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

Establishment of a functional cell line expressing both subunits of H1a and H2c of human hepatocyte surface molecule ASGPR

To better understand the effect of a new split variant of human asialoglycoprotein receptor (ASGPR H1b) on ASGPR ligands' binding ability, we established a functional cell line which expresses ASGPR. The full lengths of ASGPRH1a and H2c fragments from human liver were amplified by reverse transcript PCR (RT-PCR) and inserted into eukaryotic expression vector pIRES2EGFP, pCDNA3.1 (Zeo+) respectively. The recombinants were co-transfected into HeLa cells. After selection by using Neocin and Zeocin, a stably transfected cell line was established, which was designated 4-1-6. The transcription and expression of ASGPRH1a and H2c in 4-1-6 were confirmed by RT-PCR, Western blotting and immunofluorescence. The endocytosis function of the artificial "ASGPR" on the surface of 4-1-6 was tested by FACS. It was found that the cell line 4-1-6 could bind ASGPR natural ligand molecular asialo-orosomucoid (ASOR). After the eukaryotic plasmid H1b/pCDNA3.1 (neo) was transfected into cell line 4-1-6, H1b did not down-regulate the ligand binding ability of ASGPR. The eukaryotic expression plasmid H1b/pcDNA3.1 (neo) and H2c/pcDNA3.1 (neo) were co-transfected transiently into Hela cell. Neither single H1b nor H1b and H2c could bind ASOR. In conclusion, a functional cell line of human asialoglycoprotein receptor (ASGPR) which expresses both H1a and H2c stably was established. The new split variant H1b has no effect on ASGPR binding to ASOR. ASGPRH1b alone can't bind to ASOR, it yet can't form functional complex with ASGPRH2c.

A new splice variant of the major subunit of human asialoglycoprotein receptor encodes a secreted form in hepatocytes

BACKGROUND

The human asialoglycoprotein receptor (ASGPR) is composed of two polypeptides, designated H1 and H2. While variants of H2 have been known for decades, the existence of H1 variants has never been reported.

PRINCIPAL FINDINGS

We identified two splice variants of ASGPR H1 transcripts, designated H1a and H1b, in human liver tissues and hepatoma cells. Molecular cloning of ASGPR H1 variants revealed that they differ by a 117 nucleotide segment corresponding to exon 2 in the ASGPR genomic sequence. Thus, ASGPR variant H1b transcript encodes a protein lacking the transmembrane domain. Using an H1b-specific antibody, H1b protein and a functional soluble ASGPR (sASGPR) composed of H1b and H2 in human sera and in hepatoma cell culture supernatant were identified. The expression of ASGPR H1a and H1b in Hela cells demonstrated the different cellular loctions of H1a and H1b proteins at cellular membranes and in intracellular compartments, respectively. In vitro binding assays using fluorescence-labeled sASGPR or the substract ASOR revealed that the presence of sASGPR reduced the binding of ASOR to cells. However, ASOR itself was able to enhance the binding of sASGPR to cells expressing membrane-bound ASGPR. Further, H1b expression is reduced in liver tissues from patients with viral hepatitis.

CONCLUSIONS

We conclude that two naturally occurring ASGPR H1 splice variants are produced in human hepatocytes. A hetero-oligomeric complex sASGPR consists of the secreted form of H1 and H2 and may bind to free substrates in circulation and carry them to liver tissue for uptake by ASGPR-expressing hepatocytes.

Fibronectin and asialoglyprotein receptor mediate hepatitis B surface antigen binding to the cell surface

Both fibronectin and the asialoglycoprotein receptor (ASGPR) have been identified by some investigators as partners for hepatitis B virus (HBV) envelope proteins. Because fibronectin is a natural ligand for ASGPR, we speculated that HBV might attach to ASGPR expressed on the hepatocyte surface via fibronectin. To test this hypothesis, we first confirmed by co-immunoprecipitation that ASGPR, fibronectin and HBsAg bind to each other in HepG2.2.15 cells, and possible binding domains were identified by GST pull-down. In addition, by measuring binding of HBsAg to cells, we found that ASGPR and fibronectin enhanced the binding capability of HBsAg to HepG2 cells, and even to 293T and CHO cells, which normally do not bind HBV. In conclusion, our findings suggest that both fibronectin and ASGPR mediate HBsAg binding to the cell surface, which provides further evidence for the potential roles of these two proteins in mediating HBV binding to liver cells.

Antisense oligonucleotides targeted against asialoglycoprotein receptor 1 block human hepatitis B virus replication

Chronic hepatitis B virus (HBV) infection is a major worldwide public health problem. Better therapeutics and treatment strategies are urgently needed because of ineffective clinical treatment. Our previous study showed that asialoglycoprotein receptor 1 (ASGPR1) was upregulated by HBV but downregulated by lamivudine in HepG2.2.15 cells. It has also been reported that ASGPR is a candidate receptor for HBV attachment to hepatocytes. Therefore, as a major subunit of ASGPR, ASGPR1, might be a potential target for anti-HBV drugs. To validate this hypothesis, antisense oligonucleiotides (ASODNs) were used to downregulate ASGPR1 level in HepG2.2.15 cells. By using the MFOLD web server and BLAST searches, five ASODNs theoretically targeting ASGPR1 were selected. After 72 h post-transfection, HBV-DNA level in cell medium were examined by real-time polymerase chain reaction (PCR). Hepatitis B surface antigen (HBsAg) and Hepatitis B e antigen (HBeAg) were detected using enzyme-linked immunosorbent assay (ELISA). ASGPR1 mRNA and protein level were measured by semi-quantitative reverse transcriptase (RT)-PCR and Western blot analysis respectively. The results showed that ASODN2 significantly downregulated ASGPR1 level. It also reduced HBV-DNA, HBsAg and HBeAg level in cell medium as observed with lamivudine. In contrast, the sense sequence and scrambled sequence of ASODN2 had no effect on ASGPR1 and HBV markers in HepG2.2.15 cells. This indicated that ASODN2 could specifically reduce HBV replication in vitro. Additionally, cell proliferation and apoptosis assay suggested that downregulation of ASGPR1 did not affect cell viability. We, therefore, proposed that ASODNs targeted against ASGPR1 could block HBV replication without the influence of other changes, and ASGPR1 could be targeted for anti-HBV drug development.

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.

Study of antigenic sites on the asialoglycoprotein receptor recognized by autoantibodies

The aim of this study was to identify the epitopes recognized by antibodies to the asialoglycoprotein receptor, a specific hepatocyte protein, from sera of patients with autoimmune hepatitis. An ELISA test was used to detect anti-asialoglycoprotein receptor antibodies in the sera of patients with autoimmune hepatitis. Positive sera were tested against the same antigen by slot blot, by Western blot and by immunoprecipitation of the untreated protein and following treatment with beta-mercaptoethanol (beta-ME) and endoglycosidase F. The mature, unglycosylated and partially glycosylated forms of the asialoglycoprotein receptor synthesized by HepG2 cells were tested against positive patients' sera, as well as the in vitro translated unglycosylated form of the H1 subunit of the receptor. Sera from patients with autoimmune hepatitis recognized equally the native form, as well as the beta-ME-modified form, but less well the deglycosylated form of the human mature receptor. No reactivity was found when these sera were tested against the denatured human protein. In addition, neither the unglycosylated H1 subunit nor any of the HepG2-synthesized asialoglycoprotein receptor forms bound to the antibodies. Altogether, these results show that anti-asialoglycoprotein receptor antibodies in the sera of patients with autoimmune hepatitis are directed against conformational structures of the mature hetero-oligomeric form of the human liver protein and that at least some epitopes were located on the extracellular domain of the antigen.

Detection of the asialoglycoprotein receptor on cell lines of extrahepatic origin

The asialoglycoprotein receptor (ASGPR) is the first lectin discovered in mammals. Despite its significant biological role in binding and internalization of desialyated glycoproteins, at least in the human, little information is available regarding its tissue distribution outside of the liver. In the present study, antibodies were raised against the H1 major subunit of the human ASGPR using synthetic peptide antigens, and their binding specificity confirmed by enzyme linked immunosorbent assay. Cell surface analysis by fluorescence activated flow cytometry on various human tissue cell lines confirmed the liver parenchymal cells as the major expression site of ASGPR. Nonetheless, ASGPR was also detectable on some extrahepatic cells such as the Jurkat T-cell line. The determination of extrahepatic expression of ASGPR will have consequences in analyzing the biological role of this receptor complex as well as having implications in designing ASGPR mediated drug- or gene-delivery strategies.

Characterization of the "hepatic" asialoglycoprotein receptor in rat late-stage spermatids and epididymal sperm

Northern blot analysis of rat testicular (Te) poly(A)+RNA reveals that a transcript homologous to the major form of the asialoglycoprotein receptor (ASGP-R), designated RHL-1, is expressed as early as one week postnatally and that steady-state levels are approx. 8-times higher in the Te of an 8-week-old rat (sexually mature) as compared to an 84-week-old rat (aged). Partial cDNAs encoding RHL-1 and the minor form of the ASGP-R, designated RHL-2/3, have been cloned from two rat Te/epididymal (Ep) cDNA libraries and rat Te poly(A)+RNA. Sequence analysis of the Te/Ep RHL-1 cDNA and the Te/Ep RHL-2/3 cDNA indicates that these cDNAs are identical to the forms expressed in rat liver. Western blot analysis demonstrates the presence of a 49-kDa Te/Ep RHL-1-related protein band and a 54-kDa Te/Ep RHL-2/3-related protein band in both rat Te membrane fractions (MF) and rat Ep sperm MF. The RHL-1-related protein has been localized to late-stage Te spermatids at the time of release from the seminiferous tubules and to Ep sperm in the region of the sperm tail, referred to as the middle piece. Taken collectively, these data indicate that the authentic RHL-1 and RHL-2/3 genes of the ASGP-R are expressed in late-stage spermatids; however, the Te/Ep RHL-1-related protein differs in size from the hepatic RHL-1 polypeptide, possibly indicating a specific function of the RHL-1-related protein in spermatogenesis.

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.