Hepatobiliary function and toxicity in vitro using isolated hepatocyte couplets

Models and methods for in vitro testing of hepatic gap junctional communication

Inherent to their pivotal roles in controlling all aspects of the liver cell life cycle, hepatocellular gap junctions are frequently disrupted upon impairment of the homeostatic balance, as occurs during liver toxicity. Hepatic gap junctions, which are mainly built up by connexin32, are specifically targeted by tumor promoters and epigenetic carcinogens. This renders inhibition of gap junction functionality a suitable indicator for the in vitro detection of nongenotoxic hepatocarcinogenicity. The establishment of a reliable liver gap junction inhibition assay for routine in vitro testing purposes requires a cellular system in which gap junctions are expressed at an in vivo-like level as well as an appropriate technique to probe gap junction activity. Both these models and methods are discussed in the current paper, thereby focusing on connexin32-based gap junctions.

Primary hepatocyte cultures as prominent in vitro tools to study hepatic drug transporters

Before any drug can be placed on the market, drug efficacy and safety must be ensured through rigorous testing. Animal models are used for this purpose, though currently increasing attention goes to the use of alternative in vitro systems. In particular, liver-based testing platforms that allow the prediction of pharmacokinetic (PK) and pharmacotoxicological properties during the early phase of drug development are of interest. They also enable the screening of potential effects on hepatic drug transporters. The latter are known to affect drug metabolism and disposition, thereby possibly underlying drug-drug interactions, which, in turn, may result in liver toxicity. Clearly, stable in vivo-like functional expression of drug transporters in hepatic in vitro settings is a prerequisite to be applicable in routine PK and pharmacotoxicological testing. In the first part of the article, an updated overview of hepatic drug transporters is provided, followed by a state-of-the-art review of drug-transporter production and activity in primary hepatocyte cultures (PHCs), being the gold-standard in vitro system. Specific focus is hereby put on strategies to maintain long-term functional expression, in casu of drug transporters, in these systems. In the second part, the use of PHCs to assess hepatobiliary transport and transporter-mediated interactions is outlined.

Involvement of cell junctions in hepatocyte culture functionality

In liver, like in other multicellular systems, the establishment of cellular contacts is a prerequisite for normal functioning. In particular, well-defined cell junctions between hepatocytes, including adherens junctions, desmosomes, tight junctions, and gap junctions, are known to play key roles in the performance of liver-specific functionality. In a first part of this review article, we summarize the current knowledge concerning cell junctions and their roles in hepatic (patho)physiology. In a second part, we discuss their relevance in liver-based in vitro modeling, thereby highlighting the use of primary hepatocyte cultures as suitable in vitro models for preclinical pharmaco-toxicological testing. We further describe the actual strategies to regain and maintain cell junctions in these in vitro systems over the long-term.

Use ofin vitrotransporter assays to understand hepatic and renal disposition of new drug candidates

Hepatic and renal transporters contribute to the uptake, secretion and reabsorption of endogenous compounds, xenobiotics and their metabolites and have been implicated in drug-drug interactions and toxicities. Characterising the renal and hepatic disposition of drug candidates early in development would lead to more rational drug design, as chemotypes with 'ideal' pharmacokinetic characteristics could be identified and further refined. Because transporters are often organ specific, 'custom' transporter panels need to be identified for each major organ and chemotype to be evaluated, and appropriate studies planned. This review outlines the major renal and hepatic transporters and some of the in vitro transporter reagents, assays and processes that can be used to evaluate the renal and hepatic disposition of new chemical entities during drug discovery and development.

Effect of tauroursodeoxycholate and S-adenosyl-L-methionine on 17beta-estradiol glucuronide-induced cholestasis

BACKGROUND/AIMS

S-adenosyl-L-methionine (SAMe) and tauroursodeoxycholate (TUDC) exert an additive ameliorating effect on taurolithocholate (TLC)-induced cholestasis. The aims were to investigate the protective effect of SAMe on 17beta-estradiol-glucuronide (17betaEG) cholestasis and to find out whether SAMe and TUDC may exert an additive, ameliorating effect.

METHODS

Hepatocyte couplet function was assessed by canalicular vacuolar accumulation (cVA) of cholyllysylfluorescein (CLF). Cells were co-treated with 17betaEG and SAMe, TUDC, or both (protection study), or treated with 17betaEG and then with SAMe, TUDC or both (reversion study) before CLF uptake. Couplets were also co-treated with SAMe and dehydroepiandrosterone (DHEA), a competitive substrate for the sulfotransferase involved in 17betaEG detoxification. The effects of 17betaEG, SAMe and TUDC were also examined on intracellular distribution of F-actin.

RESULTS

Both SAMe and TUDC significantly protected against, and reversed, 17betaEG-induced cholestasis, but their effects were not additive. DHEA abolished the protective effect of SAMe. 17BetaEG did not affect the uptake of CLF into hepatocytes at the concentrations used, and also, it did not affect the intracellular distribution of F-actin.

CONCLUSIONS

17BetaEG does not affect the uptake of CLF into hepatocytes. SAMe and TUDC protect and reverse 17betaEG-induced cholestasis, but without an additive effect. Protection by SAMe may involve facilitating the sulfation of 17betaEG.

Reversible induction of rat hepatoma cell polarity with bile acids

A dynamic model for inducing and isolating polarized cell colonies from differentiated rat hepatoma was established with chenodeoxycholic acid (CDCA). Cells were treated with 75 microM CDCA in a 1% solvent mix (DMSO/ethanol: 0.5%/0.5%) for 11 days and positive Fao-BA1 and C2rev7-BA1 clones were isolated, respectively, from Fao and C2rev7. Cell polarization in these two clones was demonstrated by (i) the detection of (gamma)-glutamyl transpeptidase activity (gamma)-GT) and the presence of specific proteins, namely aminopeptidase N (APN), bile acid export pump (Bsep), multidrug resistance-associated protein 2 (Mrp2) at the canalicular pole, (ii) the expression of tight junction (ZO-1) and basolateral (1–18) marker proteins, (iii) the presence of regular microvilli in the cavities sealed by tight junctions, and (iv) functional bile canaliculi-like structures with the capacity to metabolise and secrete carboxyfluorescein diacetate dye. The polarized phenotype was maintained for more than 200 cell generations in the presence of CDCA and could be modulated by cell density or omitting the inducing agent. Hence this cellular model is well suited for studies on hepatic differentiation, polarization and bile salt trafficking with therapeutic implications.

Advances in cell separation: recent developments in counterflow centrifugal elutriation and continuous flow cell separation

Cell separation by counterflow centrifugal elutriation (CCE) or free flow electrophoresis (FFE) is performed at lower frequency than cell cloning and antibody-dependent, magnetic or fluorescence-activated cell sorting. Nevertheless, numerous recent publications confirmed that these physical cell separation methods that do not include cell labeling or cell transformation steps, may be most useful for some applications. CCE and FFE have proved to be valuable tools, if homogeneous populations of normal healthy untransformed cells are required for answering scientific questions or for clinical transplantation and cells cannot be labeled by antibodies, because suitable antibodies are not available or because antibody binding to a cell surface would induce the cell reaction which should be investigated on purified cells or because antibodies bound to the surface hamper the use of the isolated cells. In addition, the methods are helpful for studying the biological reasons for, or effects of, changes in cell size and cellular negative surface charge density. Although the value of the methods was confirmed in recent years by a considerable number of important scientific results, activities to further develop and improve the instruments have, unfortunately, declined.

Tauroursodeoxycholate and S-adenosyl-L-methionine exert an additive ameliorating effect on taurolithocholate-induced cholestasis: a study in isolated rat hepatocyte couplets

The monohydroxy bile acid, taurolithocholate (TLC), causes cholestasis in vivo and in isolated perfused livers. It is also cholestatic in vitro and, in this study using isolated rat hepatocyte couplets, causes a reduction of the accumulation of (fluorescent) bile acid in the canalicular vacuoles (cVA) of this polarized cell preparation. The hepatoprotective bile acid, tauroursodeoxycholate (TUDCA), partially protects against the action of TLC when added at the same time. It also partially reverses the cholestatic effect if added after the cells have been exposed to TLC. A second hepatoprotective compound, S-adenosyl-L-methionine (SAMe) also not only partially protects against the action of TLC when added at the same time, but it too is able to partially reverse the cholestatic effect. Neither hepatoprotective agent is fully effective alone, but their effects are additive. In combination, a full restoration of cVA is observed in moderate cholestasis, but not in severe cholestasis. We discuss briefly some possible mechanisms involved in the additive mode of action of both hepatoprotective compounds. In summary, we show for the first time that SAMe and TUDCA can exert an additive effect in the amelioration of TLC-induced cholestasis in isolated rat hepatocyte couplets. This finding may be of possible clinical relevance.

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.

BAPS prize lecture: New insight into mechanisms of parenteral nutrition-associated cholestasis: role of plant sterols. British Association of Paediatric Surgeons

BACKGROUND PURPOSE

Infants on long-term parenteral nutrition frequently will have progressive cholestatic liver disease, the cause of which remains largely unknown. The aim of this study is to establish a possible role for plant sterols (phytosterols) in the pathogenesis of parenteral nutrition-associated cholestasis (PNAC).

METHODS

Two experimental studies were used: (1) A study on neonatal piglets involved the daily injection of plant sterols; measurement of their concentrations in serum, liver, and bile during a 14-day period; and determination of serum bile acid concentrations, bile acid secretion, and bile flow at the end of the 14-day period. (2) Isolated rat hepatocyte couplets were used to study the effects of sterols on canalicular secretion.

RESULTS

The daily injection of phytosterols (in amounts similar to those given to infants who receive parenteral nutrition) led to their progressive accumulation in the piglets' serum, liver, and bile. Serum bile acid levels were significantly higher in the sterol-treated piglets. Maximal bile acid excretion was significantly lower in the sterol group. Phytosterols caused a significant inhibition of secretory function in isolated rat hepatocyte couplets.

CONCLUSION

Important contaminants of commercial lipid emulsions (phytosterols) have been identified as a possible cause of PNAC.

Effects of eicosanoids on parameters in isolated rat hepatocytes and isolated rat hepatocyte couplets: protective effects of eicosanoid receptor antagonists

A number of eicosanoids caused plasma membrane blebbing in hepatocytes and this could be inhibited in a dose-dependent fashion by the receptor antagonists AH6809 and ICI 192605. The pattern of effectiveness of eicosanoids interfering with canalicular vacuole accumulation in hepatocyte couplets differed from that causing blebbing; the two most effective eicosanoids here were PGD2 and PGF2 alpha.