Establishment of plasma membrane domains in hepatocytes. I. Characterization and localization to the bile canaliculus of three antigens externally oriented in the plasma membrane

Elf3 encodes a novel 200-kD beta-spectrin: role in liver development

beta-spectrins are crucial for the maintenance of cell shape, the establishment of cell polarity, and the formation of distinct membrane domains. Our strategy for identifying genes important for hepatocyte polarity has been to utilize subtractive hybridization of early embryonic mouse cDNA liver libraries. As a result, we have cloned three isoforms of a novel beta-spectrin elf (embryonic liver beta-fodrin), and here we report the analysis of elf3, the longest isoform (8172 nt). ELF3 comprises 2154 residues with an overall similarity of 89.0% and 95.3% to mouse beta-spectrin (betaSpIIsigma1) at the nucleotide and amino acid level, respectively. ELF3 is characterized by an actin-binding domain, a long repeat domain, and a short regulatory domain remarkable for the absence of a PH domain. Linkage analysis reveals that elf3 maps to mouse chromosome 11 between D11Bir6 and D11Xrf477, a different chromosomal locus from that of the other four spectrin genes. Northern blot analysis utilizing an elf3 3'-UTR probe demonstrates an abundant 9.0-kb transcript in brain, liver, and heart tissues. Western blot with a polyclonal antibody against ELF identifies a 200 kD protein in mouse liver, brain, kidney, and heart tissues. Immunohistochemical studies demonstrate ELF labeling of the basolateral or sinusoidal membranes surface as well as a granular cytoplasmic pattern in hepatocytes. Antisense studies utilizing cultured liver explants show a vital role of elf3 in hepatocyte differentiation and intrahepatic bile duct formation. The differential expression, tissue localization, and functional studies demonstrate the importance of elf3 in modulating interactions between various components of the cytoskeleton proteins controlling liver and bile duct development.

Staining of pancreatic centroacinar cells, liver bile canaliculi and testicular Leydig cells with a monoclonal antibody against adrenocortical cells

The immunoreactivity of a monoclonal antibody against cell suspensions from guinea pig adrenal glands was examined at light- and electron-microscopic levels. In addition to the cell surface membrane of adrenocortical cells, the antibody labeled specific sites in the pancreas, liver and testis, but did not label any of the other tissues examined. In the pancreas, microvilli-like processes and the cell surface membrane of centroacinar cells were immunoreactive to the antibody. The microvilli of interlobular duct cells and pancreatic duct cells were also immunoreactive. In the liver, bile canalicular microvilli of hepatocytes were exclusively labeled. Membrane structures of cell organelles, mainly mitochondria, in testicular Leydig cells were also labeled. Immunoblot analysis showed that the monoclonal antibody bound to two common bands at molecular weights of approximately 62 kDa and 110 kDa in the pancreas, liver, testis, and adrenal gland. The two bands reacted with the digoxigenin-conjugated lectin, Sambucus nigra agglutinin (SNA), which recognizes sialic acid linked alpha (2-6) to galactose. Reaction patterns of SNA in the pancreas, liver and testis were similar to those of the monoclonal antibody; pancreatic centroacinar cells and interlobular duct cells, hepatocyte bile canaliculi and testicular Leydig cells were densely stained with SNA. Thus, the monoclonal antibody recognizes two common membrane glycoproteins containing sialic acids in the pancreas, liver, testis and adrenal cortex.

Identification of a rat liver dipeptidyl aminopeptidase IV with a liver plasma membrane glycoprotein (gp110). A study using dipeptidyl aminopeptidase IV-deficient rats

A rat liver plasma membrane glycoprotein, gp110, was compared with dipeptidyl aminopeptidase IV (DAP IV) by using Wistar rats (DAP IV-positive rats) and Fischer 344 rats (DAP IV-negative rats). Fischer rats also lacked gp110 and gp110 of Wistar rats had DAP IV activity. Furthermore, we showed that the C-terminal sequence of gp110 was Ser-Leu-Arg, which was the same as the C-terminal amino acid sequence deduced from the nucleotide sequence of the cDNA of DAP IV. According to these results, we concluded that gp110 was identical with DAP IV.

Bile duct ligation-induced redistribution of canalicular antigen in rat hepatocyte plasma membranes demonstrated by immunogold quantitation

Extrahepatic obstructive cholestasis has been demonstrated to induce a redistribution of domain specific membrane proteins in rat hepatocytes reflecting loss or even reversal of cell polarity. In order to further characterize the redistribution of canalicular antigens, we used the Lowicryl K4M immunogold technique for examination of the effects of bile duct ligation (50 h) on the distribution of antigen in rat hepatocytes at the ultrastructural level and quantitated immuno-gold density in the three domains of the plasma membrane. In normal hepatocytes, antigen was localized almost exclusively in the canalicular domain while the sinusoidal and lateral membranes showed only weak immunoreactivity. Other localizations included organelles compatible with known pathways of biosynthesis and degradation. Bile duct ligation markedly reduced immunolabel in the canalicular and increased it slightly in the sinusoidal domain. The number and staining intensity of immunoreactive subcanalicular lysosomes and vesicles probably representing endosomes was augmented. Number of immunogold particles per micron of plasma membrane were 7.86 vs 2.46 (P less than 0.005) in the canalicular, 1.16 vs 1.38 (n.s.) in the sinusoidal, and 1.23 vs 1.08 (n.s.) in the lateral domain resulting in a canalicular decrease by 68.7% and a sinusoidal increase of 19.0%. Overall decrease in total plasma membranes was by 29.7% (P less than 0.05). Thus, our data show that the sinusoidal and lateral domains behave differently. Furthermore, quantitative immunocytochemistry demonstrates a decrease in the canalicular antigen density and suggests a sinusoidal increase. The present data agree with the concept that bile duct ligation results in a loss or even reversal of cell polarity in hepatocytes.

The antigen of bile canaliculi of the mouse hepatocyte: identification and ultrastructural localization

The AgB10 antigen of bile canaliculi of the mouse hepatocyte was identified using monoclonal antibodies. The Mr value of 116000 for AgB10 was measured by immunoblotting. The tissue localization of AgB10 was studied by light and electron microscopy using the immunoperoxidase technique. AgB10 was predominantly present on the microvillus membrane of bile canaliculi, the brush border of intestinal mucosa and apical surfaces of the epithelial cells in some other organs. A small amount of AgB10 was detected on the basolateral domain of the hepatocytes. AgB10 was specific for hepatocytes and was not found in the other cell types of the liver. In primary hepatocyte culture, AgB10 was localized on the surface of cells during the first 24 h, predominantly at the sites of cell-cell and cell-substratum contacts. After 48 h of culture AgB10 gradually disappeared from contacting cell surfaces and became concentrated only in the reconstituted bile canaliculi.

The formation of bile canaliculi in human hepatoma cell lines

Hepatocytes, known as polarized epithelial cells, are composed of sinusoid, basolateral and bile canalicular domains. Each domain contains proteins specific for it. Our studies indicate that the well-differentiated human hepatoma cell lines HepG2 and HuH-7 formed bile canaliculi in tissue culture, whereas the poorly differentiated hepatoma cell lines HA22T/VGH and SK-HEP-1 did not. We also used the 9B2 monoclonal antibody, previously shown to be specific for the human bile canalicular domain, to study formation of bile canaliculi in these human hepatoma cell lines. All four cell lines synthesize the 140-kD 9B2 antigen. Studies using peroxidase-antiperoxidase staining and immunoelectron microscopy revealed that the 9B2 antigen was first detected in cytoplasm and packaged in microvilli-lined vesicles, then vectorially transported to the cell surface and eventually fused with microvilli-lined vesicles from neighboring cells to form bile canaliculi in well-differentiated hepatoma cell lines. However, the 9B2 antigen of poorly differentiated lines was synthesized in cytoplasm, then transported directly to and evenly distributed on the cell membrane. These results lead us to conclude that human hepatoma cell lines could serve as a good in vitro model to study the formation of bile canaliculi in human hepatocytes. The bile canaliculi of human hepatocytes may be preformed and assembled in the intracellular, microvilli-lined vesicles, then vectorially transported to the cell surface, where they form the bile canaliculi through vesicles fusion. Finally, formation of bile canaliculi and transport of 9B2 antigen may be related to the differentiation of hepatocytes or progression stages of human hepatoma cells.

Identification of the bile canalicular cell surface molecule GP110 as the ectopeptidase dipeptidyl peptidase IV: an analysis by tissue distribution, purification and N-terminal amino acid sequence

This paper describes the tissue distribution, purification and N-terminal amino acid sequence of the bile canalicular cell surface molecule dipeptidyl peptidase IV. Immunoperoxidase staining of cryostat sections of rat liver with a monoclonal antibody, Medical Research Council OX-61, indicated specific binding to hepatocyte bile canalicular domains and brush borders of bile ducts. Additional staining was seen in other epithelial brush borders (small intestine, kidney, colon, pancreatic duct); acinar structures in salivary glands; endothelial structures and T cell areas in thymus, spleen and lymph node. The tissue distribution suggested that monoclonal antibody OX-61 binds to the ectoenzyme dipeptidyl peptidase IV. This was confirmed by depletion of dipeptidyl peptidase IV activity from tissue homogenates by monoclonal antibody OX-61 coupled to Sepharose. The molecule recognized by OX-61 was then purified from liver and kidney by monoclonal antibody affinity chromatography. The molecule had a molecular weight of 110 kD under reducing conditions. The purified molecule was subsequently analyzed for amino acid composition and N-terminal amino acid sequence. Thirty-one N-terminal amino acids were sequenced and indicated identity with part of the predicted N-terminus of the previously cloned bile canalicular molecule GP110. On review, other similarities between dipeptidyl peptidase IV and GP110 were detected: molecular weight, deglycosylated form and metabolic half-life. Finally, the recent cloning of dipeptidyl peptidase IV permitted a comparison between the molecule recognized by monoclonal antibody OX-61, GP110 and dipeptidyl peptidase IV. It is concluded that these three molecules are almost certainly identical.

Functional reconstitution of the canalicular bile salt transport system of rat liver

Recent studies have suggested that the canalicular bile salt transport system of rat liver corresponds to a 100-kDa membrane glycoprotein. In the present study we attempted to functionally reconstitute the 100-kDa protein into artificial proteoliposomes. Canalicular membrane proteins were solubilized with octyl glucoside in the presence of asolectin phospholipids. The extracts were treated with preimmune serum or the 100-kDa protein selectively immunoprecipitated with a polyclonal antiserum. Proteins remaining in the supernatant were then incorporated into proteoliposomes by gel-filtration chromatography. Canalicular proteoliposomes containing the 100-kDa protein exhibited transstimulatable taurocholate uptake that could be inhibited by 4,4'-diisothiocyanato-2,2'-stilbenedisulfonic acid (DIDS). In contrast, no DIDS-sensitive transstimulatable taurocholate uptake was found in 100-kDa protein-free canalicular proteoliposomes. However, when the immunoprecipitated 100-kDa protein was dissociated from the antibodies and exclusively incorporated into liposomes, reconstitution of DIDS-sensitive transstimulatable and electrogenic taurocholate anion transport was again positive. Although incorporation of solubilized basolateral membrane proteins into liposomes also resulted in a prompt reconstitution of Na+ gradient-driven taurocholate uptake, the anti-100-kDa antibodies had no effects on the reconstituted transport activity of basolateral proteins. Thus, the findings establish that the previously characterized canalicular-specific 100-kDa protein is directly involved in the transcanalicular secretion of bile salts.

Identification and characterization of a murine receptor for galactose-terminated glycoproteins

The asialoglycoprotein receptor, the hepatic binding lectin for galactose-terminated glycoproteins, has been isolated and characterized from human, rabbit and rat liver. Several recent studies have shown the existence of the same receptor in murine liver. However, the biochemical structure of the receptor in murine liver has not been resolved. In this paper, we describe the identification and purification of the receptor for asialoglycoproteins from murine liver. The purified receptor has three polypeptides with apparent molecular weights of 42,000, 45,000 and 51,000, respectively, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Furthermore, our studies suggest that the receptor from murine liver is very similar to its counterpart in rat liver, although some potential interesting differences have also been observed. Initial studies indicate that this receptor is well conserved in different mouse strains.

Isolation of domains of the plasma membrane of hepatocytes

Several recent studies have demonstrated the ability of techniques based on immunoadsorption to selectively isolate specialized subregions of membranes, termed domains, which are derived from a larger more complex parent membrane like the plasma membrane. The immunoadsorbent is directed against a specific antigen that resides exclusively or predominantly in the membrane domain to be isolated. Thus, a monospecific antibody to the domain-specific antigen is required. In the present study we developed a method employing a modified immunoblotting strategy which could utilize polyspecific antibodies to isolate membrane vesicles derived from a specific membrane domain of the hepatocyte plasma membrane. We also used specific cell surface labeling of the hepatocyte plasma membrane by lactoperoxidase-catalyzed iodination at 4 degrees C and preparation of different sized vesicles by sonication to facilitate isolation of the specific domain. For this study, polyspecific antisera were raised in goats against a membrane fraction, denoted N2u, which is enriched in bile canalicular proteins. This antiserum recognizes, among other antigens, a 110,000 Mr polypeptide previously shown to be localized in the bile canaliculus (J. Cook et al. (1983) J. Cell. Biol. 97, 1823-1833). A monospecific antiserum was raised in rabbits against the rat hepatocyte asialoglycoprotein receptor, a sinusoidal domain-specific set of glycoproteins whose major form has a Mr of 43,000. These antisera were each coupled indirectly to different pieces of nitrocellulose by the immunoblotting protocol and were used to isolate membrane vesicles from a crude extract of liver plasma membrane prepared by sonication. The ratio of iodinated asialoglycoprotein receptor to the 110,000 Mr polypeptide in vesicles isolated by the affinity nitrocellulose immunoadsorbent method indicate a 10- to 15-fold enrichment of sinusoidal-derived vesicles relative to bile canalicular-derived membrane vesicles. These results show that the affinity nitrocellulose immunoadsorbent method can be used to isolate domain-specific vesicles. Further, the affinity immunoadsorbent method described here for the isolation of domains of the plasma membrane is an integrative one allowing isolation of vesicles present in relatively small concentration in crude cell extracts and it requires minimal ultracentrifugation time.

Monoclonal antibodies identifying antigens on distinct domains of rat hepatocytes

To aid in evaluating the functional significance of various domains of the hepatocyte plasma membrane, we have developed monoclonal antibodies that react with three different antigens on the surface of rat hepatocytes. One antigen is present on the sinusoidal-lateral plasma membrane but is absent from the bile canalicular membrane. On the basis of apparent molecular weight and ability to bind a desialylated glycoprotein, this antigen may be the asialoglycoprotein receptor. It was also identified immunocytochemically on the sinusoidal-lateral plasma membrane of human hepatocytes. The second antigen is present only on the bile canalicular membrane, whereas the third is present on the entire cell surface of rat hepatocytes. Our monoclonal antibodies may be useful in investigations of the sorting mechanisms of hepatocyte membrane proteins and the pathogenesis of certain liver diseases.

Identification of different transport systems for bile salts in sinusoidal and canalicular membranes of hepatocytes

The preservation of the functional polarity of hepatocytes in liver snips (1 x 2 x 4 mm) was demonstrated by fluorescent microscopic studies using the sodium salt of (N-[7-(4-nitrobenzo-2-oxa-1,3-diazol)]-3 beta-amino-7 alpha,12 alpha- dihydroxy-5 beta-cholan-24-oyl)-2-aminoethanesulfonic acid. This fluorescent bile salt derivative is not only taken up by hepatocytes of several cell layers at the surface of the snips but also secreted into bile canaliculi. The intact hepatobiliary transport of bile salts by hepatocytes of liver snips demonstrates that they are a useful system for the investigation of those transcellular transport processes which require the integrity of hepatic structure. Photoaffinity labelling of liver snips with the sodium salt of (7,7-azo-3 alpha,12 alpha-dihydroxy-5 beta-[3 beta-3H]cholan- 24-oyl)-2-aminoethanesulfonic acid revealed that the bile-salt-binding membrane polypeptides with apparent Mr values of 54,000 and 48,000 are exclusively located in the sinusoidal membrane, whereas a single bile-salt-binding polypeptide with an apparent Mr of 100,000 is located in the bile-canalicular membrane. Photoaffinity labelling of liver snips at 4 degrees C, when transcellular bile-salt transport is insignificant, resulted in the labelling of the two sinusoidal membrane polypeptides and practically no labelling of the polypeptide with an apparent Mr of 100,000. This latter polypeptide was also not labelled when Ca2 deprivation abolished bile secretion completely. These results indicate that the directed hepatobiliary transport of bile salts in hepatocytes is accomplished by transport systems which are different for sinusoidal uptake and canalicular secretion.

Cellular events during hepatocarcinogenesis in rats and the question of premalignancy

The cellular, biochemical, and genetic changes that occur in the liver of rats exposed to chemical hepatocarcinogens are reviewed. Multiple new cell types appear in the liver of carcinogen-treated rats including foci, nodules, ducts, oval cells, and atypical hyperplastic areas. The application of phenotypic markers for these cell types suggests that hepatocellular carcinomas may arise from more than one cell type, including a putative liver stem cell that proliferates following carcinogen exposure. Study of DNA, RNA, and proteins produced by hepatocellular carcinomas and putative premalignant cells has so far failed to identify a gene or gene product clearly associated with the malignant or premalignant phenotype. Understanding the cellular lineage from normal cell through putative premalignant cell to cancer is critical to understanding the process of carcinogenesis. Application of new immunological (monoclonal antibody, transplantation) and molecular biological (gene cloning, oncogene identification) approaches to this problem holds promise that the process of hepatocarcinogenesis will be better known in the near future.

Identification of ZO-1: a high molecular weight polypeptide associated with the tight junction (zonula occludens) in a variety of epithelia

A tight junction-enriched membrane fraction has been used as immunogen to generate a monoclonal antiserum specific for this intercellular junction. Hybridomas were screened for their ability to both react on an immunoblot and localize to the junctional complex region on frozen sections of unfixed mouse liver. A stable hybridoma line has been isolated that secretes an antibody (R26.4C) that localizes in thin section images of isolated mouse liver plasma membranes to the points of membrane contact at the tight junction. This antibody recognizes a polypeptide of approximately 225,000 D, detectable in whole liver homogenates as well as in the tight junction-enriched membrane fraction. R26.4C localizes to the junctional complex region of a number of other epithelia, including colon, kidney, and testis, and to arterial endothelium, as assayed by immunofluorescent staining of cryostat sections of whole tissue. This antibody also stains the junctional complex region in confluent monolayers of the Madin-Darby canine kidney epithelial cell line. Immunoblot analysis of Madin-Darby canine kidney cells demonstrates the presence of a polypeptide similar in molecular weight to that detected in liver, suggesting that this protein is potentially a ubiquitous component of all mammalian tight junctions. The 225-kD tight junction-associated polypeptide is termed "ZO-1."

Identification of rat hepatocyte plasma membrane proteins using monoclonal antibodies

We have localized and identified five rat hepatocyte plasma membrane proteins using hybridoma technology in combination with morphological and biochemical methods. Three different membrane preparations were used as immunogens: isolated hepatocytes, a preparation of plasma membrane sheets that contained all three recognizable surface domains of the intact hepatocyte (sinusoidal, lateral, and bile canalicular), and a glycoprotein subfraction of that plasma membrane preparation. We selected monoclonal IgGs that were hepatocyte specific and localized them using both immunofluorescence on 0.5-micron sections of frozen liver and immunoperoxidase at the ultrastructural level. One antigen (HA 4) was localized predominantly to the bile canalicular surface, whereas three (CE 9, HA 21, and HA 116) were localized predominantly to the lateral and sinusoidal surfaces. One antigen (HA 16) was present in all three domains. Only one antigen (HA 116) could be detected in intracellular structures both in the periphery of the cell and in the Golgi region. The antigens were all integral membrane proteins as judged by their stability to alkaline extraction and solubility in detergents. The apparent molecular weights of the antigens were established by immunoprecipitation and/or immunoblotting. In a related study (Bartles, J.R., L.T. Braiterman, and A.L. Hubbard, 1985, J. Cell. Biol., 100:1126-1138), we present biochemical confirmation of the domain-specific localizations for two of the antigens, HA 4 and CE 9, and demonstrate their suitability as endogenous domain markers for monitoring the separation of bile canalicular and sinusoidal lateral membrane on sucrose density gradients.

Heparan sulfate proteoglycans are concentrated on the sinusoidal plasmalemmal domain and in intracellular organelles of hepatocytes

The distribution of cell surface heparan sulfate proteoglycans (HSPGs) was determined in rat liver by immunocytochemistry. A polyclonal antibody was raised against HSPGs purified from rat liver microsomes which specifically immunoprecipitated liver membrane HSPGs. It was shown to recognize both the heparin-releasable and membrane-intercalated form of membrane HSPGs and to recognize determinants on the core protein of these HSPGs. By immunocytochemistry membrane HSPGs were localized to hepatocytes. The distribution of HSPGs at the cell surface of the hepatocyte was restricted to the sinusoidal domain of the plasmalemma; there was little or no staining of the lateral or bile canalicular domains. Intracellularly, HSPGs were occasionally detected in cisternae of the rough endoplasmic reticulum and were regularly found in Golgi cisternae--usually distributed across the entire Golgi stack. HSPGs were also localized in some endosomes, lysosomes, and cytoplasmic vesicles of hepatocytes. We conclude that the HSPGs recognized by this antibody have a restricted distribution in rat liver: they are largely confined to the sinusoidal plasmalemmal domain and to biosynthetic and endocytic compartments of hepatocytes.

Studies with digitonin-treated rat hepatocytes (nude cells)

Isolated rat hepatocytes were treated with digitonin to strip the plasma membrane. The effect of digitonin concentration and exposure time on the recovery of marker enzymes for cell organelles was examined. Hepatocytes treated at room temperature for 1-2 min with 1 mg/ml of digitonin lose some 40% of their protein but retain over 95% of their intact mitochondria and peroxisomes, 90-95% of their endoplasmic reticulum, and about 80% of their lysosomal enzymes. There is little loss of the mitochondrial intermembrane content, and both oxygen uptake and phosphorylation are unimpaired by the treatment. Electron microscopy reveals a complete loss of the plasma membrane, in spite of limited loss of marker enzymes for this membrane. Scanning electron microscopy revealed the interior of the cells to be made up of a dense network of fibers and lamellae attached to the nucleus, mitochondria, and small organelles. The treated cells were stable for many hours when kept in 0.25 M sucrose containing 25 mM monovalent salts. In salt-free sucrose the cells broke up very rapidly into nuclei and other single organelles. Addition of 5 mM NaCl or KCl retards breakup, and 15-20 min were required for dissolution. Intermediate stages, illustrated by scanning electron micrographs, show structure and chains made up mainly of mitochondria held together by a lamellar network. The rapid breakdown occurred at a pH above 7.5 in an oxygen atmosphere and in the presence of phosphate and apparently is an energy-requiring process. It is slow below a pH of 7.2, and at a pH of 6.8 the treated cells remain completely stable in salt-free sucrose. Our results suggest that endoplastic reticulum is a major component of the cytostructure holding together nuclei and organelles.