Differential expression of asialoglycoprotein receptor subunits in the endocytic compartment during liver regeneration

Autoantibodies as prognostic markers in autoimmune liver disease

Certain autoantibodies in autoimmune liver disease have prognostic implications that are under-utilized and under-developed. The goals of this review are to indicate progress in characterizing the autoantibodies with prognostic connotations and to indicate the feasibility and importance of discovering other markers. Prime source and review articles in English were selected by a Medline search through 2010. Antibodies to soluble liver antigen, actin, liver cytosol type 1, asialoglycoprotein receptor, chromatin, cyclic citrullinated peptide, and uridine glucuronosyltransferases have been associated with the occurrence, severity, and progression of autoimmune hepatitis, and antibodies to Sp100, gp210, and centromere have had similar implications in primary biliary cirrhosis. Antibodies to soluble liver antigen have shown the most promise in autoimmune hepatitis as they have been associated with severe histological changes, long durations of treatment, relapse after drug withdrawal, and high frequency of liver failure. Antibodies to the nuclear rim pore protein, gp210, have shown the most promise in primary biliary cirrhosis as they have been associated with severe interface hepatitis, lobular inflammation, and progression to liver failure. The major limitations of the autoantibodies have been their lack of standardized assays, low negative predictabilities, and fluctuating levels. Performance parameters will improve as critical pathogenic pathways, comprehensive testing batteries, and standardized assays through international exchange workshops are developed. Progress has been made in identifying the serological markers of prognosis in autoimmune liver disease, and they promise to reflect critical disease mechanisms and enhance patient management.

Intracellular trafficking during liver regeneration. Alterations in late endocytic and transcytotic pathways

BACKGROUND/AIMS

Liver growth, induced by partial hepatectomy of the organ is a precisely regulated process during which a radical reorganisation of metabolism occurs as the hepatocytes become committed to enter the cell cycle. Recent studies have shown the importance of the endocytic compartment in the control of lipid and protein intracellular trafficking but also in the control of the signal transduction events, which eventually will trigger the initiation of DNA synthesis and the subsequent cell division.

METHODS

We isolated endosomes at different times after partial hepatectomy in male rats and compared with endosomes isolated from sham-operated animals. Also, bile was collected and analysed by 2D-gel electrophoresis.

RESULTS

The amount of late endosomes isolated from regenerating livers decreased, concomitant with decreased cathepsin D specific enzyme activity. Furthermore, secretion of horseradish peroxidase, pIgA and transferrin increased in the pre-replicative phase of liver regeneration.

CONCLUSIONS

At the early stages of liver regeneration, the hepatocellular transport pathway towards degradation (late endosomes and lysosomal pathway) decreases, but the transcytosis and the bile secretion of several major proteins increases.

Biochemical analysis of a caveolae-enriched plasma membrane fraction from rat liver

We isolated and characterized a subcellular fraction derived from the blood-sinusoidal plasma membrane of hepatocytes enriched in caveolin and containing several of the molecular components described to be present in caveolae isolated from other cell types. A morphological study by electron microscopy revealed that it was composed of caveolae-attached membrane profiles. Immunoelectron microscopy of isolated fraction showed the specific labeling of internal caveolae membranes with anti-caveolin antibody. Finally, one- and two-dimensional electrophoresis and Western blotting were used for the biochemical analysis of this new rat liver plasma membrane fraction. From the biochemical and the morphological characterization, we conclude that the caveolae-enriched plasma membrane fraction is a plasma membrane fraction, which originates from specialized regions of the sinusoidal plasma membrane, enriched in caveolae.

Enhanced prenyltransferase activity and Rab content in rat liver regeneration

Rabs are small GTP-binding proteins with a regulatory role in intracellular vesicular traffic. The modulation of their levels and activity in different physiological situations is poorly understood. During the first cell cycle of rat liver regeneration we observed a differential regulation of some Rabs, with a progressive increase of those involved in exocytosis and a progressive decrease of one involved in endocytosis. This could be related with the need of exposing growth factor receptors and prolonging the transduction of their signal in preparation for mitosis. Moreover, we observed an increased activity of protein prenyltransferases, the enzymes responsible for the prenylation of several proteins involved in crucial processes of proliferation, without a corresponding increase in the amount of prenyltransferase protein.

Isolated rat hepatocytes bind lactoferrins by the RHL-1 subunit of the asialoglycoprotein receptor in a galactose-independent manner

Isolated rat hepatocytes bind and internalize the iron-binding protein lactoferrin (Lf) by a set of high-affinity, recycling, Ca2+-dependent binding sites. We have purified a 45-kDa membrane protein (p45) from rat hepatocytes that exhibits Ca2+-dependent receptor activity. In this study, we found p45 to be identical to the major subunit (RHL-1) of the rat asialoglycoprotein receptor. Two tryptic fragments of p45 showed 100% identity with RHL-1 internal sequences (Leu121 --> Lys126 and Phe198 --> Lys220), and monospecific antisera against p45 and RHL-1 cross-reacted equally well with each protein. Molar excesses of anti-p45 IgG, anti-RHL-1 IgG, asialoorosomucoid, and asialofetuin competitively blocked the binding of 125I-Lf to isolated rat hepatocytes at 4 degrees C. Similarly, either excess anti-p45 or Lf blocked the binding of 125I-asialoorosomucoid to cells at 4 degrees C. We did not detect the minor subunits of the rat asialoglycoprotein receptor (RHL-2/3) in p45 preparations from Triton X-100 extracts of hepatocytes and 125I-Lf bound to purified RHL-1 but not to RHL-2/3 immobilized on nitrocellulose. Nonetheless, anti-RHL-2/3 IgG reduced the binding of 125I-Lf to hepatocytes at 4 degrees C. Exoglycosidases were used to remove terminally-exposed N-acetylneuraminyl, alpha- and beta-galactosyl, and N-acetylhexosaminyl sugars from human and bovine Lf glycans, and lectin blotting confirmed that glycosidase-treated Lfs lacked detectable terminal galactosyl sugars. Unexpectedly, these deglycosylated Lfs exhibited no loss in their ability to compete with unmodified Lfs for binding to isolated hepatocytes. In addition, molar excess of beta-lactose but not sucrose competitively blocked the binding of 125I-Lf to cells, indicating that Lf bound at or very near the carbohydrate-recognition domain of RHL-1. We conclude that RHL-1 is the Ca2+-dependent Lf receptor on hepatocytes and that it binds Lf at its carbohydrate-recognition domain yet in a galactose-independent manner.

Cyclin A is present in the endocytic compartment of rat liver cells and increases during liver regeneration

Recent studies have implicated the cell cycle kinase cdc2 and cyclin A in the inhibition of the fusion of endocytic vesicles in vitro during mitosis. However, the presence of cyclins or their associated cyclin dependent kinases (cdks) in the endocytic fractions have not been reported. Using Western-blotting and immunocytochemistry approaches with different anticyclin A antibodies we have detected cyclin A in the endocytic compartment of the rat liver. During the pre-replicative phase of liver regeneration the amount of cyclin A in endosomes increases significantly with a peak around 12 hours after partial hepatectomy. Cyclin A-dependent kinases, cdc2 and cdk2, were also found in isolated endosome fractions, showing a distinct kinetics of accumulation during the regenerative period. Finally, histone H1 kinase activity was detected associated with cyclin A in endocytic vesicles and increased in regenerating liver. These results suggest that changes in the organization and in the function of the endocytic compartment during the hepatocellular proliferation may be modulated by proteins involved in the regulation of the cell cycle.

Functional identification of three major phosphoproteins in endocytic fractions from rat liver. A comparative in vivo and in vitro study

Liver plasma membranes originating from the sinusoidal, lateral and canalicular domains and 'early' and 'late' endosomes were prepared from rats injected with [32P]orthophosphate. The phosphorylated polypeptides in these subcellular fractions, resolved by gel electrophoresis, were analysed and compared with those obtained by in vitro phosphorylation of the fractions by endogenous protein kinases. The polypeptides phosphorylated in vitro were different in plasma membranes, endosomes and lysosomes. Three of the major phosphoproteins in the endocytic membranes were shown to be the polymeric immunoglobulin receptor, the beta subunit of the insulin receptor and the 550-kDa low-density-lipoprotein-receptor-related protein (LRP). An additional 35-kDa polypeptide of unknown function was a major phosphorylated component and thus emerges as a candidate marker protein of hepatic endosomes. Phosphoserine was shown to be the major amino acid phosphorylated in vitro in the phosphoproteins of endocytic membranes. The subcellular distribution in liver tissue of protein kinase activity was also investigated and activity shown to be recovered mainly in blood-sinusoidal and lateral plasma membranes; bile canalicular plasma membranes and endosomes contained low protein kinase activities. The results show that receptor phosphorylation is an 'early' event in endocytosis and the trafficking of ligands that is sustained especially in early endosomes in liver, and emphasizes the biochemical and thus functional distinctiveness of the plasma membrane and the endosomal and lysosomal compartments with regard to their population of phosphorylated proteins.

Early induction of Na(+)-dependent uridine uptake in the regenerating rat liver

Na(+)-dependent uridine transport into liver plasma membrane vesicles from partially hepatectomized and sham-operated rats was studied. Preparations purified 6 h after 70% hepatectomy exhibited an increased Vmax of uridine uptake (3.7 vs. 1.4 pmol/mg prot/3 s) without any change in Km (6 microM). Incubation of the vesicles in the presence of monensin decreased uridine uptake although the differences between both experimental groups remained identical. It is concluded that uridine transport is induced early after partial hepatectomy by a mechanism which does not involve changes in the transmembrane Na+ gradient. This is the first evidence in favor of modulation of nucleoside transport into liver cells.

Reorganization of the endocytic compartment in regenerating liver

Antibodies raised to two membrane proteins present in rat liver endosomal fractions were used to study changes occurring in the endocytic compartment of hepatocytes during liver regeneration. Antibodies to the 42-kDa subunit (RHL-1) of the asialoglycoprotein receptor showed, by Western blotting of liver microsomes and endosomes, that there was a reduced expression of the receptor in liver 24 h following a partial hepatectomy. Immunocytochemical staining of thin sections of regenerating livers using these antibodies indicated that there was an intracellular relocation of endocytic structures in hepatocytes. The two main endocytic regions immunocytochemically stained in normal liver--one located beneath the sinusoidal plasma membrane and the other abutting the bile canaliculus--were replaced, in regenerating liver, by staining more closely associated with a region underlying the baso-lateral plasma membrane. A 140-kDa pI 4.3 calmodulin-binding protein located in endocytic and plasma membranes was also demonstrated, using a radio-iodinated calmodulin-binding assay, to be present at reduced levels in endosomes isolated from regenerating livers. Antibodies to this calmodulin-binding protein stained the hepatocyte's cytoplasm in a punctate manner. However, in regenerating liver, the staining was located in regions underlying the baso-lateral and apical plasma membrane of hepatocytes. Together, the results demonstrate that a reorganization of the endocytic compartment has occurred in hepatocytes 24 h following hepatectomy, with two endosomal proteins becoming relocated to a region below the baso-lateral-apical surface regions of hepatocytes.