Reorganization of the endocytic compartment in regenerating liver

Annexin A6 in the liver: From the endocytic compartment to cellular physiology

Annexin A6 (AnxA6) belongs to the conserved annexin family - a group of Ca²⁺-dependent membrane binding proteins. AnxA6 is the largest of all annexins and highly expressed in smooth muscle, hepatocytes, endothelial cells and cardiomyocytes. Upon activation, AnxA6 binds to negatively charged phospholipids in a wide range of intracellular localizations, in particular the plasma membrane, late endosomes/pre-lysosomes, but also synaptic vesicles and sarcolemma. In these cellular sites, AnxA6 is believed to contribute to the organization of membrane microdomains, such as cholesterol-rich lipid rafts and confer multiple regulatory functions, ranging from vesicle fusion, endocytosis and exocytosis to programmed cell death and muscle contraction. Growing evidence supports that Ca²⁺ and Ca²⁺-binding proteins control endocytosis and autophagy. Their regulatory role seems to operate at the level of the signalling pathways that initiate autophagy or at later stages, when autophagosomes fuse with endolysosomal compartments. The convergence of the autophagic and endocytic vesicles to lysosomes shares several features that depend on Ca²⁺ originating from lysosomes/late endosomes and seems to depend on proteins that are subsequently activated by this cation. However, the involvement of Ca²⁺ and its effector proteins in these autophagic and endocytic stages still remains poorly understood. Although AnxA6 makes up almost 0.25% of total protein in the liver, little is known about its function in hepatocytes. Within the endocytic route, we identified AnxA6 in endosomes and autophagosomes of hepatocytes. Hence, AnxA6 and possibly other annexins might represent new Ca²⁺ effectors that regulate converging steps of autophagy and endocytic trafficking in hepatocytes. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

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.

Plasma-kallikrein clearance during liver regeneration after partial hepatectomy in the rat

AIMS/BACKGROUND

The liver clears circulating plasma-kallikrein through a receptor-mediated endocytosis process: an initial fast phase is followed by a slow exponential phase.

METHODS

To determine whether the clearance rate of plasma-kallikrein is affected during liver regeneration, we perfused isolated rat livers with rat plasma-kallikrein (rPK) at 0, 1, 2, 3 and 7 days after partial hepatectomy or sham operation.

RESULTS

Liver regeneration was followed by the expression of the proliferating-cell nuclear antigen (PCNA) labeling index. The serum concentration of alpha2-macroglobulin, an acute phase protein in rats, was measured. At day 1, the fast phase of rPK clearance rate increased in hepatectomized rats when compared with day 0 (4.9+/-0.4 and 3.7+/-0.4 mU/g liver min, p<0.05). However, at day 2, the rPK fast phase clearance rate dropped significantly (2.6+/-0.2, p<0.05), when compared with day 1. No difference was found among the sham groups at different days of hepatectomy. These changes seem to be independent of the acute phase reaction. The regenerative liver weight increased continuously during the observation period. PCNA expression increased significantly after hepatectomy, with maximal PCNA-labeling indices at days 1 and 2, declining thereafter.

CONCLUSION

The rPK fast phase clearance rate changes during liver regeneration, with a zenith occurring when PCNA labeling index is maximal (day 1) and a nadir occurring at the mitotic phase (day 2).

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.