Characterization of the asialoglycoprotein receptor on isolated rat hepatocytes

Multifunctional and stimuli-responsive liposomes in hepatocellular carcinoma diagnosis and therapy

Hepatocellular carcinoma (HCC) is the most prevalent type of liver cancer, mainly occurring in Asian countries with an increased incidence rate globally. Currently, several kinds of therapies have been deployed for HCC therapy including surgical resection, chemotherapy, radiotherapy and immunotherapy. However, this tumor is still incurable, requiring novel strategies for its treatment. The nanomedicine has provided the new insights regarding the treatment of cancer that liposomes as lipid-based nanoparticles, have been widely applied in cancer therapy due to their biocompaitiblity, high drug loading and ease of synthesis and modification. The current review evaluates the application of liposomes for the HCC therapy. The drugs and genes lack targeting ability into tumor tissues and cells. Therefore, loading drugs or genes on liposomes can increase their accumulation in tumor site for HCC suppression. Moreover, the stimuli-responsive liposomes including pH-, redox- and light-sensitive liposomes are able to deliver drug into tumor microenvironment to improve therapeutic index. Since a number of receptors upregulate on HCC cells, the functionalization of liposomes with lactoferrin and peptides can promote the targeting ability towards HCC cells. Moreover, phototherapy can be induced by liposomes through loading phtoosensitizers to stimulate photothermal- and photodynamic-driven ablation of HCC cells. Overall, the findings are in line with the fact that liposomes are promising nanocarriers for the treatment of HCC.

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.

NanoClick: A High Throughput, Target-Agnostic Peptide Cell Permeability Assay

Macrocyclic peptides open new opportunities to target intracellular protein-protein interactions (PPIs) that are often considered nondruggable by traditional small molecules. However, engineering sufficient membrane permeability into these molecules is a central challenge for identifying clinical candidates. Currently, there is a lack of high-throughput assays to assess peptide permeability, which limits our capacity to engineer this property into macrocyclic peptides for advancement through drug discovery pipelines. Accordingly, we developed a high throughput and target-agnostic cell permeability assay that measures the relative cumulative cytosolic exposure of a peptide in a concentration-dependent manner. The assay was named NanoClick as it combines in-cell Click chemistry with an intracellular NanoBRET signal. We validated the approach using known cell penetrating peptides and further demonstrated a correlation to cellular activity using a p53/MDM2 model system. With minimal change to the peptide sequence, NanoClick enables the ability to measure uptake of molecules that enter the cell via different mechanisms such as endocytosis, membrane translocation, or passive permeability. Overall, the NanoClick assay can serve as a screening tool to uncover predictive design rules to guide structure-activity-permeability relationships in the optimization of functionally active molecules.

Synthesis of Monovalent N-Acetylgalactosamine Phosphoramidite for Liver-Targeting Oligonucleotides

Ligand-targeted drug delivery (LTDD) has emerged as an attractive option in the field of oligonucleotide drugs following the great success of N-acetylgalactosamine (GalNAc)-conjugated siRNA and antisense oligonucleotides. GalNAc is a well-known ligand of the asialoglycoprotein receptor (ASGPR), and is classified as a C-type lectin associated with the metabolism of desialylated glycoproteins. This article describes the synthesis of a non-nucleosidic monovalent GalNAc phosphoramidite-a useful reagent for facilitating the conjugation of GalNAc epitopes into oligonucleotides using DNA synthesizers-together with some important caveats. The monomeric GalNAc consists of three parts: (1) a GalNAc moiety, (2) a linker moiety, and (3) a trans-4-hydroxyprolinol (tHP) branch point. The GalNAc moiety and the tHP moiety are coupled via a condensation reaction to prepare the monovalent GalNAc phosphoramidite. © 2019 by John Wiley & Sons, Inc. Basic Protocol 1: Synthesis of N-acetylgalactosamine ligand Basic Protocol 2: Preparation of trans-4-hydroxyprolinol building block Basic Protocol 3: Preparation of GalNAc phosphoramidite.

A focus on the role of platelets in liver regeneration: Do platelet-endothelial cell interactions initiate the regenerative process?

Platelets are involved in the early phases of liver regeneration. Moreover, platelet transfusion and thrombocytosis were recently shown to enhance hepatocyte proliferation. However, the precise mechanisms remain elusive. This review discusses the latest updates regarding the mechanisms by which platelets stimulate liver regeneration, focusing on their interactions with liver sinusoidal endothelial cells and on their fate within the liver. Following liver injury, platelets are recruited to and trapped within the liver, where they adhere to the endothelium. Subsequent platelet activation results in the release of platelet granules, which stimulate hepatocyte proliferation through activation of the Akt and ERK1/2 signalling pathways. Platelets activate liver sinusoidal endothelial cells, leading to the secretion of growth factors, such as interleukin-6. Finally, liver sinusoidal cells and hepatocytes can also internalize platelets, but the effects of this alternate process on liver regeneration remain to be explored. A better understanding of the mechanisms by which platelets stimulate liver regeneration could lead to improvement in post-operative organ function and allow hepatectomies of a greater extent to be performed.

Liposomes for targeting hepatocellular carcinoma: use of conjugated arabinogalactan as targeting ligand

Present study investigates the potential of chemically modified (Shah et al., 2013) palmitoylated arabinogalactan (PAG) in guiding liposomal delivery system and targeting asialoglycoprotein receptors (ASGPR) which are expressed in hepatocellular carcinoma (HCC). PAG was incorporated in liposomes during preparation and doxorubicin hydrochloride was actively loaded in preformed liposomes with and without PAG. The liposomal systems with or without PAG were evaluated for in vitro release, in vitro cytotoxicity, in vitro cell uptake on ASGPR(+) cells, in vivo pharmacokinetic study, in vivo biodistribution study, and in vivo efficacy study in immunocompromised mice. The particle size for all the liposomal systems was below 200 nm with a negative zeta potential. Doxorubicin loaded PAG liposomes released significantly higher amount of doxorubicin at pH 5.5 as compared to pH 7.4, providing advantage for targeted tumor therapy. Doxorubicin in PAG liposomes showed superior cytotoxicity on ASGPR(+) HepG2 cells as compared to ASGPR(-), MCF7, A549, and HT29 cells. Superior uptake of doxorubicin loaded PAG liposomes as compared to doxorubicin loaded conventional liposomes was evident in confocal microscopy studies. Higher AUC in pharmacokinetic study and higher deposition in liver was observed for PAG liposomes compared to conventional liposomes. Significantly higher tumor suppression was noted in immunocompromised mice for mice treated with PAG liposomes as compared to the conventional liposomes. Targeting ability and superior activity of PAG liposomes is established pre-clinically suggesting potential of targeted delivery system for improved treatment of HCC.

Sialylation and muscle performance: sialic acid is a marker of muscle ageing

Sialic acids (Sia) are widely expressed as terminal monosaccharides on eukaryotic glycoconjugates. They are involved in many cellular functions, such as cell-cell interaction and signal recognition. The key enzyme of sialic acid biosynthesis is the bifunctional UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE), which catalyses the first two steps of Sia biosynthesis in the cytosol. In this study we analysed sialylation of muscles in wild type (C57Bl/6 GNE (+/+)) and heterozygous GNE-deficient (C57Bl/6 GNE (+/-)) mice. We measured a significantly lower performance in the initial weeks of a treadmill exercise in C57Bl/6 GNE (+/-) mice compared to wild type C57Bl/6 GNE (+/+) animals. Membrane bound Sia of C57Bl/6 GNE (+/-) mice were reduced by 33-53% at week 24 and by 12-15% at week 80 in comparison to C57Bl/6 GNE (+/+) mice. Interestingly, membrane bound Sia concentration increased with age of the mice by 16-46% in C57Bl/6 GNE (+/+), but by 87-207% in C57Bl/6 GNE (+/-). Furthermore we could identify specific morphological changes in aged muscles. Here we propose that increased Sia concentrations in muscles are a characteristic feature of ageing and could be used as a marker for age-related changes in muscle.

Molecular mechanisms of inhibition of influenza by surfactant protein D revealed by large-scale molecular dynamics simulation

Surfactant protein D (SP-D), a mammalian C-type lectin, is the primary innate inhibitor of influenza A virus (IAV) in the lung. Interactions of SP-D with highly branched viral N-linked glycans on hemagglutinin (HA), an abundant IAV envelope protein and critical virulence factor, promote viral aggregation and neutralization through as yet unknown molecular mechanisms. Two truncated human SP-D forms, wild-type (WT) and double mutant D325A+R343V, representing neck and carbohydrate recognition domains are compared in this study. Whereas both WT and D325A+R343V bind to isolated glycosylated HA, WT does not inhibit IAV in neutralization assays; in contrast, D325A+R343V neutralization compares well with that of full-length native SP-D. To elucidate the mechanism for these biochemical observations, we have determined crystal structures of D325A+R343V in the presence and absence of a viral nonamannoside (Man9). On the basis of the D325A+R343V-Man9 structure and other crystallographic data, models of complexes between HA and WT or D325A+R343V were produced and subjected to molecular dynamics. Simulations reveal that whereas WT and D325A+R343V both block the sialic acid receptor site of HA, the D325A+R343V complex is more stable, with stronger binding caused by additional hydrogen bonds and hydrophobic interactions with HA residues. Furthermore, the blocking mechanism of HA differs for WT and D325A+R343V because of alternate glycan binding modes. The combined results suggest a mechanism through which the mode of SP-D-HA interaction could significantly influence viral aggregation and neutralization. These studies provide the first atomic-level molecular view of an innate host defense lectin inhibiting its viral glycoprotein target.

Synthesis, characterization, and in vitro evaluation of palmitoylated arabinogalactan with potential for liver targeting

Arabinogalactan (AG), a water soluble polysaccharide with more than 80 mol% galactose units, was hydrophobized by covalent attachment of palmitoyl chains using a base-catalyzed esterification reaction with the objective of effective amalgamation of arabinogalactan in liposomes for targeting asialoglycoprotein receptors (ASGPR) on liver parenchymal cells. Palmitoylated AG (PAG) was characterized by physico-chemical parameters, IR, (1)H NMR, and (13)C NMR and molecular weight determination by gel permeation chromatography. PAG was incorporated in liposomes and the liposomes were characterized by dynamic light scattering, optical microscopy, zeta potential, and transmission electron microscopic (TEM) techniques. The liposomal system was evaluated for acute toxicity in swiss albino mice and was found to be safe. Targeting ability of PAG was confirmed by in vitro binding affinity to Ricinus communis agglutinin (RCA(120)), a lectin specific for galactose. The liposomal system with PAG was evaluated for cytotoxicity on HepG2, MCF7, and A549 cancer cell lines. Cytotoxicity study revealed enhanced activity on ASGPR-expressive HepG2 cells as compared to MCF7.

Asialoglycoprotein receptor (ASGPR): a peculiar target of liver-specific autoimmunity

Asialoglycoprotein receptor (ASGPR) autoantibodies have been considered specific markers of autoimmune hepatitis (AIH). The exact mechanisms responsible for the development of these autoantibodies and leading to autoimmunity to this peculiar liver receptor remain elusive. Furthermore, loss of T cell tolerance to ASGPR has been demonstrated in patients with AIH, but it is poorly understood whether such liver-specific T cell responses bear a pathogenic potential and/or participate in the precipitation of AIH. Newly developed enzyme-linked immunosorbent assays have led to the investigation of the sensitivity and specificity of anti-ASGPR antibodies for AIH. The present review provides an overview of the diagnostic and clinical relevance of anti-ASGPR antibodies. A thorough investigation of the autoreactivity against ASGPR may assist efforts to understand liver autoimmunity in susceptible individuals.

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.

Asialoglycoprotein receptor (ASGPR) as target autoantigen in liver autoimmunity: lost and found

Asialoglycoprotein receptor (ASGPR) has attracted the attention of liver immunologists for many years. This liver-specific lectin was found to be a major B and T cell autoantigenic target in patients with autoimmune liver diseases, and in particular in autoimmune hepatitis (AIH). This review discusses the biological significance of ASGPR and its relevance to the pathogenesis of autoimmune and virus-triggered liver diseases. We also discuss emerging data on the diagnostic and clinical relevance of anti-ASGPR antibodies in light of recent reports based on commercially available anti-ASGPR enzyme-linked immunosorbent assays. Finally, we critically revisit the data reporting on disease-specific cellular immune responses against ASGPR and their relevance in relation to the pathogenesis of AIH.

Ethanol feeding potentiates the pro-inflammatory response of Kupffer cells to cellular fibronectin

BACKGROUND

Excessive alcohol consumption leads to the increased extracellular matrix deposition of cellular fibronectin (cFn) in the liver, which is also implicated as an initiating event in the fibrogenic process. We propose that cFn directly stimulates Kupffer cells (KCs), which are involved in the early response to tissue damage, to produce factors that enhance the progression of alcohol-induced liver injury toward inflammation and fibrosis.

METHOD

KCs were isolated from rats fed a control or ethanol liquid diet for 4 to 6 weeks. The effect of exogenous cFn on KC viability and the secretion of the cytokines, TNF-α and IL-6, as well as of matrix remodeling factors, MMP-2 and TIMP-2, was determined after 20 hours of cell culture.

RESULTS

For KCs from both control- and ethanol-fed rats, viability remained unaffected by treatment with cFn. TNF-α and IL-6 production were increased in KCs exposed to cFn, with cells treated with 1, 2.5, and 5 μg/ml cFn secreting significantly higher levels of both cytokines compared with untreated cells (p < 0.05). Chronic ethanol administration resulted in a significantly enhanced secretion of IL-6 by KCs regardless of treatment with cFn. When MMP-2 protein and activity levels were measured by western blot analysis and gelatin zymography, respectively, we found that cFn stimulated a dramatic increase in both cells from ethanol- and control-fed rats, with the KCs from ethanol animals being more responsive to cFn at higher concentrations (p < 0.05). Significantly higher levels of TIMP-2, which inhibits both the activation and activity of MMP-2, were secreted by KCs treated with 5 μg/ml cFn. Correspondingly, more pro-MMP-2 than active-MMP-2 was detected.

CONCLUSION

Altogether, these results show that cFn stimulates KCs to produce factors that may enhance the promotion of tissue damage and that ethanol administration increases these responses.

N-Glycans on the link domain of human HARE/Stabilin-2 are needed for hyaluronan binding to purified ecto-domain, but not for cellular endocytosis of hyaluronan

The hyaluronic acid receptor for endocytosis (HARE)/Stabilin-2 is the primary systemic scavenger receptor for 13 ligands including hyaluronan (HA), heparin and chondroitin sulfates. Most ligand-binding sites are within the 190 kDa isoform, which contains approximately 25 kDa of N-glycans and is the C-terminal half of the full-length 315 kDa HARE. Glycoproteomic analyses of purified recombinant human 190-HARE ecto-domain identified a diverse population of glycans at 10 of 17 consensus sites. The most diversity (and the only sialylated structures) occurred at N(2280), within the HA-binding Link domain. To determine if these N-glycans are required for HA binding, we created human Flp-In 293 cell lines expressing membrane-bound or soluble ecto-domain variants of 190-HARE(N2280A). Membrane-bound HARE lacking Link domain N-glycans mediated rapid HA endocytosis, but purified 190-HARE(N2280A) ecto-domain showed little or no HA binding in ELISA-like, HA-HARE pull-down assays or by surface plasmon resonance analysis (which detected very high apparent affinity for 190-HARE ecto-domain binding to HA; K(d) = 5.2 nM). The results indicate that Link domain N-glycans stabilize interactions that facilitate HA binding to HARE.

The ligand-binding profile of HARE: hyaluronan and chondroitin sulfates A, C, and D bind to overlapping sites distinct from the sites for heparin, acetylated low-density lipoprotein, dermatan sulfate, and CS-E

The hyaluronic acid receptor for endocytosis (HARE)/ Stabilin-2 is the primary systemic scavenger receptor for hyaluronan (HA), the chondroitin sulfates (CS), dermatan sulfate (DS), and nonglycosaminoglycan (GAG) ligands such as acetylated low-density lipoprotein (AcLDL), pro-collagen propeptides, and advanced glycation end products. We recently discovered that HARE is also a systemic scavenger receptor for heparin (Hep) (Harris EN, Weigel JA, Weigel PH. 2008. The human hyaluronan receptor for endocytosis [HARE/Stabilin-2] is a systemic clearance receptor for heparin. J Biol Chem. 283:17341-17350). Our goal was to map the binding sites of eight different ligands within HARE. We used biotinylated GAGs and radio-iodinated streptavidin or AcLDL to assess the binding activities of ligands directly or indirectly (by competition with unlabeled ligands) in endocytosis assays using stable cell lines expressing the 315 or 190 kDa HA receptor for endocytosis (315- or 190-HARE) isoforms, and ELISA-like assays, with purified recombinant soluble 190-HARE ecto-domain. For example, Hep binding to HARE was competed by DS, CS-E, AcLDL, and dextran sulfate, but not by other CS types, HA, dextran, or heparosan. (125)I-AcLDL binding to HARE was partially competed by Hep and dextran sulfate, but not competed by HA. Two ligands, DS and CS-E, competed with both Hep and HA to some degree. Hep and HA binding or endocytosis is mutually inclusive; binding of these two GAGs occurs with functionally separate, noncompetitive, and apparently noninteracting domains. Thus, HARE binds to HA and Hep simultaneously. Although the domain(s) responsible for Hep binding remains unknown, the Link domain was required for HARE binding to HA, CS-A, CS-C, and CS-D. These results enable us to outline, for the first time, a binding activity map for multiple ligands of HARE.

The human hyaluronan receptor for endocytosis (HARE/Stabilin-2) is a systemic clearance receptor for heparin

The hyaluronic acid receptor for endocytosis (HARE; also designated Stabilin-2) mediates systemic clearance of hyaluronan and chondroitin sulfates from the vascular and lymphatic circulations. The internalized glycosaminoglycans are degraded in lysosomes, thus completing their normal turnover process. Sinusoidal endothelial cells of human liver, lymph node, and spleen express two HARE isoforms of 315 and 190 kDa. Here we report that the 190- and 315-kDa HARE isoforms, expressed stably either in Flp-In 293 cell lines or as soluble ectodomains, specifically bind heparin (Hep). The K(d) for Hep binding to purified 190- and 315-kDa HARE ectodomains was 17.2 +/- 4.9 and 23.4 +/- 5.3 nm, respectively. Cells expressing HARE readily and specifically internalized (125)I-streptavidin-biotin-Hep complexes, which was inhibited >70% by hyperosmolar conditions, confirming that uptake is mediated by the clathrin-coated pit pathway. Internalization of Hep occurred for many hours with an estimated HARE recycling time of approximately 12 min. Internalized fluorescent streptavidin-biotin-Hep was present in a typical endocytic vesicular pattern and was delivered to lysosomes. We conclude that HARE in the sinusoidal endothelial cells of lymph nodes and liver likely mediates the efficient systemic clearance of Hep and many different Hep-binding protein complexes from the lymphatic and vascular circulations.

Properties of cryopreserved fetal liver stem/progenitor cells that exhibit long-term repopulation of the normal rat liver

We have previously achieved a high level of long-term liver replacement by transplanting freshly isolated embryonic day (ED) 14 rat fetal liver stem/progenitor cells (FLSPCs). However, for most clinical applications, it will be necessary to use cryopreserved cells that can effectively repopulate the host organ. In the present study, we report the growth and gene expression properties in culture of rat FLSPCs cryopreserved for up to 20 months and the ability of cryopreserved FLSPCs to repopulate the normal adult rat liver. After thawing and placement in culture, cryopreserved FLSPCs exhibited a high proliferation rate: 49.7% Ki-67-positive on day 1 and 34.7% Ki-67-positive on day 5. The majority of cells were also positive for both alpha-fetoprotein and cytokeratin-19 (potentially bipotent) on day 5. More than 80% of cultured cells expressed albumin, the asialoglycoprotein receptor, and UDP-glucuronosyltransferase (unique hepatocyte-specific functions). Expression of glucose-6-phosphatase, carbamyl phosphate synthetase 1, hepatocyte nuclear factor 4alpha, tyrosine aminotransferase, and oncostatin M receptor mRNAs was initially negative, but all were expressed on day 5 in culture. After transplantation into the normal adult rat liver, cryopreserved FLSPCs proliferated continuously, regenerated both hepatocytes and bile ducts, and produced up to 15.1% (mean, 12.0% +/- 2.0%) replacement of total liver mass at 6 months after cell transplantation. These results were obtained in a normal liver background under nonselective conditions. This study is the first to show a high level of long-term liver replacement with cryopreserved fetal liver cells, an essential requirement for future clinical applications.

The asialoglycoprotein receptor clears glycoconjugates terminating with sialic acid alpha 2,6GalNAc

Endogenous ligands have not, to date, been identified for the asialoglycoprotein receptor (ASGP-R), which is abundantly expressed by parenchymal cells in the liver of mammals. On the basis of the rapid clearance of BSA bearing multiple chemically coupled sialic acid (Sia)alpha2,6GalNAcbeta1,4GlcNAcbeta1,2Man tetrasaccharides (SiaGGnM-BSA) from the circulation, and the ability of the ASGP-R hepatic lectin-1 subunit to bind SiaGGnM-BSA, we previously proposed that glycoproteins modified with structures terminating with Siaalpha2,6GalNAc may represent previously unrecognized examples of endogenous ligands for this receptor. Here, we have taken a genetic approach using wild-type and ASGP-R-deficient mice to determine that the ASGP-R in vivo does indeed account for the rapid clearance of glycoconjugates terminating with Siaalpha2,6GalNAc. We have also determined that the ASGP-R is able to bind core-substituted oligosaccharides with the terminal sequence Siaalpha2,6Galbeta1,4GlcNAc but not those with the terminal Siaalpha2,3Galbeta1,4GlcNAc. We propose that glycoproteins bearing terminals Siaalpha2,6GalNAc and Siaalpha2,6Gal are endogenous ligands for the ASGP-R, and that the ASGP-R helps to regulate the relative concentration of serum glycoproteins bearing alpha2,6-linked Sia.

Cellular disposition of arabinogalactan in primary cultured rat hepatocytes

To characterize a targeting property of arabinogalactan (AG) as a carrier to the liver, we examined cellular disposition, such as binding and internalization in primary cultured rat hepatocytes, comparing them to those of asialofetuin (AF). A tyramine derivative of AG was synthesized to allow labeling with 125I. Binding of AG to the cells was concentration-dependent and saturable. The number of binding sites (n) of AG on the cell surface was 4.0 x 10(5) +/- 0.1 x 10(5) sites per cell which was about similar to that of AF. The value of Ka of AG was 2.2 x 10(8) +/- 0.1 x 10(8) M-1 being seven-fold higher than that of AF. The binding of AG was competitively inhibited by AF and was decreased by calcium depletion. These results indicate that AG can bind strongly to hepatocytes probably through the recognition by the asialoglycoprotein receptor (ASGP-R). Both 125I-labeled AG and fluorescein-labeled AG were internalized into the cells. The rate of internalization of AG was faster than that of AF, indicating that AG is effectively endocytosed. Microscopic observations showed that FITC labeled AG accumulated in granules within the primary cultured rat hepatocytes. Subcellular fractionation indicated that the internalized AG was mainly associated with the lysosomal fraction. However, the internalized AG seemed to remain intact in the hepatocytes. In conclusion, we found that AG is effectively internalized in primary cultured rat hepatocytes. Although AG seems a good candidate for targeting to the liver due to its high affinity binding and rapid internalization, it remains to be established whether the apparent lack of biodegradation will result in cytotoxic effects at chronic administration in vivo.

Assay of Rab4-dependent trafficking on microtubules

We present an in vitro method to measure how Rab4 and other regulatory proteins affect microtubule-based organelle motility. The protocols utilize small-volume, disposable "microchambers" designed for epifluorescence, confocal, or other microscope platforms and into which microtubules, organelles, and primary and fluorescent secondary antibodies are added. Our work has focused on the isolation and use of endocytic vesicles from rat liver, and we present these protocols. However, the techniques can be adapted for other organelles or cell types. Multiple fluorescent probes, rapid image capture, and immunofluorescence under non-fixation conditions allow for measurements of the location and intensity changes of endogenous proteins upon addition of ATP or upon addition of other proteins or regulatory factors. We review measurements of microtubule-based motility as well as measurements for protein localization and protein segregation in vitro.

Rapid clearance of sialylated glycoproteins by the asialoglycoprotein receptor

The asialoglycoprotein-receptor (ASGP-R) located on liver parenchymal cells was originally identified and characterized on the basis of its ability to bind glycoproteins bearing terminal galactose (Gal) or N-acetylgalactosamine (GalNAc); however, endogenous ligands for the ASGP-R have not to date been definitively identified. We have determined that the rat ASGP-R specifically binds oligosaccharides terminating with the sequence Siaalpha2,6GalNAcbeta1,4GlcNAcbeta1,2Man. Bovine serum albumin chemically modified with 10-15 tetrasaccharides with the sequence Siaalpha2,6GalNAcbeta1,4GlcNAcbeta1,2Man is cleared from the blood of the rat with a half-life of <1 min by a receptor located in the liver. We have isolated the receptor and identified it as the ASGP-R. Furthermore, we have determined that subunit 1 of the ASGP-R accounts for the binding of terminal Siaalpha2,6GalNAcbeta. Based on the newly defined specificity of the rat ASGP-R we hypothesize that glycoproteins bearing structures that are selectively modified with terminal Siaalpha2,6GalNAcbeta and are released into the blood may be endogenous ligands for the rat ASGP-R.

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.

Enhanced cell aggregation and liver functions using polymers modified with a cell-specific ligand in primary hepatocyte cultures

Hepatocytes cultured as multicellular aggregates called spheroids exhibit enhanced liver functions and maintain them over a long period compared with monolayer culture. We previously reported the induction of hepatocyte spheroids using the synthetic polymer Eudragit (a copolymer of methacrylic acid and methylmethacrylate) as an artificial matrix in a cell suspension system (Yamada et al., J. Biochem., 123, 1017-1023, 1998). In this method, hepatocyte aggregation was promoted by the effects of electrostatic and hydrophobic interactions between cells and the polymer. To enhance the cell aggregation ability and cell-specificity of the polymer, in the present study, we prepared hepatocyte-targeting polymers containing lactone, a ligand of the asialoglycoprotein receptor. Addition of the lactone-modified polymers to the medium promoted cell aggregation and spheroid formation more effectively than unmodified Eudragit. The spheroids induced by the polymers exhibited enhanced liver functions, i.e., albumin secretion, ammonia removal, and urea synthesis, from early in the culture. We also investigated the induction of hetero-spheroids composed of various liver constitutive cells by this method. The hetero-spheroids induced by the polymers showed improved liver functions.

Impairment of the asialoglycoprotein receptor by ethanol oxidation

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

The major subunit of the asialoglycoprotein receptor is expressed on the hepatocellular surface in mice lacking the minor receptor subunit

The mammalian asialoglycoprotein receptor (ASGPR) is located on the sinusoidal membrane of hepatocytes where it binds and endocytoses galactose-terminated glycoproteins (asialoglycoproteins). ASGPR is composed of two highly homologous subunits, termed hepatic lectin 1 and 2. Despite numerous studies the contribution of both subunits to biosynthesis and functional activity of ASGPR in vivo has remained controversial. Mice lacking the murine hepatic lectin (MHL)-2 subunit are viable and fertile without obvious phenotypic abnormalities. In the absence of MHL-2, knockout mice express MHL-1 protein at reduced levels. Here, we examine the intracellular fate and function of this remaining subunit. The results show that MHL-1 reaches the hepatocellular surface in knockout mice but is unable to effectively remove any one of three different radiolabeled ligands within 30 min. A small but detectable residual ligand clearance in knockout mice at 4 h is apparently not mediated by remaining MHL-1. Serum concentrations of galactose-terminating glycoproteins are not elevated in these ASGPR-deficient mice. However, competitive in vitro degradation experiments suggest that other endogenous ASGPR ligands, the nature of which remain to be determined, accumulate in serum of knockout animals.

Hepatocyte culture on carbohydrate-modified star polyethylene oxide hydrogels

We describe the synthesis and in vitro biological characterization of a new class of carbohydrate-modified hydrogels based on radiation-cross-linked star polyethylene oxide (PEO). Hydrogels were synthesized from either of two types of PEO star molecules in order to vary the terminal hydroxyl content of the gels while keeping other gel properties such as molecular weight between cross-links and water content constant. The resulting gels were covalently modified with monosaccharide ligands and the behaviour of primary rat hepatocytes on the modified gels was evaluated under culture conditions. Hepatocytes exhibited a sugar-specific adhesion to the modified gels, adhering to gels bearing galactose but not glucose. Cell spreading was observed on both types of galactose-modified PEO star gels; moreover, the gels supported long-term (6 d) culture and differentiated function of primary hepatocytes. Further, on comparing the cell spreading behaviour observed on the PEO star gels with that reported previously for galactose-modified polyacrylamide, we find that our gels elicit spreading at ligand concentrations lower by an order of magnitude. A simple mechanistic analysis indicates that this enhanced ability of PEO star gels to support spreading of primary hepatocytes on low concentrations of immobilized galactose derives from freedom of the immobilized ligands to come within sufficiently close proximity to mimic a high-affinity branched oligosaccharide.

Effects of fusogenic and DNA-binding amphiphilic compounds on the receptor-mediated gene transfer into hepatic cells by asialofetuin-labeled liposomes

Effects of fusogenic and DNA-binding amphiphilic compounds on the receptor-mediated gene transfer using asialofetuin-labeled liposomes (AF-liposomes) were examined with HepG2 cells and rat hepatocytes in primary culture. AF-liposomes were sufficiently taken up by both types of cells through the asialoglycoprotein receptor-mediated endocytosis. In HepG2 cells, bacterial beta-galactosidase (beta-Gal) gene expression was observed by transfection using AF-liposomes encapsulating plasmid pCMV beta DNA (AF-liposome-pCMV beta). By addition of dioleoylphosphatidylethanolamine (DOPE) to the liposomal lipid composition (AF-liposome(DOPE)-pCMV beta), the transfection efficiency was clearly increased. The effects of DOPE were more conspicuous in the presence of chloroquine in the medium throughout the transfection. When pCMV beta complexed with gramicidin S (pCMV beta (GrS)) was encapsulated (AF-liposome(DOPE)-pCMV beta (GrS) and was transfected to HepG2 cells, an significantly high beta-Gal activity in the cells was observed as compared with that in the cells transfected with AF-liposome(DOPE)-pCMV beta. No effects of GrS were found in the transfection using AF-non-labeled control liposomes. In primary culture of rat hepatocytes, no beta-Gal gene expression was observed even though AF-liposome(DOPE)-pCMV beta was introduced into the cells prepared from adult rats. However, following the transfection with AF-liposome(DOPE)-pCMV beta, the beta-Gal activity was expressed in the cells from immature rats cultured in the medium supplemented with epidermal growth factor and insulin, and the transfection efficiency was 2-fold higher than that transfected with pCMV beta encapsulated in AF-non-labeled control liposomes. By the complex formation of pCMV beta with GrS, the transfection efficiency of AF-liposome(DOPE)-pCMV beta (GrS) increased according to the increase of GrS in the complex. It was shown that AF-liposome(DOPE)-pCMV beta (GrS) did efficiently introduce and express beta-Gal gene in both HepG2 cells and primary hepatocytes in the receptor mediated manner.

Membrane estrogen receptor-enriched GH(3)/B6 cells have an enhanced non-genomic response to estrogen

We immunoselected GH(3)/B6 cells for a membrane estrogen receptor (mER) using antibodies generated against the rat intracellular ER (iER). Immunocytochemistry with anti-ER antibodies revealed bright fluorescence distributed in patches over the surface of mER-enriched cells, while cells immuno-depleted for mER showed only low-level membrane immunofluorescence. Quantitation via digital image analysis confirmed that immunoenriched populations show increases in both stained cell numbers and intensity of staining. Short-term culturing with serum reversibly decreased the intensity of immunostaining in mER-enriched cells to immuno-depleted cell levels. The mER-enriched populations initially contained ∼85% immunopositive cells in defined medium, but when cultured continuously with serum gradually decline to ∼22% immunopositive cells by 10 weeks. Cells enriched for mER showed a significant increase in rapid (after 2 or 5 min) prolactin release when treated with 17β-estradiol, while mER-depleted cells lacked this response. Immunoprecipitabie membrane proteins isolated from mER-enriched cells were 60,000, 74,000 and ∼ 200,000 MW, compared to an iER size of 67,000. Therefore, the presence and level of an mER that is antigenically related to iER is correlated with the ability of GH(3)/B6 cells to mediate a rapid action of estrogen.

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.

Zonal differences in ethanol-induced impairments in hepatic receptor binding

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

Effect of monensin and diabetes on asialoglycoprotein degradation in rat hepatocytes

We have studied the effects of two modulations--streptozotocin-induced diabetes in vivo, and the presence of the carboxylic proton ionophore monensin in vitro--on the degradation of 3H-asialoorosomucoid ligand in isolated rat hepatocytes. The ligand was internalized by means of a synchronous wave procedure. Diabetes was associated with a marked decrease in the amount of total degraded radioactive ligand compared to that in normal cells (3.6% and 37.3% of internalized ligand respectively, at 60 min), together with increased secretion of degradation products into the incubation medium (87% and 46.3% of the total degraded ligand was secreted by diabetic and normal cells, respectively). Monensin induced similar effects in normal cells, but had no apparent effect in diabetic cells.

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

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

The role of capillarization in hepatic failure: studies in carbon tetrachloride-induced cirrhosis

During the cirrhotic process, the hepatic microvascular phenotype is transformed from sinusoids (discontinuous capillaries) into continuous capillaries. This transformation has been termed capillarization. Many hepatic functions depend on the rapid, bidirectional exchange of macromolecules between plasma and hepatocytes. To determine whether capillarization contributes to hepatic failure in cirrhosis, we decided to study the plasma clearance (125I) and hepatocyte uptake (electron microscopy) of three tracers in normal and cirrhotic rats. The tracers chosen were a hemeundecapeptide with peroxidatic activity (fluid-phase pinocytosis), asialofetuin (receptor-mediated endocytosis of a medium size protein) and ferritin (receptor-mediated endocytosis of a large size protein). The results demonstrate a decreased hepatocyte uptake of hemeundecapeptide; a significant delay in plasma clearance of asialofetuin; and a minor delay in plasma clearance of ferritin, but a striking trapping of ferritin in the cirrhotic capillary basement membrane. The delayed plasma clearance in cirrhosis cannot be ascribed to a decreased number of surface receptors because, in isolated hepatocytes, the number of molecules bound per cell was equivalent in normal and cirrhotic livers. These findings support the concept of capillarization, with the formation of continuous diffusion and filtration barriers between plasma and hepatocytes, representing a significant hindrance to the bidirectional macromolecular exchange normally taking place between these two compartments. Furthermore, at least in the case of ferritin, the capillary basement membrane of cirrhotic livers seems to be the major filtration barrier. This hindrance to hepatocyte uptake, and presumably also to secretion, may be the cause (or at least a major determinant) of the hepatic failure characteristic of cirrhosis.

Vanadate modulates the activity of a subpopulation of asialoglycoprotein receptors on isolated rat hepatocytes: active surface receptors are internalized and replaced by inactive receptors

In the absence of ligand, sodium vanadate causes a time- and dose-dependent loss of up to approximately 50% of the surface galactosyl receptor (GalR) activity in rat hepatocytes at 37 degrees C. The effect on total (surface plus intracellular) GalR activity is also dependent on exposure time and vanadate concentration. At less than 1 mM, vanadate induces a transient decrease and then partial recovery of cell surface GalR activity. At greater than 3 mM vanadate, surface GalR activity decreases rapidly (t1/2 approximately 2 min). Lost surface activity is initially recovered in digitonin-permeabilized cells, indicating that active surface GalRs redistribute to the cell interior. However, an antibody assay for GalR protein showed that although surface activity decreased, there was no decrease in surface receptor protein. The active intracellular GalRs then slowly inactivate over 30-60 min. With 8 mM vanadate, the loss of both surface and total cellular GalR activity is more rapid and coincident; no lag is observed. Maximal activity loss, however, was still only approximately 50%. Again, no net change was seen in the distribution of GalR protein between the cell surface and the interior. These results indicate that vanadate causes active GalRs to move from the surface to the inside and be replaced by inactive receptors moving from the inside to the cell surface. The Gal receptor system is comprised of two functionally different receptor subpopulations that operate via two distinct intracellular pathways. Only the State 2 GalRs, which recycle constitutively, are sensitive to modulation by vanadate. Consistent with this, vanadate inhibits the endocytosis of 125I-asialoorosomucoid (ASOR) only partially. The rate of uptake and the steady state level of ASOR intracellular accumulation were maximally inhibited by 50 and 70%, respectively, at 0.2 mM vanadate. The rate and extent of degradation of 125I-ASOR were also inhibited by 50-70%. Residual ASOR uptake and degradation is accounted for by the minor vanadate-resistant State 1 Gal receptor pathway.

Mouse zygotes express endogenous lectins

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

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

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

The liver and IgA: immunological, cell biological and clinical implications

Secretory immunoglobulin A is the characteristic and predominant immunoglobulin of the mucosal immune system; it participates in immunological protection at the level of mucous membrane surfaces. During the past 10 to 15 years, a great deal of experimental and clinical evidence has shown that the liver is very much involved in the sIgA system. In certain animals (rats, mice, rabbits), polymeric forms of IgA are efficiently cleared by the liver and transported into bile by a receptor-mediated vesicular pathway across hepatocytes. Taking advantage of this easily accessible pathway, investigators have defined many of the events in the external secretion of pIgA, including details about the synthesis and secretion of its receptor, secretory component. In the rat hepatocyte, secretory component is synthesized as a transmembrane glycoprotein and is expressed preferentially on the sinusoidal plasma membrane; circulating pIgA that binds to secretory component is internalized into endocytic vesicles and transported across the hepatocyte to the bile canalicular membrane, where the pIgA is released into bile as a soluble complex with a portion of the secretory component, the complex being secretory IgA. In some other animals (dog, guinea pig, sheep) as well as man, biliary epithelial cells, not hepatocytes, express secretory component and perform the transcytosis and secretion of pIgA into bile. In those species, much of the pIgA that reaches bile is synthesized locally in plasma cells that populate the biliary tree; this design is analogous to the release of sIgA into various mucosae in the body. The major biological functions ascribed to the secretion of IgA into bile are enhancement of immunological defense of the biliary and upper intestinal tracts and the clearance of harmful antigens from the circulation as IgA-antigen complexes. However, the importance of biliary IgA antibodies is largely unclarified, and man lacks the capacity for effective clearance of IgA-antigen complexes via the secretory component-mediated transhepatocellular pathway; whether this deficit contributes to the propensity for man to develop IgA immune complex diseases should be clarified. Among liver diseases, alcoholic disease is most closely linked to alterations in IgA metabolism. This association is manifested principally by the deposition of IgA along the sinusoids in the livers of the majority of alcoholics and in the renal mesangium of many.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of receptors in the liver that mediate endocytosis of circulating tissue kallikreins

The liver plays an important role in the clearance, by receptor-mediated endocytosis, of circulating glycoproteins. It has been demonstrated that tissue kallikreins, which are acid glycoproteins, circulate in plasma, where they are poorly inhibited by plasma proteins. We have shown that the liver is the main organ that clears tissue kallikreins from the circulation. We now report the identification of receptors involved in this clearance. Using a perfused rat-liver system, and as models, pig pancreatic (PPK) and horse urinary (HoUK) kallikreins, we have found that: (a) the binding of PPK to the perfused liver was inhibited by 50 mM methyl alpha-D-mannoside and 20 microM mannan, was partially inhibited by 50 mM mannose and was unaffected by 1.5 microM asialofetuin; (b) binding of HoUK to the perfused liver was inhibited by 1.5 microM asialofetuin, 50 mM galactose and 50 mM lactose and was unaffected by 50 mM mannose; (c) the clearance rate of both kallikreins followed the equation y = a.xb; (d) their binding was Ca2+-dependent and their clearance was inhibited by 3 mM chloroquine and 10 mM methylamine. Using isolated liver cells and tritiated HoUK, we calculated that 500,000 receptors/cell were present and the Scatchard plot showed that there were two apparent affinity constants: 0.24.10(9) l/M) (high-affinity) and 0.3.10(8) l/M (low-affinity). These results show that PPK is recognized by a liver mannose receptor and HoUK by the galactose receptor. The liver uptake of native and circulating tissue kallikreins thus emerges as a mechanism by which their levels in plasma are regulated.

The role of the liver in translocation of IgA into the gastrointestinal tract

The liver plays a key role in the translocation of IgA into the upper gastrointestinal tract. The amount of IgA transported and the mechanisms involved, however, vary widely among species. In some, best defined in the rat, large amounts of polymeric IgA (pIgA) are cleared from the plasma by hepatocytes, which synthesize the polymeric immunoglobulin receptor, secretory component (SC), and express it on their sinusoidal plasma membranes. Circulating pIgA binds to SC, is internalized into endocytic vesicles and transported across the hepatocyte to the bile canalicular membrane, where the pIgA is released into bile in complex with a portion of the SC, i.e., secretory sIgA (sIgA). In some other species, including man, there is much less hepatic transport of circulating IgA, at least in part because SC is present only in biliary epithelium, and there is relatively more local synthesis of IgA within hepatobiliary tissues. On the other hand, certain IgA1 myeloma proteins appear to bind to and enter human hepatocytes via an asialoglycoprotein receptor. These species differences have implications for the biological significance of the biliary secretion of IgA, including the disposal of circulating IgA-antigen complexes into bile.

Ligand binding and internalization by the rat hepatic asialoglycoprotein receptor does not generate polyphosphoinositide derived second messengers

Recent studies suggest that protein kinase C and, thus, possibly the rate of inositol phospholipid hydrolysis may regulate the function and distribution of the asialoglycoprotein (or galactosyl) receptor on isolated rat hepatocytes (Takahashi et al., Biochem. Biophys. Res. Commun., 1985, 126, 1054; Fallon and Schwartz, J. Biol. Chem., 1986, 261, 15081). We have studied the effects of asialoorosomucoid (ASOR) on the hydrolysis of [32P]-inositol phospholipids in isolated rat hepatocytes. When internalization of ASOR is maximal at 310 molecules/cell/sec, there is neither a decrease in the amount of [32P]-phosphatidylinositol-4,5-bisphosphate (PIP2) nor an increase in [32P]-phosphatidic acid (PA) up to 30 min after stimulation. On the other hand, 10(-6)M vasopressin, which was used as a positive control, caused a 35-40% decrease in the level of [32P]-PIP2 and a 70-80% increase in [32P]-PA within 30 sec. Addition of orosomucoid or ASOR, even at concentrations 1000-times the Kd, did not change the levels of any of the six phospholipids tested. Similarly, addition of ASOR did not increase the levels of soluble [3H]-inositol phosphates, whereas vasopressin caused a 6-fold increase in [3H]-inositol-1,4-diphosphate (IP2) and a 4-fold increase in [3H]-inositol-1,4,5-triphosphate (IP3) in isolated rat hepatocytes prelabeled with [3H]-inositol. We conclude that the receptor mediated endocytosis of asialoglycoproteins by rat hepatocytes does not stimulate hydrolysis of the inositol phospholipids.