Ligand-induced modulation of the hepatic receptor for asialoglycoproteins in the human hepatoblastoma cell line, Hep G2

Interaction Study between Synthetic Glycoconjugate Ligands and Endocytic Receptors Using Flow Cytometry

Flow cytometric analysis of synthetic galactosyl polymers, asialofetuin and LDL derivatives labeled with FITC (Fluorescein Isothiocyanate) was carried out to determine the phenotypes of endocytic receptors, such as asialoglycoprotein (ASPG) and the LDL receptor, on various types of cells. When FITC-labeled galactosyl polystyrene (GalCPS), being a synthetic ligand of ASPG, was applied to rat hepatocytes and human cancer cells (Hep G2 and Chang Liver), surface fluorescence intensities varied according to receptor expression on the cells. The fluorescence intensity originates from the calcium-dependent binding of the FITC-labeled GalCPS. Although unaltered by pre-treatment with glucosyl polystyrene (GluCPS), fetuin and LDL, the fluorescence intensity was suppressed by pre-treatment with (non-labeled) GalCPS and asialofetuin. Flow cytometry allowed us to demonstrate that the calcium-dependent binding of FITC-labeled LDL (prepared from rabbits) upon the addition of 17alpha-ethinyl estradiol enhances LDL receptor expression, and the expression is suppressed upon the addition of a monoclonal antibody to the LDL receptor. The binding efficiency based on the combination of FITC-labeled ligands suggests a possible application for the classification of cell types and conditions corresponding to endocytic receptor expression without the need for immuno-active antibodies or radiolabeled substances. Furthermore, the synthetic glycoconjugate (GalCPS) is shown to be a sensitive and useful marker for classification based on cell phenotype using flow cytometry.

Neisseria gonorrhoeae utilizes and enhances the biosynthesis of the asialoglycoprotein receptor expressed on the surface of the hepatic HepG2 cell line

One of the lipooligosaccharide (LOS) structures of Neisseria gonorrhoeae contains a terminal Gal(beta 1-4)GlcNAc residue which is a good candidate to serve as a ligand for human asialoglycoprotein receptors (ASGP-R). These receptors have been shown to be present on macrophages, sperm cells, and hepatocytes. The human tissue culture cell line used most often to study this receptor, HepG2, was used in our investigations only as a model. We also chose N. gonorrhoeae 1291 for these studies because, unlike many other gonococcal strains, this strain expresses one main species of LOS. The LOS structure expressed by this strain has also been fully characterized. Using well-established assays for the utilization of the ASGP-R, we found that incubation of HepG2 cells with gonococci expressing the terminal Gal(beta 1-4)GlcNAc asialo-LOS carbohydrate structure competitively inhibited the ASGP-R from binding to one of its well-known ligands, asialo-alpha-acid-1-glycoprotein. The inhibition was specific to the ASGP-R, since binding of two other ligands to their specific receptors in the same model cell system was not affected. Immunoblot analysis for the ASGP-R suggested that gonococci seemed to stimulate the HepG2 cells to increase the expression of the major (46-kDa) receptor species. This observation was confirmed both by functional analysis, which showed that the concentration of total receptor molecules, as well as surface receptors, was about 60% higher after incubation with gonococci than in control cells and by Northern (RNA) blot analysis using a cDNA probe of the major human H1 subunit. Poly(A) RNA purified from control and HepG2 cells exposed to gonococci indicated the presence of increased amounts of mRNA coding for the ASGP-R after incubation with gonococci. This result supports the idea that the molecular mechanism controlling the receptor level after gonococcal exposure is under transcriptional regulation.

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.

An improved polarized rat hepatoma hybrid cell line. Generation and comparison with its hepatoma relatives and hepatocytes in vivo

Studies of hepatocyte polarity, an important property of liver epithelial cells, have been hampered by the lack of valid in vitro models. We report here that a new polarized hepatoma-derived hybrid cell line, called WIF-B, has improved characteristics to those of its parent, WIF12-1. This latter line originated from the fusion of non-polarized rat hepatoma Fao cells with human fibroblasts (WI-38) and selection for a polarized phenotype. We generated the WIF-B line by growing WIF12-1 cells as unattached aggregates for three weeks and selecting for survivors. Karyotype analysis showed a broad chromosome pattern in the initial WIF-B population, but this pattern stabilized after a few passages. The growth and phenotypic properties of these cells were quite different from those of their polarized WIF12-1 parent. WIF-B cells attained a 4-fold higher maximal density in monolayer culture, survived at this density for > 5 days rather than 1 day, and exhibited two to three times more apical structures during this period (80 to 95%). We compared several parameters of liver differentiation in the WIF-B cells with those of a related hybrid clone, WIF12-E, which is extinguished for most liver-specific functions, and with the common hepatoma parent, Fao. By immunoblot analysis, the levels of expression of eight plasma membrane proteins were higher in the WIF-B cells than in either of the other two cell lines and ranged from 10 to 200% of those in vivo. Two plasma membrane proteins were not detected in WIF12-E cells. By immunofluorescence, the apical membrane proteins in WIF-B displayed different cellular localizations than in either of the other two cell lines. In WIF-B cells, apical proteins were confined to a plasma membrane region that we have identified as the apical domain by several criteria (Ihrke, G., Neufeld, E.D., Meads, T., Shanks, M.R., Cassio, D., Laurent, M., Schroer, T.A., Pagano, R. E. and Hubbard, A. L. J. Cell Biol., 123, 1761–1765). The same molecules were distributed over the entire plasma membrane of Fao and WIF12-E cells and also (for Fao cells) in intracellular punctate structures that did not colocalize with the majority of structures containing a secretory protein, albumin. Our results indicate that the WIF-B cells are more highly differentiated than any of their ancestors (Fao or WIF12-1 cells) and thus, are promising candidates for in vitro studies of hepatocyte polarity.

Diversity among the beta subunits of heterotrimeric GTP-binding proteins: characterization of a novel beta-subunit cDNA

Heterotrimeric guanine nucleotide binding proteins transduce signals from cell surface receptors to intracellular effectors. The alpha subunit is believed to confer receptor and effector specificity on the G protein. This role is reflected in the diversity of genes that encode these subunits. The beta and gamma subunits are thought to have a more passive role in G protein function; biochemical data suggests that beta-gamma dimers are shared among the alpha subunits. However, there is growing evidence for active participation of beta-gamma dimers in some G protein mediated signaling systems. To further investigate this role, we examined the diversity of the beta subunit family in mouse. Using the polymerase chain reaction, we uncovered a new member of this family, G beta 4, which is expressed at widely varying levels in a variety of tissues. The predicted amino acid sequence of G beta 4 is 79% to 89% identical to the three previously known beta subunits. The diversity of beta gene products may be an important corollary to the functional diversity of G proteins.

The NH2-terminal alpha subunit attenuator domain confers regulation of G protein activation by beta gamma complexes

Gs and Gi, respectively, activate and inhibit the enzyme adenylyl cyclase. Regulation of adenylyl cyclase by the heterotrimeric Gs and Gi proteins requires the dissociation of GDP and binding of GTP to the alpha s or alpha i subunit. The beta gamma subunit complex of Gs and Gi functions, in part, to inhibit GDP dissociation and alpha subunit activation by GTP. Multiple beta and gamma polypeptides are expressed in different cell types, but the functional significance for this heterogeneity is unclear. The beta gamma complex from retinal rod outer segments (beta gamma t) has been shown to discriminate between alpha i and alpha s subunits (Helman et al: Eur J Biochem 169:431-439, 1987). beta gamma t efficiently interacts with alpha i-like G protein subunits, but poorly recognizes the alpha s subunit. beta gamma t was, therefore, used to define regions of the alpha i subunit polypeptide that conferred selective regulation compared to the alpha s polypeptide. A series of alpha subunit chimeras having NH2-terminal alpha i and COOH-terminal alpha s sequences were characterized for their regulation by beta gamma t, measured by the kinetics of GTP gamma S activation of adenylyl cyclase. A 122 amino acid NH2-terminal region of the alpha i polypeptide encoded within an alpha i/alpha s chimera was sufficient for beta gamma t to discriminate the chimera from alpha s. A shorter 54 amino acid alpha i sequence substituted for the corresponding NH2-terminal region of alpha s was insufficient to support the alpha i-like interaction with beta gamma t. The findings are consistent with our previous observation (Osawa et al: Cell 63:697-706, 1990) that a region in the NH2-terminal moiety functions as an attenuator domain controlling GDP dissociation and GTP activation of the alpha subunit polypeptide and that the attenuator domain is involved in functional recognition and regulation by beta gamma complexes.

Asialoglycoprotein receptor in human isolated hepatocytes from normal liver and its apparent increase in liver with histological alterations

We have characterized a binding site for galactosyl terminal glycoproteins in hepatocytes isolated from human biopsies. The binding of asialoorosomucoid on hepatocytes previously treated by Triton X-100 was saturable, calcium-dependent and highly affine (Ka = 1.11 +/- 0.87.10(9) M-1) thus corresponding to a ligand-receptor binding. The total number of receptors in the normal human liver was 140,000 +/- 65,000 sites per cell. This corresponded to the value obtained in the human hepatoma cell line HepG2, but was significantly lower than for isolated rat hepatocytes. Furthermore, in hepatocytes isolated from livers with histological features of either fibrosis, cirrhosis, hepatocarcinoma with cirrhosis or nodular regenerative hyperplasia, the number of asialoglycoprotein receptors per cell was increased, while the binding affinity was unchanged.

A G protein gamma subunit shares homology with ras proteins

Guanine nucleotide binding proteins (G proteins) that transduce signals from cell surface receptors to effector molecules are made up of three subunits, alpha, beta, and gamma. A complementary DNA clone that encodes a 71-amino acid protein was isolated from bovine brain; this protein contains peptide sequences that were derived from the purified gamma subunit of Gi and Go. The primary sequence of this G protein gamma subunit (G gamma) has 55 percent homology to the gamma subunit of transducin (T gamma) and also has homology to functional domains of mammalian ras proteins. The probe for isolating the clone was generated with the use of the polymerase chain reaction (PCR). The extent of divergence between T gamma and G gamma, the isolation of homologous PCR-generated fragments, and the differences between the predicted amino acid sequence of G gamma and that derived from the gamma subunit of Gi and Go indicate that gamma subunits are encoded by a family of genes.

Characterization and biological implications of membrane lectins in tumor, lymphoid and myeloid cells

Complex carbohydrates and sugar receptors at the surface of eukaryotic cells are involved in recognition phenomena. Membrane lectins have been characterized, using biochemical, biological and cytological methods. Their biological activities have been assessed using labeled glycoproteins or neoglycoproteins. Specific glycoproteins or neoglycoproteins have been used to inhibit their binding capacity in both in vitro and in vivo experiments. In adults, lymphoid and myeloid cells as well as tumor cells grow in a given organ and eventually migrate and home in another organ; these phenomena are known as the homing process or metastasis, respectively. In specific cases, membrane lectins of endothelial cells recognize cell surface glycoconjugates of lymphocytes or tumor cells, while membrane lectins of lymphocytes and of tumor cells recognize glycoconjugates of extracellular matrices or of non-migrating cells. Therefore, membrane lectins are involved in cell-cell recognition phenomena. Membrane lectins are also involved in endocytosis and intracellular traffic of glycoconjugates. This property has been demonstrated not only in hepatocytes, fibroblasts, macrophages and histiocytes but also in tumor cells, monocytes, thyrocytes, etc. Upon endocytosis, membrane lectins are present in endosomes, whose luminal pH rapidly decreases. In cells such as tumor cells or macrophages, endosomes fuse with lysosomes; it is therefore possible to target cytotoxic drugs or activators, by binding them to specific glycoconjugates or neoglycoproteins through a linkage specifically hydrolyzed by lysosomal enzymes. In cells such as monocytes, the delivery of glycoconjugates to lysosomes is not active; in this case, it would be preferable to use an acid-labile linkage. Cell surface membrane lectins are developmentally regulated; they are present at given stages of differentiation and of malignant transformation. Cell surface membrane lectins usually bind glycoconjugates at neutral pH but not in acidic medium: their ligand is released in acidic specialized organelles; the internalized ligand may be then delivered into lysosomes, while the membrane lectin is recycled. Some membrane lectins, however, do bind their ligand in relatively acidic medium as in the case of thyrocytes. The presence of cell surface membrane lectins which recognize specific sugar moieties opens the way to interesting applications: for instance, isolation of cell subpopulations such as human suppressor T cells, targeting of anti-tumor or anti-viral drugs, targeting of immunomodulators or biological response modifiers.