complex with H1 as H2b does. H2a is therefore not a subunit of the receptor but a precursor for a secreted form of the protein; signal peptidase is probably responsible for the cleavage to the soluble fragment. Therefore, the juxtamembrane sequence regulates the function of the transmembrane domain of a type II membrane protein as either a signal-anchor sequence (H2b) or as a cleaved signal sequence, which generates a secreted product (H2a).

Enhanced folding and processing of a disulfide mutant of the human asialoglycoprotein receptor H2b subunit

Unfolded forms of the H2b subunit of the human asialoglycoprotein receptor, a galactose-specific C-type lectin, are degraded in the endoplasmic reticulum (ER), whereas folded forms of the protein can mature to the cell surface (Wikström, L., and Lodish, H. F. (1993) J. Biol. Chem. 268, 14412-14416). There are eight cysteines in the exoplasmic domain of the protein, forming four disulfide bonds in the folded protein. We have constructed double cysteine to alanine mutants for each of the four disulfide bonds and examined the folding and metabolic fate of each of the mutants in transfected 3T3 fibroblasts. We find that mutation of the two cysteines nearest to the transmembrane region (C1) does not prevent proper folding of the protein, whereas mutations of the other three disulfides prevent proper folding of the protein and all of the mutant proteins are degraded in the ER. A normal (approximately 20%) fraction of the C1 mutant protein exists the endoplasmic reticulum and is processed in the Golgi complex, and it does so at a faster rate compared to the wild-type. Furthermore, the folded form of this mutant protein is more resistant to unfolding by dithiothreitol than the wild-type. The C1 mutant protein is expressed on the cell surface and can form a functional receptor with the H1 subunit with similar binding affinities for natural ligands as that of the wild-type receptor. The same fraction of newly made mutant and wild-type proteins (approximately 80%) remain in the ER, but the mutant protein is degraded more quickly. Thus, the presence of the C1 disulfide bond in the wild-type receptor both reduces the rate of protein folding and exit to the Golgi and slows the rate of ER degradation of the portion (approximately 80%) of the receptor that never folds properly.

Rat sperm galactosyl receptor: purification and identification by polyclonal antibodies raised against multiple antigen peptides

We have previously reported the purification of rats testis galactosyl receptor, an equivalent to the Ca(2+)-dependent (C-type) minor variant of rat hepatic lectin-2/3 (RHL-2/3). We now report the purification of galactosyl receptor from rat sperm and its immunolocalization in the intact rat testis and sperm by polyclonal antibodies prepared using multiple antigen peptides (MAP) as immunogens. Two MAP antigens (designated 27-mer and 28-mer), corresponding to amino acid sequences of the carbohydrate-recognition domain (galactose) and adjacent Ca(2+)-binding sites of RHL-2/3, were used for immunization. Anti-RHL-2/3, anti-p27, and anti-p28 sera crossreacted with rat hepatocyte RHL-2/3 and its rat testis and sperm equivalent, galactosyl receptor, purified by chromatofocusing followed by galactose-Hydropore-EP affinity chromatography. Neither anti-p27 nor anti-p28 sera cross-reacted with the major hepatocyte variant, RHL-1. A RHL-1-equivalent was not detected in rat testis and sperm. Immunofluorescence studies demonstrated that anti-p27 and anti-p28 sera recognize galactosyl receptor sites at the Sertoli cell-spermatogenic cell interface and on the dorsal surface of the sperm head, overlying the acrosome. The characteristic crescent-shaped immunoreactive pattern in sperm was lost after induction of the acrosome reaction. Further studies should determine whether antisera to MAP antigens 27-mer and 28-mer, corresponding to specific protein motifs, can serve as immunological probes for examining cell-cell interaction events during spermatogenesis and at fertilization.

The differences in structural specificity for recognition and binding between asialoglycoprotein receptors of liver and macrophages

The Gal/GalNAc-specific lectin on the surface of rat peritoneal macrophages (macrophage asialoglycoprotein binding protein, M-ASGP-BP), which consists of a single polypeptide chain of 42 kDa, can form a homo-oligomeric receptor exhibiting high affinity for asialoorosomucoid (ASOR) [Ozaki K., Ii M., Itoh N., Kawasaki T. (1992) J Biol Chem 267: 9229-35]. In this study, the binding affinity of M-ASGP-BP was studied by using a series of synthetic or natural glycosides as inhibitors of 125I-ASOR binding to recombinant M-ASGP-BP expressed on COS-1 cells (rM-ASGP-BP), and the results were compared with those of human hepatic lectin (HHL) on Hep G2 cells. Clustering of multiple Gal (or GalNAc) residues increased the binding affinity to M-ASGP-BP as well as to HHL. In contrast to HHL and other mammalian hepatic lectins, rM-ASGP-BP bound Gal residues tighter than GalNAc residues. A galactose-terminated triantennary N-glycoside, having one N-acetyl-lactosamine unit on the 6 branch and two N-acetyl-lactosamine units on the 3 branch of the trimannosyl core structure, showed affinity enhancement of approximately 10(5) over a monovalent ligand for HHL, while the same glycopeptide showed enhancement of about 2000-fold for rM-ASGP-BP. These results suggest that spatial arrangements of sugar combining sites and subunit organization of macrophage and hepatic lectins are different.

Expression of an endogenous asialoglycoprotein receptor in a human intestinal epithelial cell line, Caco-2

We have previously shown that rat asialoglycoprotein receptor expressed in the intestine and liver differ in mRNA size, cell surface distribution, and ratio of compositional protein subunits. In this study, we examined a well characterized intestinal epithelial cell line, Caco-2, as a potential model for studying endogenous receptor in a polarized cell line. Both subunits H1 and H2 of human asialoglycoprotein receptor were detected in Caco-2 cells by Western blots using subunit-specific antisera raised against the hepatic receptor. Antigenic receptor level in fully differentiated Caco-2 cells was approx. 1/3 to 1/2 the level of hepatic HepG2 cells H1 was the dominant subunit in both cell lines. The apparent size of H1 and H2 in Caco-2 cells was not the same as that in HepG2 cells, due to differences in N-linked glycosylation. Consistent with this finding, Northern blot analysis showed that receptor mRNA in the two cell types was of identical size. In pulse-chase experiments H1 was first detected as a 'high-mannose' precursor (40 kDa) in Caco-2 cells that was converted to mature H1 (43 kDa) with a half-life of approx. 60 min. Antigenic levels of H1 and H2 in undifferentiated Caco-2 cells were low, but increased rapidly during cell differentiation, reaching a peak level at 7 days after confluence. Immunocytochemical staining and domain-selective cell surface biotinylation assays showed that the ASGP-R was predominantly localized in the basolateral domain. The receptor in Caco-2 cells was capable of mediating specific uptake and degradation of [125I]asialoorosomucoid. The ligand uptake capacity of the basolateral surface of was approx. 10-fold higher than the apical. These characteristics (H1 subunit and basolateral predominance) of the receptor in Caco-2 cells, resembles the hepatic receptor. We conclude that Caco-2 cells endogenously express in ectopic hepatic-type functional asialoglycoprotein receptor.

Regulation of asialoglycoprotein receptor synthesis by inflammation-related cytokines in HepG2 cells

The asialoglycoprotein receptor (AGPR) is responsible for the catabolism of acute phase proteins. The effects of inflammation-related cytokines on the expression of AGPR were investigated in HepG2 cells derived from a human hepatoblastoma cell line. Binding studies, using a [125I]-labeled asialo-orosomucoid ligand, revealed that AGPR numbers on cell surfaces were increased by interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor (TNF). In cells treated with IL-1, IL-6, or TNF, immunohistochemical staining with an anti-AGPR monoclonal antibody demonstrated augmented expression. Pulse labeling analysis, using [35S]-labeled methionine, showed newly synthesized AGPR in both the precursor and the mature forms. When IL-1, IL-6, and TNF were added to the culture medium, total synthesis of AGPR (sum of the mature and precursor forms) was increased. In addition, the relative proportion of the synthesized precursor form of AGPR was higher in cytokine-treated than in untreated cells, suggesting that these cytokines augment the synthesis of AGPR, particularly in the stage prior to glycosylation.

Characterization of a human cervical carcinoma-associated antigen by lectin binding and immuno-electron microscopy

The specific binding and nature of the epitope recognized by monoclonal antibody (Mab) 1H10, which binds an antigen expressed on human cervical tumors, was characterized by enzyme digestion, lectin competition assay and immuno-electron microscopy. Membrane homogenates of CaSki cervical carcinoma cells were digested with various enzymes, then analysed by SDS-PAGE and immunoblotting. Cells grown on coverslips were treated with various enzymes and in situ binding of Mab 1H10 to cells was analysed by electron microscopy. The ability of lectin-conjugates to block Mab 1H10 binding to CaSki cells was also examined. Treatment of samples with sodium periodate abrogated antigen recognition by Mab 1H10. Neuraminidase and hyaluronidase digestion decreased but did not eliminate Mab 1H10 binding to cells in situ. Chondroitinase ABC digestion, in contrast, removed Mab 1H10 binding sites both in vitro and in situ. Trypsin and chymotrypsin digestion of cell membrane homogenates decreased the molecular weight of the Mab 1H10 antigen but did not decrease the binding intensity. Wheat germ agglutinin (WGA) strongly bound to CaSki cells and partially blocked Mab 1H10 binding, indicating that the antigen contains N-acetyl-galactosamine residues at or near the epitope recognized by Mab 1H10. Ricinus communis agglutinin (RCA) exhibited a similar binding pattern to WGA. However, concanavalin A bound only weakly to CaSki cells and was ineffective at blocking Mab 1H10 binding. The tumor-associated antigen recognized by Mab 1H10 is concluded to be a chondroitin sulphate glycoprotein or proteoglycan rather than a mucopolysaccharide or lipoprotein.

Abnormal surface distribution of the human asialoglycoprotein receptor in cirrhosis

Serum concentrations of asialoglycoproteins are increased in cirrhosis. We hypothesized that this increase results from abnormalities in the asialoglycoprotein receptor, which is located on the sinusoidal and lateral membrane of hepatocytes. Therefore we searched for morphological alterations in the distribution of the asialoglycoprotein receptor in human liver, using a light microscopic immunoperoxidase method in autopsy livers. In 24 of 25 (96%) of patients without liver disease, the asialoglycoprotein receptor was located on the sinusoidal and, less prominently, the lateral surface of hepatocytes but not the canalicular surface. In contrast, in 12 of 18 (67%) patients with cirrhosis of various causes, the receptor also was localized strikingly along the canalicular surface, with a corresponding decrease on the sinusoidal and lateral surfaces. We conclude that an abnormal cell-surface distribution of the asialoglycoprotein receptor commonly occurs in cirrhosis. This abnormality might result in impaired clearance of desialylated glycoproteins from plasma.

Differences in the abundance of variably spliced transcripts for the second asialoglycoprotein receptor polypeptide, H2, in normal and transformed human liver

The human hepatic asialoglycoprotein receptor comprises two homologous polypeptides designated H1 and H2. Two distinct complementary DNA clones encoding these receptor subunits have been previously isolated from the human hepatoblastoma cell line HepG2. We discovered that multiple variants of H2 transcripts exist both in HepG2 cells and in the normal human liver that, at least in part, appear to be the result of alternative splicing events. We have found that (a) the complementary DNA clone for H2 previously isolated from HepG2 cells, characterized by a 57-nucleotide insertion within the 5' end of the complementary DNA that is absent from H1, represented only one third of H2-related sequences in an unamplified normal human liver complementary DNA library and less than 10% of H2 clones in HepG2 cells; (b) the predominant message for H2 expressed in the liver and HepG2 cells, designated L-H2, appeared to represent the fully processed product of the gene encoding both L-H2 and H2; and (c) a variant H2 transcript existed in HepG2 cells, designated H2', that contained a novel, 5' 88-bp nucleotide insertion. Poly(A+) RNA analysis of the normal liver and HepG2 cells by complementary RNA hybridization and ribonuclease protection corroborated the observations made during the screening of complementary DNA libraries regarding the abundance of the various messages. A striking incongruity was found between the levels of messenger RNA containing the H2-specific 57-nucleotide sequence and the levels of polypeptide expressed in the liver and HepG2 cells as recognized by antiserum specifically raised against this sequence.

Recognition of complex oligosaccharides by the multi-subunit asialoglycoprotein receptor

The hepatic asialoglycoprotein receptor, a galactose lectin, is an oligomer of two types of similar polypeptide chains, each of which weakly binds galactose. High-affinity binding of complex oligosaccharides requires a precise geometric arrangement of receptor subunits. The two subunits have different functions in receptor assembly, ligand binding and endocytosis.