The water channel aquaporin-8 is mainly intracellular in rat hepatocytes, and its plasma membrane insertion is stimulated by cyclic AMP.

We previously found that water transport across hepatocyte plasma membranes occurs mainly via a non-channel mediated pathway. Recently, it has been reported that mRNA for the water channel, aquaporin-8 (AQP8), is present in hepatocytes. To further explore this issue, we studied protein expression, subcellular localization, and regulation of AQP8 in rat hepatocytes. By subcellular fractionation and immunoblot analysis, we detected an N-glycosylated band of approximately 34 kDa corresponding to AQP8 in hepatocyte plasma and intracellular microsomal membranes. Confocal immunofluorescence microscopy for AQP8 in cultured hepatocytes showed a predominant intracellular vesicular localization. Dibutyryl cAMP (Bt(2)cAMP) stimulated the redistribution of AQP8 to plasma membranes. Bt(2)cAMP also significantly increased hepatocyte membrane water permeability, an effect that was prevented by the water channel blocker dimethyl sulfoxide. The microtubule blocker colchicine but not its inactive analog lumicolchicine inhibited the Bt(2)cAMP effect on both AQP8 redistribution to cell surface and hepatocyte membrane water permeability. Our data suggest that in rat hepatocytes AQP8 is localized largely in intracellular vesicles and can be redistributed to plasma membranes via a microtubule-depending, cAMP-stimulated mechanism. These studies also suggest that aquaporins contribute to water transport in cAMP-stimulated hepatocytes, a process that could be relevant to regulated hepatocyte bile secretion.

Taurocholate-induced inhibition of hepatic lysosomal degradation of horseradish peroxidase.

Endocytosed proteins in hepatocytes are transported to lysosomes for degradation. Metabolites accumulating in these organelles are released into bile by exocytosis, a process that seems to be regulated by the bile salt taurocholate (TC). In this study we examined if TC is also involved in the control of the lysosomal degradation of endocytosed proteins. We used [(14)C]sucrose-labeled horseradish peroxidase ([(14)C]S-HRP), a probe suitable to evaluate lysosomal proteolysis. TC-infused rats as well as isolated rat hepatocytes exposed to TC showed a significant inhibition in the lysosomal degradation of [(14)C]S-HRP (approximately 30%), with no change in either the uptake or the amount of protein reaching lysosomes. Under these conditions, the in vitro assay of lysosomal cathepsins B, L, H, and D revealed no change in their activities, suggesting that a reversible inhibition (lysosomal alkalinization?) was taking place in hepatocytes. Nevertheless, lysosomal pH measured using fluorescein isothiocyanate-dextran was shown not to be altered by TC. In addition, TC was unable to inhibit proteolysis in [(14)C]S-HRP loaded lysosomes or interfere in cathepsin assays. The results suggest that TC inhibits the lysosomal degradation of endocytosed proteins in hepatocytes and that the mechanism does not involve an effect of the bile salt per se or a rise in lysosomal pH.

Taurolithocholate can inhibit the biliary discharge of lysosomes in the rat.

The natural bile salt taurolithocholate (TLC) impairs the biliary excretion of lipids and proteins, which are known to reach the canaliculus via vesicles. In this study we examined whether these observations could be extended to the exocytic discharge of lysosomal contents into bile. The single intravenous injection of a cholestatic dose of TLC, 3 micromol/100 g body wt., markedly inhibited the biliary excretion of the lysosomal enzymes acid phosphatase and beta-glucuronidase, despite the excretion of bile salts being normalized after a transient diminution. Under such a condition, TLC did not affect the normal transport to and the processing in lysosomes of the exogenously administered [14C]sucrose-labeled horseradish peroxidase. However, the biliary excretion of the radioactive lysosomal metabolites of the protein was significantly reduced. The results indicate that TLC can inhibit the biliary discharge of lysosomes in the rat without altering the functional integrity of these organelles. Possible explanations for these findings are discussed.

Assessment of the in vivo hepatic lysosomal processing of horseradish peroxidase.

The lysosomal processing of horseradish peroxidase (HRP) was assessed in this study, i.e., its lysosomal proteolysis and the biliary output of its possible lysosomal metabolites by rat liver in vivo. HRP was covalently linked to [14C]sucrose to provide a label that remains trapped within lysosomes after proteolysis. The [14C]sucrose-labelled HRP was injected into the portal vein of rat, and after 30 min about 34% of the injected radiolabel was present in the liver. Subcellular fractionation by differential centrifugation and further purification of lysosomes in a Percoll gradient showed that radiolabel was concentrated in lysosomes and indicated that about 91% of the total proteolysis of HRP in liver could be attributed to these organelles. The in vivo lysosomal degradation rate of HRP at 30 min was about 40%/h, decreasing over time. The lysosomal inhibitors chloroquine and leupeptin suppressed proteolysis of HRP by about 30 and 60%, respectively. Analysis of the 14C excreted in bile by trichloroacetic acid precipitation and by SDS-polyacrylamide gel electrophoresis showed a minor fraction, which was intact HRP (40 kDa), and a major fraction, which was associated with material smaller than 3 kDa. The biliary output of these low molecular mass products, in contrast to that of intact HRP, did not gradually decline with time and represented about 3% of the corresponding amounts in liver. Chloroquine and leupeptin specifically decreased their biliary excretion (about 60%), giving additional support to their lysosomal origin. In addition, the overall hepatic processing of [14C]sucrose-labelled HRP did not differ from that of the native HRP measured by enzyme assay, indicating no significant alteration caused by the labelling procedure.