Taurocholate stimulates transcytotic vesicular pathways labeled by horseradish peroxidase in the isolated perfused rat liver

Bile formation and secretion

Bile is a unique and vital aqueous secretion of the liver that is formed by the hepatocyte and modified down stream by absorptive and secretory properties of the bile duct epithelium. Approximately 5% of bile consists of organic and inorganic solutes of considerable complexity. The bile-secretory unit consists of a canalicular network which is formed by the apical membrane of adjacent hepatocytes and sealed by tight junctions. The bile canaliculi (∼1 μm in diameter) conduct the flow of bile countercurrent to the direction of portal blood flow and connect with the canal of Hering and bile ducts which progressively increase in diameter and complexity prior to the entry of bile into the gallbladder, common bile duct, and intestine. Canalicular bile secretion is determined by both bile salt-dependent and independent transport systems which are localized at the apical membrane of the hepatocyte and largely consist of a series of adenosine triphosphate-binding cassette transport proteins that function as export pumps for bile salts and other organic solutes. These transporters create osmotic gradients within the bile canalicular lumen that provide the driving force for movement of fluid into the lumen via aquaporins. Species vary with respect to the relative amounts of bile salt-dependent and independent canalicular flow and cholangiocyte secretion which is highly regulated by hormones, second messengers, and signal transduction pathways. Most determinants of bile secretion are now characterized at the molecular level in animal models and in man. Genetic mutations serve to illuminate many of their functions.

Receptor-mediated and fluid-phase transcytosis of horseradish peroxidase across rat hepatocytes

Horseradish peroxidase (HRP) is often used as a fluid-phase marker to characterize endocytic and transcytotic processes. Likewise, it has been applied to investigate the mechanisms of biliary secretion of fluid in rat liver hepatocytes. However, HRP contains mannose residues and thus binds to mannose receptors (MRs) on liver cells, including hepatocytes. To study the role of MR-mediated endocytosis of HRP transport in hepatocytes, we determined the influence of the oligosaccharid mannan on HRP biliary secretion in the isolated perfused rat liver. A 1-minute pulse of HRP was applied followed by marker-free perfusion. HRP appeared in bile with biphasic kinetics: a first peak at 7 minutes and a second peak at 15 minutes after labeling. Perfusion with 0.8 mg/mL HRP in the presence of a twofold excess of mannan reduced the first peak by 41% without effect on the second one. Together with recently published data on MR expression in rat hepatocytes this demonstrates two different mechanisms for HRP transcytosis: a rapid, receptor-mediated transport and a slower fluid-phase transport.

Biliary secretion of fluid phase markers is modified under post-cholestatic conditions

Hepatocytes take up macromolecules from the circulation by receptor-mediated and/or fluid-phase endocytosis. These molecules are either selectively or nonspecifically transported through the cell (transcytosis) and are subsequently secreted into bile. As transcytosis of diverse fluid-phase markers (FPM) is still poorly characterized, biliary secretion of two FPMs (horseradish peroxidase (HRP), FITC-Dextran) was studied in the isolated perfused rat liver following short-term (1 min) single-pulse administration. HRP was secreted into bile with a fast (5 min) and slow (15 min) transit time, while FITC-dextran appeared in bile in a single peak at 7 min. Short-time reversible cholestasis, induced by bile duct ligation (BDL), had been shown to affect HRP secretion. Here, we compare the influence of 2 h BDL on post-cholestatic biliary secretion of HRP and FITC-dextran. BDL drastically stimulated the fast component of HRP secretion into bile, but had an effect neither on the second HRP peak nor on the appearance of FITC-dextran in bile. Perfusion at low temperature (16 degrees C) under control and post-cholestatic conditions suppressed both, the second HRP peak and the appearance of FITC-dextran in bile, but uptake of FPM by endocytosis was not inhibited as the markers were secreted upon re-warming to 37 degrees C. In addition, perfusion at low temperature under control and post-cholestatic conditions delayed the appearance of the fast HRP peak in bile and it abrogated the stimulating effect of BDL on the first HRP peak. These data indicate that BDL boosts HRP secretion along a temperature-sensitive transcellular pathway and/or a paracellular route. This fast route is taken only by HRP but not by FITC-dextran, the latter being exclusively transported by a transcellular route under all conditions investigated.

Differential effects of hydroxyacetophenone analogues on the transcytotic vesicular pathway in rat liver

Insertion of transporter proteins into the apical canalicular membrane via vesicular transport is one of several choleretic mechanisms. Based on different choleretic activities of hydroxyacetophenone analogues including 4-mono; 2,6-di and 2,4,6-trihydroxy-acetophenone (MHA, DHA and THA), the present study aims to determine if these compounds stimulated vesicular transport in hepatocytes. Hydroxyacetophenone was continuously infused into the duodenum of the bile fistula rat. Bile flow rate was allowed to stabilize and then followed by an intraportal injection of horseradish peroxidase, a marker of the transcytotic vesicle pathway. MHA which stimulates bile acid independent flow, showed a dose-dependent increase in both the early (paracellular) and late (transcellular) peak of horseradish peroxidase excretion in bile. THA, which stimulates both bile acid dependent flow and bile acid independent flow, did not alter the pattern of horseradish peroxidase excretion into bile. However, DHA, which is more hydrophobic and increases only bile acid dependent flow, decreased the late peak. The stimulating effects of MHA on bile flow and horseradish peroxidase excretion were markedly inhibited by colchicine, suggesting that its choleretic action involves stimulation of exocytosis, as well as increase in paracellular permeability. In contrast, the lack of a stimulatory effect of THA and DHA on biliary horseradish peroxidase excretion suggested that their choleretic action is not associated with vesicular exocytosis. These results demonstrate a variable effect of hydroxyacetophenones on the transcytotic vesicular pathway reflecting different choleretic mechanisms and therapeutic potential.

Bile salt transporters: molecular characterization, function, and regulation

Molecular medicine has led to rapid advances in the characterization of hepatobiliary transport systems that determine the uptake and excretion of bile salts and other biliary constituents in the liver and extrahepatic tissues. The bile salt pool undergoes an enterohepatic circulation that is regulated by distinct bile salt transport proteins, including the canalicular bile salt export pump BSEP (ABCB11), the ileal Na(+)-dependent bile salt transporter ISBT (SLC10A2), and the hepatic sinusoidal Na(+)- taurocholate cotransporting polypeptide NTCP (SLC10A1). Other bile salt transporters include the organic anion transporting polypeptides OATPs (SLC21A) and the multidrug resistance-associated proteins 2 and 3 MRP2,3 (ABCC2,3). Bile salt transporters are also present in cholangiocytes, the renal proximal tubule, and the placenta. Expression of these transport proteins is regulated by both transcriptional and posttranscriptional events, with the former involving nuclear hormone receptors where bile salts function as specific ligands. During bile secretory failure (cholestasis), bile salt transport proteins undergo adaptive responses that serve to protect the liver from bile salt retention and which facilitate extrahepatic routes of bile salt excretion. This review is a comprehensive summary of current knowledge of the molecular characterization, function, and regulation of bile salt transporters in normal physiology and in cholestatic liver disease and liver regeneration.

Bile acid regulation of hepatic physiology: I. Hepatocyte transport of bile acids

Bile acids are cholesterol derivatives that serve as detergents in bile and the small intestine. Approximately 95% of bile acids secreted by hepatocytes into bile are absorbed from the distal ileum into the portal venous system. Extraction from the portal circulation by the hepatocyte followed by reexcretion into the bile canaliculus completes the enterohepatic circulation of these compounds. Over the past few years, candidate bile acid transport proteins of the sinusoidal and canalicular plasma membranes of the hepatocyte have been identified. The physiology of hepatocyte bile acid transport and its relationship to these transport proteins is the subject of this Themes article.

Protein kinase C-dependent inhibition of the lysosomal degradation of endocytosed proteins in rat hepatocytes

We studied the role of protein kinase C (PKC) in the lysosomal processing of endocytosed proteins in isolated rat hepatocytes. We used [14C]sucrose-labeled horseradish peroxidase ([14C]S-HRP) to simultaneously evaluate endocytosis and lysosomal proteolysis. The PKC activator phorbol 12-myristate 13-acetate (PMA) inhibited the lysosomal degradation of [14C]S-HRP (1 microM PMA: 40% inhibition, P<.05), without affecting either the endocytic uptake or the delivery to lysosomes. However, PMA was not able to affect the lysosomal processing of the beta-galactosidase substrate dextran galactosyl umbelliferone. The PKC inhibitors, chelerytrine (Che), staurosporine (St) and Gö 6976, prevented PMA inhibitory effect on lysosomal proteolysis. Nevertheless, purified PKC failed to alter proteolysis in [14C]S-HRP-loaded isolated lysosomes, suggesting that intracellular intermediates are required. PMA induced phosphorylation and hepatocyte membrane-to-lysosome redistribution of the myristoylated alanine-rich C kinase substrate (MARCKS) protein, raising the possibility that MARCKS mediates the PKC-induced inhibition of lysosomal proteolysis.

Control by signaling modulators of the sorting of canalicular transporters in rat hepatocyte couplets: role of the cytoskeleton

Hormonal control of the restoration of hepatocanalicular polarity in short-term cultured hepatocyte couplets was analyzed. One hour following isolation, couplets were unable to accumulate the fluorescent bile acid analogue, cholyl-lysyl-fluorescein (CLF), and showed a nonpolarized distribution of F-actin and mrp2 over the cell body. A progressive, time-dependent restoration of couplet-polarized function and morphology was reached after 4 hours of culture. Both dibutyryl cyclic adenosine monophosphate (DBcAMP) and the Ca(2+)-elevating compound, thapsigargin, accelerated restoration of normal couplet morphology and function. The DBcAMP-mediated stimulus was inhibited by the Ca(2+) chelator, 1, 2-bis-(o-aminophenoxy)-ethene-N,N,N',N'-tetra-acetate tetra-(acetomethyl)ester (BAPTA/AM), but not by the protein kinase A (PKA) inhibitors, KT5720 or H89, suggesting that Ca(2+) elevation rather than PKA activation is involved. N-(6-aminohexyl-5-chloro-1-napththalenesulfonamide (W-7), a calmodulin inhibitor, and the protein kinase C (PKC) activator, phorbol dibutyrate, inhibited both the basal and the DBcAMP-stimulated recovery of functional polarity, whereas staurosporine and Gö 6976, 2 PKC inhibitors, accelerated the basal recovery of polarized function. Disruption of the microtubule cytoskeleton by colchicine induced only minor changes under basal, but not under DBcAMP-stimulated, conditions. The Golgi complex disruptor, brefeldin A, significantly delayed, and the microfilament-disrupting agent, cytochalasin D, fully blocked, both processes. However, DBcAMP stimulated trafficking of vesicles containing CLF to the pericanalicular region under the last condition. Our results indicate that restoration of couplet polarity following isolation occurs via a Ca(2+)-calmodulin-mediated mechanism, which depends on microfilament, but not on microtubule integrity. A second pathway is activated by DBcAMP activation via Ca(2+)-calmodulin formation, whose requirements with respect to cytoskeletal components are opposite. PKC has a negative regulatory role in both pathways.

Rab3D, a small GTP-binding protein implicated in regulated secretion, is associated with the transcytotic pathway in rat hepatocytes

Rab3 isotypes are expressed in regulated secretory cells. Here, we report that rab3D is also expressed in rat hepatocytes, classic models for constitutive secretion. Using reverse transcriptase polymerase chain reaction (RT-PCR) with primers specific for rat rab3D, we amplified a 151 base pair rab3D fragment from total RNA extracted from primary cultures of rat hepatocytes. Immunoblot analysis using polyclonal antibodies to peptides representing the N- and C-terminal hypervariable regions of murine rab3D recognized a protein of approximately 25 kd in hepatocyte lysates, hepatic subcellular fractions, and tissue extracts. The distribution of rab3D was primarily cytosolic; however, only membrane-associated rab3D significantly bound guanosine triphosphate (GTP) in overlay assays. Several lines of investigation indicate that rab3D is associated with the transcytotic pathway. First, rab3D was enriched in a crude vesicle carrier fraction (CVCF), which includes transcytotic carriers. Vesicular compartments immunoisolated from the CVCF on magnetic beads coated with anti-rab3D antibody were enriched in the transcytosed form of the polymeric IgA receptor (pIgA-R), but lacked not only the pIgA-R precursor form associated with the secretory pathway, but also a Golgi marker protein. Second, indirect immunofluorescence on frozen liver sections and in polarized cultured hepatocytes localized rab3D-positive sites at or near the apical plasma membrane and to the pericanalicular cytoplasm. Finally, cholestasis induced by bile duct ligation (BDL), a manipulation known to slow transcytosis, caused rab3D to accumulate in the pericanalicular cytoplasm of cholestatic hepatocytes. Our results indicate that rab3D plays a role in the regulation of apically directed transcytosis in rat hepatocytes.

Role of glutathione and oxidative stress in phalloidin-induced cholestasis

BACKGROUND/AIMS

Biliary glutathione is an important generator of the bile-salt independent flow, and is known to be regulated by the hepatic glutathione availability. We investigated, in an acute model of phalloidin-induced cholestasis, biliary glutathione secretion and the role of hepatic glutathione, oxidative stress, and protein kinase c activation, which have been implicated in many hepatotoxin-induced hepatic dysfunctions.

METHODS

Rats were given a single dose of 80 microg/100 g body weight of phalloidin and the hepatic thiols and glutathione content, redox state and vesicular activity were evaluated during both development of and recovery from cholestasis. The prophylactic effect of N-acetylcysteine (a precursor of glutathione synthesis and an antioxidant) was also examined. In addition, in the isolated perfused rat liver, we studied the prophylactic effect of the PKc inhibitor H7 on phalloidin-induced cholestasis.

RESULTS

In the early stages of cholestasis, phalloidin induced a decline in bile flow, mainly related to a disruption of biliary glutathione secretion. The decline in biliary glutathione content was not associated with increased glutathione degradation, indicated by a parallel decline in biliary non-protein thiols and by the lack of an increase in biliary gamma-glutamyltranspeptidase. There was also no evidence of hepatic depletion of glutathione or of oxidative stress, as measured by the oxidized-to-reduced glutathione ratio. Moreover, phalloidin resulted in inhibition of vascular transcytosis as assessed by horseradish peroxidase labeling. Pre-treatment with N-acetylcysteine did not counteract the decline in biliary glutathione secretion and bile flow produced by phalloidin, supporting the view that the hepatic availability of glutathione and oxidative stress injury are not implicated in the early stages of cholestatic injury. Moreover, treatment with H-7 did not alter the biliary glutathione output, or the decline in bile flow induced by the toxin.

CONCLUSIONS

This study suggests that the phalloidin-induced inhibition of bile formation may be attributed to rapid disruption of the hepatocanalicular transport of glutathione.

Canalicular export pumps traffic with polymeric immunoglobulin A receptor on the same microtubule-associated vesicle in rat liver

Basolateral to apical vesicular transcytosis in the hepatocyte is an essential pathway for the delivery of compounds from the sinusoidal blood to the bile and to traffic newly synthesized resident apical membrane proteins to their site of function at the canalicular membrane front. To characterize this pathway better, microtubules in a hepatocyte homogenate were polymerized by addition of taxol, and associated membrane-bound vesicles were isolated. This fraction was enriched in polymeric immunoglobulin A receptor and contained apical membrane proteins. Immunoelectron microscopy demonstrated that polymeric immunoglobulin A receptor was localized predominantly on vesicles ranging from 100 to 160 nm and that the multidrug resistance protein 2 and the bile salt export pump co-localized on these vesicles. The minus-ended microtubule motor, dynein, was highly enriched in the fraction, and its intermediate chain could be released effectively by incubation with 1 mM ATP or GTP. However, the association of the transcytotic vesicles with the microtubules was not sensitive to hydrolyzable or non-hydrolyzable nucleotides. This study characterizes a fraction of microtubule-associated vesicles from rat hepatocytes and demonstrates that several resident apical membrane transport proteins and the polymeric immunoglobulin A receptor traffic on the same vesicle.

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.

Carbon monoxide-mediated alterations in paracellular permeability and vesicular transport in acetaminophen-treated perfused rat liver

This study aimed to examine whether acetaminophen (AAP), an anti-inflammatory agent producing hepatocellular damages with its overdose, evokes hepatocellular dysfunction through mechanisms involving carbon monoxide (CO) generated by heme oxygenase (HO). In perfused rat livers, CO and bilirubin were determined in venous perfusate and bile samples as indices of heme degradation. Biliary excretion of transportally injected horseradish peroxidase was also determined to assess paracellular junctional permeability and vesicular transport across hepatocytes. AAP at 20 mmol/L induced a transient choleresis, followed by a reduction of bile output. Under these circumstances, the release of CO and bilirubin IXalpha, terminal products of the HO-mediated heme degradation, became 2. 5-fold greater than the control. The rate of CO production appeared stoichiometric to the degradation rate of microsomal cytochrome P-450. Mechanisms for the AAP-induced cholestasis involved an increase in the junctional permeability that coincided with a reduction of vesicular transport across hepatocytes. Clotrimazole, a cytochrome P-450 inhibitor, or zinc protoporphyrin IX, an HO inhibitor, but not copper protoporphyrin IX, which did not inhibit HO, attenuated these AAP-induced changes. Furthermore, administration of CO at concentrations comparable with those induced by AAP elicited a marked elevation of the paracellular junctional permeability concurrent with a reduction of transcellular vesicular transport, mimicking effects of the AAP administration. Thus, CO serves as a putative regulator of hepatocellular function that is overproduced through acute heme degradation during xenobiotic transformation.

Hepatic sequestration and modulation of the canalicular transport of the organic cation, daunorubicin, in the Rat

In contrast to organic anions, substrates for the canalicular mdr1a and b are usually organic cations and are often sequestered in high concentrations in intracellular acidic compartments. Because many of these compounds are therapeutic agents, we investigated if their sequestration could be regulated. We used isolated perfused rat liver (IPRL), isolated rat hepatocyte couplets (IRHC), and WIF-B cells to study the cellular localization and biliary excretion of the fluorescent cation, daunorubicin (DNR). Despite rapid (within 15 minutes) and efficient (>90%) cellular uptake in the IPRL, only approximately 10% of the dose administered (0.2-20 micromol) was excreted in bile after 85 minutes. Confocal microscopy revealed fluorescence predominantly in vesicles in the pericanalicular region in IPRL, IRHC, and WIF-B cells. Treatment of these cells with chloroquine and bafilomycin A, agents that disrupt the pH gradient across the vesicular membrane, resulted in a loss of vesicular fluorescence, reversible in the case of bafilomycin A. Taurocholate (TC) and dibutyryl cAMP (DBcAMP), stimulators of transcytotic vesicular transport, increased the biliary recovery of DNR significantly above controls, by 70% and 35%, respectively. The microtubule destabilizer, nocodazole, decreased biliary excretion of DNR. No effect on secretion was noted in TR- mutant rats deficient in mrp2. Coadministration of verapamil, an inhibitor of mdr1, also decreased DNR excretion. While TC and DBcAMP did not affect the fluorescent intensity or pattern of distribution in IRHC, nocodazole resulted in redistribution of DNR to peripheral punctuate structures. These findings suggest that the organic cation, DNR, is largely sequestered in cells such as hepatocytes, yet its excretion can still be modulated.

Enhanced bile formation induced by experimental dicrocoeliosis in the hamster

The purpose of this investigation was to determine the effects of experimental dicrocoeliosis on bile formation in the hamster. Studies were carried out at 120 days after infection with an oral dose of 40 metacercariae of Dicrocoelium dendriticum. A significant elevation in bile flow (+20%) and in the biliary output of glutathione (+34%), bile acid (+59%), cholesterol (+108%), phospholipids (+99%) and alkaline phosphatase (+36%) was observed in the infected animals. The bile-to-plasma [14C] mannitol ratio increased to values greater than 1 and there was a reduced contribution (-26%) of biliary tree to bile formation. Those data suggest that enhancement in choleresis had a canalicular origin. The presence of oxidative stress, evidenced by the increased oxidized/reduced glutathione ratio and TBARS concentrations, may contribute to the elevated glutathione efflux into bile. Enhancement in bile acid output was not due to qualitative or quantitative changes in bile acid metabolism, as indicated by the absence of significant modification in liver cholesterol 7alpha-hydroxylase activity and bile acid profile in bile. Increase in the ability of the canalicular membrane to export bile acids was not involved, since maximal secretion rate for exogenously administered taurocholate was decreased. When bile flow, bile acid and biliary lipid secretion was determined in colchicine-pretreated animals differences between control and infected animals were abolished, suggesting that stimulation of the transcytotic vesicle pathway plays an important role in the alteration of the biliary function caused by dicrocoeliosis.

Mechanisms and functional features of polarized membrane traffic in epithelial and hepatic cells

Epithelial cells express plasma-membrane polarity in order to meet functional requirements that are imposed by their interaction with different extracellular environments. Thus apical and basolateral membrane domains are distinguished that are separated by tight junctions in order to maintain the specific lipid and protein composition of each domain. In hepatic cells, the plasma membrane is also polarized, containing a sinusoidal (basolateral) and a bile canalicular (apical)-membrane domain. Relevant to the biogenesis of these domains are issues concerning sorting, (co-)transport and regulation of transport of domain-specific membrane components. In epithelial cells, specific proteins and lipids, destined for the apical membrane, are sorted in the trans-Golgi network (TGN), which involves their sequestration into cholesterol/sphingolipid 'rafts', followed by 'direct' transport to the apical membrane. In hepatic cells, a direct apical transport pathway also exists, as revealed by transport of sphingolipids from TGN to the apical membrane. This is remarkable, since in these cells numerous apical membrane proteins are 'indirectly' sorted, i.e. they are first transferred to the basolateral membrane prior to their subsequent transcytosis to the apical membrane. This raises intriguing questions as to the existence of specific lipid rafts in hepatocytes. As demonstrated in studies with HepG2 cells, it has become evident that, in hepatic cells, apical transport pathways can be regulated by protein kinase activity, which in turn modulates cell polarity. Finally, an important physiological function of hepatic cells is their involvement in intracellular transport and secretion of bile-specific lipids. Mechanisms of these transport processes, including the role of multidrug-resistant proteins in lipid translocation, will be discussed in the context of intracellular vesicular transport. Taken together, hepatic cell systems provide an important asset to studies aimed at elucidating mechanisms of sorting and trafficking of lipids (and proteins) in polarized cells in general.

Microtubule-dependent vesicle transport: modulation of channel and transporter activity in liver and kidney

Microtubule-based vesicle transport driven by kinesin and cytoplasmic dynein motor proteins facilitates several membrane-trafficking steps including elements of endocytosis and exocytosis in many different cell types. Most early studies on the role of microtubule-dependent vesicle transport in membrane trafficking focused either on neurons or on simple cell lines. More recently, other work has considered the role of microtubule-based vesicle transport in other physiological systems, including kidney and liver. Investigation of the role of microtubule-based vesicle transport in membrane trafficking in cells of the kidney and liver suggests a major role for microtubule-based vesicle transport in the rapid and directed movement of ion channels and transporters to and from the apical plasma membranes, events essential for kidney and liver function and homeostasis. This review discusses the evidence supporting a role for microtubule-based vesicle transport and the motor proteins, kinesin and cytoplasmic dynein, in different aspects of membrane trafficking in cells of the kidney and liver, with emphasis on those functions such as maintenance of ion channel and transporter composition in apical membranes that are specialized functions of these organs. Evidence that defects in microtubule-based transport contribute to diseases of the kidney and liver is also discussed.

Apical plasma membrane proteins and endolyn-78 travel through a subapical compartment in polarized WIF-B hepatocytes

We studied basolateral-to-apical transcytosis of three classes of apical plasma membrane (PM) proteins in polarized hepatic WIF-B cells and then compared it to the endocytic trafficking of basolaterally recycling membrane proteins. We used antibodies to label the basolateral cohort of proteins at the surface of living cells and then followed their trafficking at 37 degreesC by indirect immunofluorescence. The apical PM proteins aminopeptidase N, 5'nucleotidase, and the polymeric IgA receptor were efficiently transcytosed. Delivery to the apical PM was confirmed by microinjection of secondary antibodies into the bile canalicular-like space and by EM studies. Before acquiring their apical steady-state distribution, the trafficked antibodies accumulated in a subapical compartment, which had a unique tubulovesicular appearance by EM. In contrast, antibodies to the receptors for asialoglycoproteins and mannose-6-phosphate or to the lysosomal membrane protein, lgp120, distributed to endosomes or lysosomes, respectively, without accumulating in the subapical area. However, the route taken by the endosomal/lysosomal protein endolyn-78 partially resembled the transcytotic pathway, since anti-endolyn-78 antibodies were found in a subapical compartment before delivery to lysosomes. Our results suggest that in WIF-B cells, transcytotic molecules pass through a subapical compartment that functions as a second sorting site for a subset of basolaterally endocytosed membrane proteins reaching this compartment.

Effect of bile acids on hepatobiliary transport of cisplatin by perfused rat liver

The liver and kidney collaborate in the excretion of the cytostatic drug, cis-diamminedichloroplatinum(II) (cisplatin) from the body. Enhancement of this process is envisaged as a way of reducing cisplatin toxicity, thus allowing increases in the doses administered. In this sense, using different compounds, several attempts have been made to enhance cisplatin biliary excretion. In this study, the ability of endogenous compounds belonging to the bile acid family to improve cisplatin excretion by the isolated perfused rat liver was investigated. A highly choleretic bile acid (ursodexoycholic acid) and two others bile acids with marked micelle-forming properties (glycocholic acid and chenodeoxycholic acid) were chosen for study. When these drugs were given at concentrations (1 microM) that did not affect the viability of liver preparations, a correlation between the biliary excretion of platinum and bile acid output was found. This was not due to the incorporation of cisplatin into mixed micelles because no correlation between the biliary output of lecithin or cholesterol and platinum was observed. Moreover, a wash-out effect of bile acids was probably not the cause of bile acid-induced platinum output into bile because no correlation between this and bile flow was found. An enhancement in cisplatin transport processes by the hepatocyte or by direct binding of cisplatin to bile acid monomers or aggregates cannot be ruled out. In spite of the biliary induction of cisplatin output, the net excretion of platinum was reduced under bile acid administration. This was related to lower platinum contents in the liver tissue, probably due to an inhibition of the ability of the hepatocyte to take up and/or retain cisplatin while subject to bile acid infusion. In summary, our results indicate that bile acids reduce the net excretion of cisplatin by the liver even though they induce an enhancement in the transport of this compound from the hepatocyte into bile.

Adaptive regulation of hepatic bile salt transport: role of bile salt hydrophobicity and microtubule-dependent vesicular pathway

BACKGROUND/AIMS

The hepatic transport of bile salts can be regulated by changes in bile salt pool size and/or in the flux of bile salts through the liver. Prolonged bile salt pool depletion is associated with down-regulation of maximum taurocholate transport and decreased canalicular membrane specific bile salt binding sites. This study was undertaken to investigate: a) whether adaptive down-regulation of maximum hepatic bile salt transport occurs to the same extent for bile acids of different hydrophobicity; and b) the role of microtubule-dependent vesicular pathway in the adaptive changes of bile salt transport capacity.

METHODS

Male rats were subjected to 24-h or 48-h external biliary diversion to induce bile salt pool depletion. Basal bile flow, bile salt secretion and lipid secretion, maximum secretory rate of three bile salts of different hydrophobicity (tauroursodeoxycholate, taurocholate and taurochenodeoxycholate) and changes in the biliary excretion of two markers of the microtubule-dependent vesicular pathway (horseradish peroxidase and polyethyleneglycol molecular weight-900) were measured in control and bile salt-depleted rats. Taurocholate-stimulated horseradish peroxidase biliary excretion was also assessed in order to define whether the restoration of bile salt flux across the hepatocytes increased the excretion of this marker in bile salt-depleted rats.

RESULTS

The reduction in the maximum secretory rate of the three bile salts under study observed after prolonged biliary diversion was clearly related to their hydrophobicity, with greater reduction for taurochenodeoxycholate and smaller reduction for tauroursodeoxycholate, compared with taurocholate. The biliary excretion of vesicular transport markers was significantly reduced in bile salt-depleted rats. However, when stimulated by taurocholate, biliary excretion of horseradish peroxidase was similar to controls.

CONCLUSIONS

The magnitude of the decrease of the hepatic bile salt maximum transport capacity seen after bile salt pool depletion is directly related to the hydrophobicity of the bile salt infused. A functionally depressed vesicular transport pathway appears to be also a contributing factor to this phenomenon.

Protective effect of the intravenous administration of ursodeoxycholic acid against endotoxemia in rats with obstructive jaundice

This study was undertaken to elucidate the effect of the intravenous administration of ursodeoxycholic acid (UDCA) on endotoxemia in rats with obstructive jaundice from the viewpoint of the biliary excretion of lipopolysaccharide (LPS) through hepatocytes. In rats with obstructive jaundice, fluorescein isothiocyanate-labeled LPS was administered via the portal vein and then its biliary excretion was examined. A significant increase in the LPS excretion was thus noticed in UDCA-treated rats at a dose of 0.1 micromol/100 g body wt. per min. In place of UDCA, sodium taurocholate at the same dose also enhanced the biliary excretion of LPS. Secondly, we also examined whether or not UDCA protects against endotoxemia. In this experiment, nonlabeled LPS was administered via the portal vein and its peripheral concentration was then measured. In UDCA-treated rats, the endotoxin concentration was significantly lower. Finally, to elucidate the effect of UDCA on Kupffer cells, serum tumor necrosis factor (TNF-alpha) was measured. But UDCA had no effect on the TNF-alpha level. These findings thus demonstrate that the intravenous administration of UDCA protects against endotoxemia by enhancing the transport of LPS across the hepatocytes from blood to bile without affecting Kupffer cells, and that this biliary excretion of LPS is dependent on bile acids.

Vasopressin reduces taurochenodeoxycholate-induced hepatotoxicity by lowering the hepatocyte taurochenodeoxycholate content

BACKGROUND/AIMS

Vasopressin has been reported to reduce bile flow, but its effects on bile acid secretion and bile acid-related hepatotoxicity are still unclear. We therefore investigated the influence of vasopressin on the hepatotoxicity and biliary excretion of taurochenodeoxycholic acid in primary cultured rat hepatocytes and isolated perfused rat liver models.

METHODS/RESULTS

1) Addition of vasopressin to hepatocyte cultures significantly decreased lactate dehydrogenase release as compared to cultures exposed to 1 mM taurochenodeoxycholic acid alone, and also reduced intracellular taurochenodeoxycholic acid content from 19.3 +/- 2.2 to 13.0 +/- 1.6 nmol/mg protein. After 30 min of preincubation with 1 mM taurochenodeoxycholic acid, rinsing and reculture of hepatocytes in bile acid-free medium resulted in gradual decrease in the intracellular level of the bile acid, and addition of vasopressin (10(-9) M) to the reculture medium accelerated this process. 2) Superimposition of vasopressin (330 pmol/l) for 10 min on taurochenodeoxycholic acid infusion (1.0 mumol/min: 25 mumol/l) caused a rapid increase in bile flow and biliary excretion of taurochenodeoxycholic acid (697 +/- 42 vs 584 +/- 27 nmol/10 min per g liver) from perfused rat livers, and significantly reduced lactate dehydrogenase release. 3) Superimposition of the PKC blocker H-7 (5 mumol/l) on taurochenodeoxycholic acid infusion (1.0 mumol/min: 25 mumol/l) caused a gradual increase in bile flow and biliary excretion of taurochenodeoxycholic acid. Furthermore, an additional infusion of vasopressin (100 pmol/l) for 10 min in the presence of H-7 produced a greater increase in bile flow and biliary excretion of taurochenodeoxycholic acid as compared with H-7 alone (754 +/- 71 vs. 657 +/- 26 nmol/g liver). 4) Continuous infusion of vasopressin (330 pmol/l) significantly increased the late peak (10-50 min) of horseradish peroxidase excretion from perfused livers (from 8.48 +/- 1.02 to 21.7 +/- 6.02 ng/g liver).

CONCLUSIONS

These findings suggest that vasopressin exerts a protective effect against taurochenodeoxycholic acid-induced hepatotoxicity by stimulating the secretion of this bile acid via intracellular vesicular transport systems.

Enhancing effects of vasoconstrictors on bile flow and bile acid excretion in the isolated perfused rat liver

The effects of vasoconstrictors on bile flow and bile acid excretion were examined in single-pass isolated perfused rat livers. Administration of norepinephrine (NE), 4 nmol/min, plus continuous infusion of taurocholate (TC) (1.0 mumol/min) rapidly increased bile flow in 1 min, and from min 5 until the end of NE administration (late period) bile flow remained above the basal level (111.7 +/- 2.2%), as did bile acid output (114.6 +/- 1.8%). Without TC infusion, administration of NE produced no increase in the late period. Administration of NE plus taurochenodeoxycholate (1.0 mumol/min) increased bile flow and bile acid output in the late period to 121.9 +/- 7.0 and 137.1 +/- 6.8%, respectively. With NE plus taurodehydrocholate, the respective values were only 105.4 +/- 1.6 and 104.1 +/- 4.0%. When horseradish peroxidase (HRP) (25 mg) was infused over 1 min with continuous NE, the late peak (20-25 min) of HRP elimination into bile significantly exceeded that of untreated controls (P < 0.01). These observations suggest that vasoconstrictors enhance biliary excretion of more hydrophobic bile acids, in part by stimulating vesicular transport.

Mechanism of bile salt-induced mucin secretion by cultured dog gallbladder epithelial cells

1. Hypersecretion of gallbladder mucin has been proposed to be a pathogenic factor in cholesterol gallstone formation. Using cultured gallbladder epithelial cells, we demonstrated that bile salts regulate mucin secretion by the gallbladder epithelium. In the present study we have investigated whether established second messenger pathways are involved in bile salt-induced mucin secretion. 2. The effect of activators and inhibitors on mucin secretion was studied by measuring the secretion of [3H]N-acetyl-D-glucosamine-labelled glycoproteins. Intracellular cAMP content of the cells was measured using a radioimmunoassay. 3. Incubation of the cells with 10 mM taurocholate did not increase the intracellular cAMP content (25.7 versus control 22.8 pmol of cAMP/mg of protein). No stimulation of mucin secretion was observed after incubation with 1-100 microM concentrations of the calcium ionophores ionomycin and A23187. The stimulatory effect of 10 mM tauroursodeoxycholate (TUDC) on mucin secretion could not be inhibited by the addition of EDTA. Activation of protein kinase C (PKC) by 1 microgram/ml phorbol 12-myristate 13-acetate (PMA) caused an increase in mucin secretion (342% versus control 100%), comparable with the effect of 40 mM TUDC. The effect of 10 ng/ml PMA could partially be inhibited by a concentration of 2 microM of the PKC inhibitor staurosporin. Staurosporin had no inhibitory effect on mucin secretion induced by TUDC. 4. In gallbladder epithelial cells bile salts do not stimulate mucin secretion via one of the classical signal transduction pathways. We hypothesize that bile salts act on mucin secretion via a direct interaction with the apical membrane.

Polarised membrane traffic in hepatocytes

The liver was used widely in early studies of polarised transport but has been largely overlooked in recent years, mostly because of the development of epithelial cell lines which provide more tractable experimental systems. The majority of membrane proteins and lipids reach the hepatocyte apical membrane by transcytosis and it remains unclear whether there is a direct route for apical targeting, although the pathways present have yet to be fully characterised. The recent development of systems that allow hepatocyte transport processes to be studied in culture and the observation that transcytosis can be significantly stimulated under physiological conditions suggest that hepatocytes have a role to play in future studies of polarised transport. This review discusses the known features of polarised membrane traffic in hepatocytes and contrasts them with the characteristics of vesicular transport in other epithelial cell types.

Vesicle targeting to the apical domain regulates bile excretory function in isolated rat hepatocyte couplets

BACKGROUND & AIMS

Plasma membrane solute transport may be regulated in many epithelial cells by vesicle traffic to and from the site of residence of the transporter. The aim of this study was to determine if this phenomenon may also play a role in the regulation of canalicular transport of bile acids.

METHODS

Confocal microscopy and image analysis were performed to quantitatively assess changes in secretory capacity and vesicle targeting in isolated rat hepatocyte couplets that had been exposed to fluorescent bile acid after pretreatment with dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) and/or nocodazole.

RESULTS

DBcAMP stimulated bile acid secretion by 240% while significantly increasing canalicular circumference. Nocodazole decreased secretion by 410% and significantly decreased canalicular circumference. When DBcAMP was added to nocodazole-treated couplets, a slight but significant increase was found in both fluorescent bile acid secretion and canalicular circumference as compared with nocodazole alone. Finally, DBcAMP stimulated translocation of vesicles to the canalicular membrane as determined by immunocytochemical localization of a putative bile acid transporter, Ca2+, Mg2+-ecto-adenosine triphosphatase.

CONCLUSIONS

The findings support the view that apical membrane transport activity in the rat hepatocyte is highly regulated by the insertion of vesicles into this domain and that this process involves both microtubule-dependent and -independent mechanisms.

Hepatobiliary function and toxicity in vitro using isolated hepatocyte couplets

1. Hepatocyte couplets can be routinely prepared from rat liver to produce a suitable in vitro model for polarized primary cells. 2. Centrifugal elutriation provides a means of producing enriched subpopulations of periportal and perivenous couplets from the same liver, thus providing a means of studying the influence of zonal heterogeneity on hepatobiliary function. 3. The maintenance of structural and secretory polarity demonstrated by hepatocyte couplets provides a convenient in vitro system for mechanistic studies of factors both regulatory and adversely affecting hepatobiliary functions. 4. Couplets are also uniquely appropriate for specific studies of regulation at the biliary pole, on the performance of junctions and on the maintenance and rate of transcytotic movement. 5. The possibility also exists that effects of an in vivo pre-exposure to agents causing hepatobiliary dysfunction can be assessed in couplets ex vivo.

Enhancement of Mdr2-mediated phosphatidylcholine translocation by the bile salt taurocholate. Implications for hepatic bile formation

Expression of the Mdr2-protein in secretory vesicules (SVs) from the yeast mutant sec6-4 causes a time- and temperature-dependent enhancement of phosphatidylcholine (PC) translocation from the outer to the inner leaflet of the SV lipid bilayer. We show that this activity is independent of changes either in the membrane potential or the pH gradient (inside positive) generated in these SVs by the yeast proton-translocating PMA1 ATPase. However, loading of the SVs with the primary bile salt taurocholate results in an apparent enhancement of Mdr2-mediated PC translocation activity. Reducing the intravesicular taurocholate (TC) concentration by dissipating the electrochemical potential across the SV membranes eliminates the enhancing effect of TC. Three lines of evidence suggest that the enhanced Mdr2-mediated PC translocation activity is not caused by a regulatory effect of TC on Mdr2 but rather reflected the formation of TC/PC aggregates or micelles in the lumen of SVs. First, significantly higher detergent concentrations are required to reveal the fluorescence of (7-nitro-2-1,3-benzoxadiazol-4-yl)amino-PC molecules translocated in Mdr2-SV under conditions of TC stimulation than under control conditions; second, the nonmicelle-forming bile salt taurodehydrocholate does not cause enhancement of PC translocation in Mdr2-SVs; third, enzyme marker studies indicate that TC behaves as a potent lipid solubilizer directly extracting PC molecules out of the bilayer without causing leakage. This results in the formation of intravesicular aggregates or mixed micelles, and provokes the apparent stimulation of Mdr2 activity. These data demonstrate a unique relationship between Mdr2, PC, and TC in the process of bile formation and secretion.

Effect of cholephilic dyes on hepatic tight junctional permeability in the rat

Changes in biliary permeability during cholephilic dye-induced choleresis, as assessed by measuring the movement into bile of two permeability probes, [14C]sucrose and horseradish peroxidase, were analyzed following an i.v. infusion (60 nmol/min per 100 g body wt) of the model cholephilic organic anion sulfobromophthalein in rats. Dye infusion led to a progressive increase of the [14C]sucrose bile-to-plasma ratio, which reached a maximum value after 100 min of dye infusion (+97%). Paracellular entry of horseradish peroxidase, as evaluated by the early peak of its biliary appearance curve, was also selectively increased (+69%), without changes in the later (transcytotic) access of the protein. Additional dose-response studies of biliary permeability to [14C]sucrose, using sulfobromophthalein and rose bengal, showed that this effect was dose-dependent and rapidly reversed by interruption of dye administration. The influence of hydrophobic/hydrophilic balance on this effect was also studied by infusing four dyes covering a broad range of hydrophobicity (phenol red, bromocresol green, sulfobromophthalein, and rose bengal), so as to attain a similar value of dye hepatic content at the end of the experiment (approximately 150 nmol/g liver wt). Under these conditions, a strong positive correlation was found between the increase in biliary permeability to [14C]sucrose and dye hydrophobicity. These results suggest that cholephilic dyes increase tight junctional permeability in a reversible and dose-dependent manner, and that this effect depends on the hydrophobic/hydrophilic balance of the dye.

Effect of cerulein hyperstimulation on the paracellular barrier of rat exocrine pancreas

BACKGROUND/AIMS

Cerulein-induced pancreatitis causes a rapid increase in pancreatic enzyme levels in serum and decreases in pancreatic duct secretion and interstitial edema. One mechanism to explain these early events is disruption of the actin tight junction paracellular seal of acinar and intralobular pancreatic duct cells.

METHODS

To examine the paracellular barrier of the proximal exocrine pancreas, rats were hyperstimulated with 5.0 micrograms.kg-1.h-1 of cerulein. Actin was visualized with rhodamine phalloidin and by electron microscopy and tight junctions were visualized with antibodies to the tight-junction protein ZO-1. Paracellular permeability was measured by movement of horseradish peroxidase from interstitium into duct or acinar lumens.

RESULTS

In controls, linear actin and ZO-1 staining occurred along the apical membrane of intralobular duct cells and extended to the apical pole of acinar cells. Hyperstimulation caused progressive disruption of the linear staining of f-actin and ZO-1. Actin disruption in duct cells was confirmed by electron microscopy. Horseradish peroxidase entered intralobular ducts and acinar lumens of hyperstimulated animals more frequently than those of controls.

CONCLUSIONS

The structure and function of the paracellular barrier of acinar and intralobular pancreatic duct cells are disrupted early during cerulein pancreatitis and may contribute to early clinical features.

Isolation of the microtubule-vesicle motor kinesin from rat liver: selective inhibition by cholestatic bile acids

BACKGROUND/AIMS

Vesicular transport is supported by microtubule-based, force-transducing adenosine triphosphatases (ATPases), such as kinesin, a ubiquitous motor enzyme that has been well studied in neuronal tissues. Although vesicular transport is important for hepatocellular secretory and clearance activities, the role of kinesin in liver function is poorly understood. Furthermore, the effects of bile acids on kinesin are unknown.

METHODS

Kinesin was purified from rat liver cytosol by conventional chromatography and microtubule affinity binding and was characterized by immunoblotting with domain-specific kinesin antibodies and amino acid sequencing of tryptic fragments. Kinesin activity was measured with and without bile acids using an in vitro motility assay and ATPase assays.

RESULTS

Immunoblot analysis and partial amino acid sequencing of purified kinesin showed that the sequence at the heavy chain of hepatic kinesin is nearly identical to that of brain kinesin. Purified kinesin transported microtubules in vitro with a velocity of approximately 0.5 microns/s; this activity was significantly inhibited by 0.5-1 mmol/L taurochenodeoxycholate but not by tauroursodeoxycholate. At a dose of 1 mmol/L, chenodeoxycholate conjugates, but not ursodeoxycholate or cholate conjugates, directly inhibited the ATPase activities of kinesin and another microtubule motor, cytoplasmic dynein.

CONCLUSIONS

Cholestatic concentrations of chenodeoxycholate conjugates directly inhibit the activity of microtubule motors, suggesting a possible mechanism for impairment of vesicular transport in cholestasis.

Regulation of biliary lipid secretion by mdr2 P-glycoprotein in the mouse

Disruption of the mdr2 gene in mice leads to a complete absence of phospholipid from bile (Smit, J. J. M., et al. 1993. Cell. 75:451-462). We have investigated the control of both mdr2 P-glycoprotein (Pgp) expression and bile salt secretion on biliary lipid secretion in the mouse. Lipid secretion was monitored at various bile salt output rates in wild-type mice (+/+), heterozygotes (+/-), and homozygotes (-/-) for mdr2 gene disruption. In (-/-) mice, phospholipid secretion was negligible at all bile salt output rates. In (+/-) mice, a curvilinear relation between bile salt and phospholipid secretion was observed similar to that in (+/+) mice; however, at all bile salt secretion rates phospholipid secretion was reduced compared to (+/+) mice, indicating that mdr2 Pgp exerts a strong control over secretion. Infusion of increasing amounts of taurocholate up to maximal secretory rate led to a decline in the phospholipid and cholesterol secretion in both (+/+) and (+/-) mice in accordance to what has been observed in other species. In contrast, in (-/-) mice cholesterol secretion increased under these conditions while phospholipid output remained extremely low. The increased cholesterol secretion may represent extraction of cholesterol from the canalicular plasma membrane by taurocholate micelles as opposed to the concomitant secretion of both phospholipid and cholesterol in the presence of a functional mdr2 Pgp. Increased bile flow in (-/-) mice could be attributed completely to an increase in the bile salt-independent fraction and may therefore be caused by the bile duct proliferation in these mice.

Effect of taurohyodeoxycholic acid, a hydrophilic bile salt, on bile salt and biliary lipid secretion in the rat

Taurohyodeoxycholic acid is a natural 6 alpha-hydroxylated bile acid with an apparent hydrophilicity intermediate between those of tauroursodeoxycholic and taurocholic acids. We investigated in the rat the hepatobiliary metabolism, choleretic properties, and biliary maximum secretory rate (SRmax) of taurohyodeoxycholic in comparison with these two bile salts. Each compound was infused intravenously, at a rate increased in a stepwise manner from 100 to 300 nmol/min/100 g body wt, in bile salt-depleted bile fistula rats. The three bile salts appeared rapidly starting with the infusion and increased to represent more than 95% of the total bile salts. No apparent biliary metabolites were formed. All the bile salts caused a dose-dependent increase in bile flow and biliary lipid output. The absolute increase in bile flow was lower in rats infused with taurohyodeoxycholic acid, yet the volume of bile formed per nanomole of secreted bile salt was 13.8 nl for taurohyodeoxycholic, 6.4 nl for tauroursodeoxycholic acid, and 10.9 nl for taurocholic. The SRmax values were 1080, 3240, and 960 nmol/min/100 g, respectively. At all infusion rates, taurohyodeoxycholic acid caused a greater (P < 0.001) secretion of biliary lecithin compared to the other bile salts. There were no significant differences in the biliary secretion of cholesterol and proteins. Electron microscopy showed the recruitment of vesicles and lamellar bodies around and within bile canaliculi. In conclusion, taurohyodeoxycholic promotes a biliary lecithin secretion greater than expected from physicochemical predictions, representing a novel secretory property with potential pharmacological relevance.

Fluorescent choleretic and cholestatic bile salts take different paths across the hepatocyte: transcytosis of glycolithocholate leads to an extensive redistribution of annexin II

We have used fluorescent derivatives of the choleretic bile salts cholate and chenodeoxycholate, the cholestatic salt lithocholate, and the therapeutic agent ursodeoxycholate to visualize distinct routes of transport across the hepatocyte and delivery to the canalicular vacuole of isolated hepatocyte couplets. The cholate and chenodeoxycholate derivatives produced homogeneous intracellular fluorescence and were rapidly transported to the vacuole, while the lithocholate analogue accumulated more slowly in the canalicular vacuole and gave rise to punctate fluorescence within the cell. Fluorescent ursodeoxycholate showed punctate intracellular fluorescence against a high uniform background indicating use of both pathways. Inhibition of vesicular transport by treatment with colchicine and Brefeldin A had no effect on the uptake of any of the compounds used, but it dramatically impaired delivery of both the lithocholate and the ursodeoxycholate derivatives to the canalicular vacuole. We conclude that while the chenodeoxycholate and cholate analogues traverse the hepatocyte by a cytoplasmic route, lithocholate and ursodeoxycholate analogues are transported by vesicle-mediated transcytosis. Treatment of couplets with glycine derivatives of lithocholate and ursodeoxycholate, but not cholate or chenodeoxycholate, led to a marked relocalization of annexin II, which initially became concentrated at the basolateral membrane, then moved to a perinuclear distribution and finally to the apical membrane as the incubation progressed. This suggests that lithocholate and ursodeoxycholate treatment leads to a rapid induction of transcytosis and that annexin II exchange occurs upon membrane fusion at all stages of the hepatocyte transcytotic pathway. These results indicate that isolated hepatocyte couplets may provide an inducible model system for the study of vesicle-mediated transcytosis.

Effects of protein kinase C and cytosolic Ca2+ on exocytosis in the isolated perfused rat liver

Both protein kinase C and cytosolic Ca2+ are involved in the regulation of exocytosis in a number of cell types. However, the relative importance of each of these for apical exocytosis in the hepatocyte is unknown. To investigate this, we studied the effects of protein kinase C and Ca2+ agonists on horseradish peroxidase excretion in the isolated perfused rat liver. Vasopressin increased both horseradish peroxidase concentration and net horseradish peroxidase excretion in bile, and these effects were abolished by the protein kinase C inhibitor H-7. The protein kinase C activator phorbol dibutyrate also increased both net excretion and the concentration of biliary horseradish peroxidase. In contrast, the Ca2+ ionophore A23187 and the Ca2+ mobilizing agent 2,5'-di(tertbutyl)-1,4-benzohydroquinone both had minimal effects on horseradish peroxidase concentration and inhibited the rate of horseradish peroxidase excretion. These results suggest that protein kinase C stimulates apical exocytosis in the hepatocyte, whereas increased Cai2+ per se does not influence exocytosis and inhibits excretion only transiently by reducing bile flow.

The mutant Eisai hyperbilirubinemic rat is resistant to bile acid-induced cholestasis and cytotoxicity

We investigated bile flow and biliary excretion of bile acids in the Eisai hyperbilirubinemic rat, a Sprague-Dawley mutant rat with conjugated hyperbilirubinemia, using both in vivo and in vitro models. In vivo bile flow was lower in Eisai hyperbilirubinemic rats than in the control rats before and after taurocholate was infused. After taurocholate was infused, bile acid output was similar in the Eisai hyperbilirubinemic rats and control rats. In the isolated perfused rat liver, biliary excretion of bile acids was higher in the Eisai hyperbilirubinemic rats than in the control rats after a high-dose infusion of taurocholate (0.33 mumol/min/gm liver). Infusion of taurochenodeoxycholate (0.22 mumol/min/gm liver) did not produce cholestasis and did not reduce the biliary excretion of bile acids in the Eisai hyperbilirubinemic rats. Taurochenodeoxycholate significantly increased the phospholipid/bile acid molar ratio and slightly reduced bile acid-induced alkaline phosphatase output into bile. The release of lactate dehydrogenase from the perfused liver 30 min after the start of the taurochenodeoxycholate infusion was 10 times lower in the Eisai hyperbilirubinemic rats than in the control rats (2.0 +/- 0.8 vs. 28.7 +/- 6.8 mU/min/gm liver). When the isolated perfused rat liver was infused with a 1-min pulse of horseradish peroxidase (25 mg), we observed an early and late peak of biliary excretion of horseradish peroxidase. The Eisai hyperbilirubinemic rats showed a significant increase in the late peak.(ABSTRACT TRUNCATED AT 250 WORDS)

Tubulovesicular transport of horseradish peroxidase in isolated rat hepatocyte couplets: effects of low temperature, cytochalasin B and bile acids

The transcytotic vesicular pathway in isolated rat hepatocyte couplets was investigated using horseradish peroxidase. Ten to 20 min after horseradish peroxidase labeling, vesicles and tubules containing horseradish peroxidase were observed to be predominantly around the bile canaliculi. In hepatocytes incubated in a 4 degrees C medium for 10 min after horseradish peroxidase labeling, few horseradish peroxidase-containing structures were observed around the bile canaliculi, and the fine reticular immunofluorescence of microtubules was reduced. Cells treated with cytochalasin B (a microfilament inhibitor) showed a fair number of horseradish peroxidase-containing structures around the markedly dilated bile canaliculi and the distribution of microtubules was preserved. Cells labeled by horseradish peroxidase and then incubated for 10 min in a horseradish peroxidase-free medium containing 50 mumol/L of taurocholic acid, ursodeoxycholic acid or tauroursodeoxycholic acid had more tubular structures containing horseradish peroxidase around the bile canaliculi than control cells, whereas 50 mumol/L of taurochenodeoxycholic acid, taurodeoxycholic acid, dehydrocholic acid and taurodehydrocholic acid each failed to increase the number of tubular structures. These findings show that horseradish peroxidase was transported in hepatocyte couplets from the cell periphery to the bile canalicular front through the tubulovesicular pathway, depending on cytoplasmic microtubules. Cytoplasmic microfilaments appeared to play a minor role in this transport. Several specific bile acids such as taurocholic acid, ursodeoxycholic acid and tauroursodeoxycholic acid each promoted the tubular transformation.

Crosstalk between calcium- and cyclic AMP-mediated signalling systems and the short-term modulation of bile flow in normal and cholestatic rat liver

The flow of bile is subject to short-term modulation by glucagon and calcium-mobilizing hormones. Of potential relevance is the crosstalk between the second messenger-mediated signal transducing systems of these agonists. This latter point has revealed an area of investigation that should enable further insights to be made into a physiological network that interrelates bile flow, hepatocellular calcium movements and hormone action. This information in turn may provide insights into the etiology and treatment of human and animal diseases in which cholestasis is an underlying feature.

Tauroursodeoxycholic acid stimulates hepatocellular exocytosis and mobilizes extracellular Ca++ mechanisms defective in cholestasis

To assess the effects of tauroursodeoxycholic acid (TUDCA) on bile excretory function, we examined whether TUDCA modulates vesicular exocytosis in the isolated perfused liver of normal rats in the presence of high (1.9 mM) or low (0.19 mM) extracellular Ca++ and in cholestatic rats 24 h after bile duct ligation. In addition, the effects of TUDCA on Ca++ homeostasis were compared in normal and in cholestatic hepatocytes. In the isolated perfused rat liver, TUDCA (25 microM) stimulated a sustained increase in the biliary excretion of horseradish peroxidase, a marker of the vesicular pathway, in the presence of high, but not low extracellular Ca++ or in the cholestatic liver. In contrast, TUDCA stimulated bile flow to the same extent regardless of the concentration of extracellular Ca++ or the presence of cholestasis. In indo-1-loaded hepatocytes, basal cytosolic free Ca++ ([Ca++]i) levels were not different between normal and cholestatic cells. However, in cholestatic cells [Ca++]i increases induced by TUDCA (10 microM) and its 7 alpha-OH epimer taurochenodeoxycholic acid (50 microM) were reduced to 22% and 26%, respectively, compared to normal cells. The impairment of TUDCA-induced [Ca++]i increase in cholestatic cells could be mimicked by exposing normal cells to low extracellular Ca++ (21%) or to the Ca++ channel blocker NiCl2 (23%). These data indicate that (a) dihydroxy bile acid-induced Ca++ entry may be of functional importance in the regulation of hepatocellular vesicular exocytosis, and (b) this Ca++ entry mechanism across the plasma membrane is impaired in cholestatic hepatocytes. We speculate that the beneficial effect of ursodeoxycholic acid in cholestatic liver diseases may be related to the Ca+(+)-dependent stimulation of vesicular exocytosis by its conjugate.

Bile acid-dependent vesicular transport of lysosomal enzymes into bile in the rat

BACKGROUND

Bile acids may stimulate the movement of hepatocyte vesicles and enhance their fusion with the biliary canaliculus. The present study examined the effects of various bile acids on the exocytosis of the contents of hepatocyte lysosomes into the biliary canaliculus.

METHODS

The effects of various bile acids on hepatocyte lysosome movement and on exocytosis of the contents of hepatocyte lysosomes into the biliary canaliculus were determined from the distribution of fluorescein isothiocyanate-dextran--labeled lysosomes in hepatocyte couplets and by quantitating biliary lysosomal enzyme output in rats.

RESULTS

Hydrophobic as well as hydrophilic and nonmicellar bile acids were found to stimulate to a similar degree the output of lysosomal enzymes into bile, indicating that bile acid-induced change of canalicular or lysosomal membrane fluidity is not responsible for enhanced exocytosis. The taurocholate-dependent increase in lysosomal enzyme excretion was completely blocked by either microtubule or microfilament inhibition, suggesting that these subcellular structures are involved in bile acid-dependent vesicular transport. Fluorescent microscopy studies showed that taurocholate causes a microtubule-dependent translocation of lysosomes towards the canaliculus in hepatocyte couplets, which occurred at the same time as increased output of lysosomal enzymes into bile.

CONCLUSIONS

The results suggest that bile acids modulate vesicle traffic towards the canaliculus by a mechanism unrelated to bile acid interaction with the vesicle membrane.

Biliary hyperpressure in rat extrahepatic cholestasis alters horseradish peroxidase biliary excretion

1. The authors investigated the effect of two extrahepatic cholestasis models (one by bile duct ligation and the other by choledocho-jugular fistula) on the hepatic clearance of horseradish peroxidase in male Sprague-Dawley rats divided into four groups. 2. In groups A (n = 5 rats) and B (n = 5), bile duct ligation was performed, while a choledocho-jugular fistula was created in groups C (n = 5) and D (n = 7). A 10 mg intravenous bolus of horseradish peroxidase was injected after 24 h (groups A and C), 48 h (groups B and D) or 1 h (Group E; five sham-operated rats). Serum and bile samples were then serially collected for 2 h. 3. In all groups, serum horseradish peroxidase levels increased soon after injection and then rapidly decreased, the curves being similar. Biliary excretion increased for 30 min and then slowly decreased. The highest horseradish peroxidase biliary concentrations and outputs were found in Group B followed by Group A; both groups had significantly higher levels than Group E. No difference was found between horseradish peroxidase biliary excretion of groups C and D and that of sham-operated rats. 4. When each group was considered separately, sampling times correlated with the corresponding ratios of bile/plasma HRP. Significant differences were found between the relative slopes of groups A, B and E, but not between those of groups C, D and E. 5. In conclusion, bile duct obstruction greatly affects the plasma-bile transfer of fluid phase markers, such as horseradish peroxidase, while single retention, caused by choledocho-jugular fistula, has no influence.(ABSTRACT TRUNCATED AT 250 WORDS)