Actin filament alteration as a potential marker for cholestasis: a study in isolated rat hepatocyte couplets

Effects of Phalloidin on Hepatic Gene Expression in Mice

An attempt has been made to identify molecular markers of intrahepatic cholestasis in mice employing phalloidin as a cholestatic agent. Phalloidin was administered to BALB/c mice at three predetermined dose: 250 μg/kg, 500 μg/kg, and 1 mg/kg for 1, 3, and 7 days. Liver function was estimated to confirm cholestasis. Histopathological observations on liver were also made to confirm liver injury. Phalloidin at 1 mg/kg for 7 days was found to induce cholestasis. Therefore gene expression studies were confined to this group only. A total of 88 genes were found to be affected by phalloidin. These were the genes associated with cytoskeleton regulation as well as tight junction, focal adhesion, and ATP-binding cassette transporters. Such proteins obstruct the removal of bile components from hepatocytes to the bile canaliculus or blood. Phalloidin treatment did not affect the proteins responsible for cell maintenance or death. The authors show that phalloidin-induced intrahepatic cholestasis is manifested by disturbing the cytoskeleton. The set of genes up-regulated by phalloidin can be considered as molecular markers of intrahepatic cholestasis. The observations are further expected to be helpful in the management of cholestatic pharmaceuticals and associated problems of liver diseases in humans.

A glyphosate-based herbicide induces histomorphological and protein expression changes in the liver of the female guppy Poecilia reticulata

Glyphosate-based herbicides (GBH) are among the most common herbicides found in aquatic systems, but limited data are available about their mode of action and hepatotoxicity in fish. This study investigated the hepatotoxicity induced by GBH in the guppy Poecilia reticulata using a histopathological assessment associated with a proteomic approach. Guppies were exposed to GBH for 24 h at 1.8 mg of glyphosate L-1, corresponding to 50% of the LC50, 96 h. The results indicate that the GBH at 1.8 mg of glyphosate L-1 induce the development of hepatic damage in P. reticulata, which is exposure-time dependent. The histopathological indexes demonstrate that GBH cause inflammatory, regressive, vascular and progressive disorders in the liver of guppies. Using 2D gel electrophoresis associated with mass spectrometry, 18 proteins that changed by GBH were identified and were related to the cellular structure, motility and transport, energy metabolism and apoptosis. The results show that the acute exposure to GBH causes hepatic histopathological damage related to protein expression profile changes in P. reticulata, indicating that a histopathological assessment associated with a proteomic analysis provides a valuable approach to assess the toxic effects of GBH in sentinel fish species.

Early Alterations of Bile Canaliculi Dynamics and the Rho Kinase/Myosin Light Chain Kinase Pathway Are Characteristics of Drug-Induced Intrahepatic Cholestasis

Intrahepatic cholestasis represents 20%-40% of drug-induced injuries from which a large proportion remains unpredictable. We aimed to investigate mechanisms underlying drug-induced cholestasis and improve its early detection using human HepaRG cells and a set of 12 cholestatic drugs and six noncholestatic drugs. In this study, we analyzed bile canaliculi dynamics, Rho kinase (ROCK)/myosin light chain kinase (MLCK) pathway implication, efflux inhibition of taurocholate [a predominant bile salt export pump (BSEP) substrate], and expression of the major canalicular and basolateral bile acid transporters. We demonstrated that 12 cholestatic drugs classified on the basis of reported clinical findings caused disturbances of both bile canaliculi dynamics, characterized by either dilatation or constriction, and alteration of the ROCK/MLCK signaling pathway, whereas noncholestatic compounds, by contrast, had no effect. Cotreatment with ROCK inhibitor Y-27632 [4-(1-aminoethyl)-N-(4-pyridyl) cyclohexanecarboxamide dihydrochloride] and MLCK activator calmodulin reduced bile canaliculi constriction and dilatation, respectively, confirming the role of these pathways in drug-induced intrahepatic cholestasis. By contrast, inhibition of taurocholate efflux and/or human BSEP overexpressed in membrane vesicles was not observed with all cholestatic drugs; moreover, examples of noncholestatic compounds were reportedly found to inhibit BSEP. Transcripts levels of major bile acid transporters were determined after 24-hour treatment. BSEP, Na⁺-taurocholate cotransporting polypeptide, and organic anion transporting polypeptide B were downregulated with most cholestatic and some noncholestatic drugs, whereas deregulation of multidrug resistance-associated proteins was more variable, probably mainly reflecting secondary effects. Together, our results show that cholestatic drugs consistently cause an early alteration of bile canaliculi dynamics associated with modulation of ROCK/MLCK and these changes are more specific than efflux inhibition measurements alone as predictive nonclinical markers of drug-induced cholestasis.

Destruction of the hepatocyte junction by intercellular invasion of Leptospira causes jaundice in a hamster model of Weil's disease

Weil's disease, the most severe form of leptospirosis, is characterized by jaundice, haemorrhage and renal failure. The mechanisms of jaundice caused by pathogenic Leptospira remain unclear. We therefore aimed to elucidate the mechanisms by integrating histopathological changes with serum biochemical abnormalities during the development of jaundice in a hamster model of Weil's disease. In this work, we obtained three-dimensional images of infected hamster livers using scanning electron microscope together with freeze-cracking and cross-cutting methods for sample preparation. The images displayed the corkscrew-shaped bacteria, which infiltrated the Disse's space, migrated between hepatocytes, detached the intercellular junctions and disrupted the bile canaliculi. Destruction of bile canaliculi coincided with the elevation of conjugated bilirubin, aspartate transaminase and alkaline phosphatase levels in serum, whereas serum alanine transaminase and γ-glutamyl transpeptidase levels increased slightly, but not significantly. We also found in ex vivo experiments that pathogenic, but not non-pathogenic leptospires, tend to adhere to the perijunctional region of hepatocyte couplets isolated from hamsters and initiate invasion of the intercellular junction within 1 h after co-incubation. Our results suggest that pathogenic leptospires invade the intercellular junctions of host hepatocytes, and this invasion contributes in the disruption of the junction. Subsequently, bile leaks from bile canaliculi and jaundice occurs immediately. Our findings revealed not only a novel pathogenicity of leptospires, but also a novel mechanism of jaundice induced by bacterial infection.

Disruption of blastomeric F-actin: a potential early biomarker of developmental toxicity in zebrafish

The expression of at least some biomarkers of toxicity is generally thought to precede the appearance of frank pathology. In the context of developmental toxicity, certain early indicators may be predictive of later drastic outcome. The search for predictive biomarkers of toxicity in the cells (blastomeres) of an early embryo can benefit from the fact that for normal development to proceed, the maintenance of blastomere cellular integrity during the process of transition from an embryo to a fully functional organism is paramount. Actin microfilaments are integral parts of blastomeres in the developing zebrafish embryo and contribute toward the proper progression of early development (cleavage and epiboly). In early embryos, the filamentous actin (F-actin) is present and helps to define the boundary of each blastomere as they remain adhered to each other. In our studies, we observed that when blastomeric F-actin is depolymerized by agents like gelsolin, the blastomeres lose cellular integrity, which results in abnormal larvae later in development. There are a variety of toxicants that depolymerize F-actin in early mammalian embryos, the later consequences of which are, at present, not known. We propose that very early zebrafish embryos (~5-h old) exposed to such toxicants will also respond in a like manner. In this review, we discuss the potential use of F-actin disruption as a predictive biomarker of developmental toxicity in zebrafish.

Inositol trisphosphate analogues selective for types I and II inositol trisphosphate receptors exert differential effects on vasopressin-stimulated Ca2+ inflow and Ca2+ release from intracellular stores in rat hepatocytes

Previous studies have shown that adenophostin A is a potent initiator of the activation of SOCs (store-operated Ca2+ channels) in rat hepatocytes, and have suggested that, of the two subtypes of Ins(1,4,5)P3 receptor predominantly present in rat hepatocytes [Ins(1,4,5)P3R1 (type I receptor) and Ins(1,4,5)P3R2 (type II receptor)], Ins(1,4,5)P3R1s are required for SOC activation. We compared the abilities of Ins(1,4,6)P3 [with higher apparent affinity for Ins(1,4,5)P3R1] and Ins(1,3,6)P3 and Ins(1,2,4,5)P4 [with higher apparent affinities for Ins(1,4,5)P3R2] to activate SOCs. The Ins(1,4,5)P3 analogues were microinjected into single cells together with fura 2, and dose-response curves for the activation of Ca2+ inflow and Ca2+ release from intracellular stores obtained for each analogue. The concentration of Ins(1,4,6)P3 which gave half-maximal stimulation of Ca2+ inflow was substantially lower than that which gave half-maximal stimulation of Ca2+ release. By contrast, for Ins(1,3,6)P3 and Ins(1,2,4,5)P3, the concentration which gave half-maximal stimulation of Ca2+ inflow was substantially higher than that which gave half-maximal stimulation of Ca2+ release. The distribution of Ins(1,4,5)P3R1 and Ins(1,4,5)P3R2 in rat hepatocytes cultured under the same conditions as those employed for the measurement of Ca2+ inflow and release was determined by immunofluorescence. Ins(1,4,5)-P3R1s were found predominantly at the cell periphery, whereas Ins(1,4,5)P3R2s were found at the cell periphery, the cell interior and nucleus. It is concluded that the idea that a small region of the endoplasmic reticulum enriched in Ins(1,4,5)P3R1 is required for the activation of SOCs is consistent with the present results for hepatocytes.

Regulation of F-actin and endoplasmic reticulum organization by the trimeric G-protein Gi2 in rat hepatocytes. Implication for the activation of store-operated Ca2+ inflow

The roles of the heterotrimeric G-protein, G(i2), in regulating the actin cytoskeleton and the activation of store-operated Ca(2+) channels in rat hepatocytes were investigated. Galpha(i2) was principally associated with the plasma membrane and microsomes. Both F-actin and Galpha(i2) were detected by Western blot analysis in a purified plasma membrane preparation, the supernatant and pellet obtained by treating the plasma membrane with Triton X-100, and after depolymerization and repolymerization of F-actin in the Triton X-100-insoluble pellet. Actin in the Triton X-100-soluble supernatant co-precipitated with Galpha(i2) using either anti-Galpha(i2) or anti-actin antibodies. The principally cortical location of F-actin in hepatocytes cultured for 0.5 h changed to a pericanalicular distribution over a further 3.5 h. Some Galpha(i2) co-localized with F-actin at the plasma membrane. Pretreatment with pertussis toxin ADP-ribosylated 70-80% of Galpha(i2) in the plasma membrane and microsomes, prevented the redistribution of F-actin, caused redistribution and fragmentation of the endoplasmic reticulum, and inhibited vasopressin-stimulated Ca(2+) inflow. It is concluded that (i) a significant portion of hepatocyte Galpha(i2) associates with, and regulates the arrangement of, cortical F-actin and the endoplasmic reticulum and (ii) either or both of these regulatory roles are likely to be required for normal vasopressin activation of Ca(2+) inflow.

Vasopressin-induced disruption of actin cytoskeletal organization and canalicular function in isolated rat hepatocyte couplets: possible involvement of protein kinase C

The effect of vasopressin (VP) on canalicular function and hepatocellular morphology, with particular regard to actin cytoskeletal organization and the concomitant plasma membrane bleb formation, was studied in isolated rat hepatocyte couplets. VP induced the concentration-dependent formation of multiple plasma membrane blebs as well as simultaneous impairment in both canalicular vacuolar accumulation (cVA) and retention (cVR) of the fluorescent bile acid, cholyl-lysyl-fluorescein (CLF), which evaluate couplet secretory function and tight-junction integrity, respectively. These effects were mimicked by the protein kinase C (PKC) activator, phorbol dibutyrate (PDB), but not by the protein kinase A (PKA) activator, dibutyryl-cAMP. VP-induced bleb formation and canalicular dysfunction were fully prevented by the protein kinase inhibitor, H-7, but not by the PKA inhibitor, KT5720, further suggesting a specific role of PKC. VP-induced alterations were also prevented by pretreatment with the Ca2+-buffering agent, BAPTA/AM, but not with the calmodulin-dependent protein kinase II antagonist, calmidazolium. Neither the Ca2+-activated neutral protease inhibitor, leupeptin, nor the antioxidants, alpha-tocopherol or deferoxamine, were able to prevent either VP-induced plasma membrane blebbing or canalicular dysfunction. The Ca2+-ionophore, A23187, mimicked the VP-induced alterations, but its harmful effects were completely prevented by H-7. Bleb formation induced by VP and PDB was accompanied by an extensive redistribution of filamentous actin from the pericanalicular area to the cell body, and this effect was fully prevented by H-7. These results suggest that VP-induced canalicular and cytoskeletal dysfunction is mediated by PKC and that classical (Ca2+-dependent) PKC appear to be involved because intracellular Ca2+ is required for VP to induce its harmful effects.

Hepatobiliary effects of tertiary-butylhydroperoxide (tBOOH) in isolated rat hepatocyte couplets

The organic hydroperoxide, tertiary-butylhydroperoxide (tBOOH), causes oxidative damage in a number of cell types. It is used here in an isolated rat hepatocyte couplet preparation to study adverse hepatobiliary effects of peroxidative damage in vitro. At subcytotoxic concentrations (as determined by lactate dehydrogenase release and maintenance of cytoplasmic ATP concentrations) tBOOH caused decreased accumulation of a fluorescent bile acid analogue, cholyl-lysyl-fluorescein (CLF), in the canalicular vacuole of couplets (a hepatobiliary effect; cholestasis). This was dose dependent in the range 100-200 microM. At the same concentrations it brought about release of preaccumulated CLF, suggesting that its effect was more likely to be on sealing properties of the vacuole than processes of uptake, transcytosis, and secretion. Pretreatment of tBOOH-treated couplets with the antioxidants deferoxamine mesylate (iron chelator) and dimethyl sulfoxide (free radical scavenger) resulted in the prevention of both canalicular vacuolar accumulation (cVA, which assesses canalicular function) and canalicular vacuolar retention (cVR, which assesses the retaining ability of couplets) depression at 100 microM tBOOH but not at higher concentrations. This indicates that the cholestatic effect of tBOOH has a preventable and nonpreventable phase and that free radicals are involved in these processes. By selectively generating the two types of tBOOH radical, peroxyl (tBOO.) and alkoxyl (tBO.), using suitable catalysts, we were able to determine that the peroxyl radical was most probably involved in tBOOH-induced cholestasis. This was further supported by experiments employing specific peroxyl and alkoxyl radical scavengers; only the peroxyl scavenger reduced the effect of tBOOH upon canalicular function under the conditions studied.

Hepatotoxicity in drug development: detection, significance and solutions

Despite considerable progress in the understanding of the mechanism of liver toxicity we are not yet able to design non-hepatotoxic molecules rationally. Also, there is no "universal" in vitro primary screening approach for early identification of hepatotoxic molecules. In most cases hepatotoxicity is detected at later stages of drug development in animal toxicity studies or clinical trials. Although the liver is the most common target organ for drug candidates in animal toxicity studies, hepatotoxicity rarely leads to cessation of drug development during the preclinical phase. Indeed, contrary to other target organs, liver toxicity is usually reversible and can be monitored in man by sensitive serum enzyme tests. Therefore in many cases a compound found hepatotoxic in an animal species will be tested in man for a definitive assessment of its hepatotoxic potential. Liver toxicity in man may be acceptable when a drug has major therapeutic potential. In this situation mechanistic studies are essential to assess the risk in man and in some cases to identify protective agents. When liver toxicity leads to project termination a secondary screening approach may be envisaged if biologically active analogs are available.

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.

Effect of cholestasis and bile acids on interferon-induced 2',5'-adenylate synthetase and NK cell activities

BACKGROUND/AIMS

The mechanisms involved in resistance to interferon alfa in patients with chronic hepatitis C are unclear. Both cirrhosis and cholestasis have been shown to be predictive of resistance. The aim of this study was to evaluate the influence of cholestasis and bile acids on 2',5'-oligoadenylate synthetase and natural killer activities, which are both involved in the antiviral activity of interferon.

METHODS

2',5'-Oligoadenylate synthetase activity was evaluated in spleen, liver, and isolated hepatocytes from bile duct-ligated rats, and the effect of bile acids in vitro on interferon-induced 2',5'-oligoadenylate synthetase and natural killer activities was examined in fresh mononuclear cells from healthy subjects.

RESULTS

Cholestasis had a time-dependent inhibitory effect on 2',5'-oligoadenylate synthetase activity in liver, spleen, and isolated hepatocytes from cholestatic rats (-70%, 86%, and 70% relative to baseline, respectively). In vitro, endogenous bile acids had a concentration-dependent inhibitory effect on interferon-induced 2',5'-oligoadenylate synthetase and natural killer activities, which was related to their structure. This inhibitory effect correlated with the surface activity index.

CONCLUSIONS

Cholestasis and bile acids diminish the biological activity of interferon and natural killer activity. The results suggest a decrease in the antiviral defenses in cholestatic conditions.

Disruption of canalicular function in isolated rat hepatocyte couplets caused by cyclosporin A

Isolated rat hepatocyte couplets were used to study the effects of different concentrations of cyclosporin A in relation to canalicular function. Canalicular function was assessed by counting the percentage of couplets which were able to accumulate the fluorescent cholephile cholyl lysyl fluorescein (CLF) into the canalicular vacuole between the two cells, i.e. canalicular vacuole accumulation (CVA). At lower doses, the immunosuppressor increased the CVA, reaching 121 +/- 3.86% of control at 25 nM cyclosporin A. However, higher doses of cyclosporin A induced a concentration-dependent inhibition of CVA to 64.0 +/- 3.51% of control at 100 nM. Modifications in canalicular area (as % couplet area) were also observed. Image analysis of the fluorescent image showed that cyclosporin A (25 nM) increased canalicular area by 25% (of control); however, this parameter decreased to 36% of control at 100 nM cyclosporin A. In addition, at 100 nM, cyclosporin A reduced the proportion of couplets retaining CLF within the canaliculus to 75.0 +/- 6.59% of control. Treatment of couplets with cyclosporin A (0-2 microM) for 15 min revealed that reduced glutathione (GSH) intracellular content does not change significantly at these doses. However, alteration in pericanalicular F-actin at 100 nM cyclosporin A may be an important factor in the disruption of the canalicular function induced by higher doses of the immunosuppressor.

Scanning laser cytometry: alterations induced by cholestatic agents in isolated rat hepatocyte couplets

Scanning laser cytometry, an analytic technique that provides an accurate fluorescent measurement in adherent cells, was used to study cholestatic mechanisms in isolated rat hepatocyte couplets (IRHC). Treatment of IRHC with cholestatic compounds induced a pericanalicular F-actin accumulation and an increase in cytosolic free calcium. These data obtained with a scanning cytometer used in conjunction with an in vitro model representing the primary secretory unit suggest that abnormalities of pericanalicular F-actin filaments and calcium homeostasis play a key role in cholestasis. Considering the necessity for the development of mechanistic studies in toxicology, this technique should prove to be an outstanding tool.

Effect of oxidative stress and disruption of Ca2+ homeostasis on hepatocyte canalicular function in vitro

Isolated rat hepatocyte couplets were used to study the effects of menadione and a rise in the intracellular concentration of calcium on biliary canalicular function. Canalicular function was assessed by counting the percentage of couplets which were able to accumulate the fluorescent cholephile, cholyl lysyl fluorescein (CLF) into the canalicular vacuole between the two cells. Menadione induced a concentration-dependent inhibition of the canalicular vacuole accumulation (CVA) of CLF reaching 7.6 +/- 1.8% of control at 100 microM menadione. This disruption was not prevented by blocking receptor-operated calcium channels with Ni2+ (300 microM). The concentration range of menadione used did not deplete cellular ATP content. In contrast glutathione content was reduced to 52% of its control value by 100 microM menadione. A rise in cytosolic calcium induced by the calcium ionophore, A23187 (up to 30 microM) also disrupted CVA in a concentration-dependent manner. Release of endoplasmic reticulum calcium stores by thapsigargin (50 nM) affected the retention of canalicular contents to a much lesser extent, although it was able to stimulate a reduction in canalicular area to 40% of its original value, assumed to be due to canalicular contraction. Menadione (30 and 100 microM) reduced the fluorescence of phalloidin-FITC-labelled F-actin in both the total and pericanalicular cytoskeleton. Canalicular function was therefore disrupted by non-lethal concentrations of menadione via a mechanism which does not appear to involve ATP depletion or the entry of extracellular calcium, but is associated with a depletion of both cellular glutathione and F-actin. An increase in the concentration of intracellular calcium can stimulate canalicular contraction, and at relatively high concentrations calcium can also disrupt canalicular function.