The Use of Long-term Hepatocyte Cultures for Detecting Induction of Drug Metabolising Enzymes: The Current Status

Perspectives on:In Vitro Evaluation of Biomedical Polymers

Recently polymeric materials have gained tremendous attention in a wide variety of applications spanning from electronics to environmental and biomedical fields. In this paper, current in vitro methods for polymers biocompatibility assessment are reviewed in combination with new concepts and techniques that appear promising for the development and improvement of in vitro methods with the purpose of reducing animal experimentation. The utilization of medical devices, for example, has always been subordinate to the assessment of their biocompatibility. This aspect, as well as the methods for evaluating biocompatibility have changed over the years as a result of new developments in cell biology that have revolutionized in vitro techniques for assaying polymeric materials for bioapplications.

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.

In vitro models for liver toxicity testing

Over the years, various liver-derived in vitro model systems have been developed to enable investigation of the potential adverse effects of chemicals and drugs. Liver tissue slices, isolated microsomes, perfused liver, immortalized cell lines, and primary hepatocytes have been used extensively. Immortalized cell lines and primary isolated liver cells are currently most widely used in vitro models for liver toxicity testing. Limited throughput, loss of viability, and decreases in liver-specific functionality and gene expression are common shortcomings of these models. Recent developments in the field of in vitro hepatotoxicity include three-dimensional tissue constructs and bioartificial livers, co-cultures of various cell types with hepatocytes, and differentiation of stem cells into hepatic lineage-like cells. In an attempt to provide a more physiological environment for cultured liver cells, some of the novel cell culture systems incorporate fluid flow, micro-circulation, and other forms of organotypic microenvironments. Co-cultures aim to preserve liver-specific morphology and functionality beyond those provided by cultures of pure parenchymal cells. Stem cells, both embryonic- and adult tissue-derived, may provide a limitless supply of hepatocytes from multiple individuals to improve reproducibility and enable testing of the individual-specific toxicity. This review describes various traditional and novel in vitro liver models and provides a perspective on the challenges and opportunities afforded by each individual test system.

Interaction between hepatocytes and collagen gel in hollow fibers

Gel entrapment culture of primary mammalian cells within collagen gel is one important configuration for construction of bioartificial organ as well as in vitro model for predicting drug situation in vivo. Gel contraction in entrapment culture, resulting from cell-mediated reorganization of the extracellular matrix, was commonly used to estimate cell viability. However, the exact influence of gel contraction on cell activities has rarely been addressed. This paper investigated the gel contraction under varying culture conditions and its effect on the activities of rat hepatocyte entrapped in collagen gel within hollow fibers. The hepatocyte activities were reflected by cell viability together with liver-specific functions on urea secretion and cytochrome P450 2E1. Unexpectedly, no gel contraction occurred during gel entrapment culture of hepatocyte under a high collagen concentration, but hepatocytes still maintained cell viability and liver-specific functions at a similar level to the other cultures with normal gel contraction. It seems that cell activities are unassociated with gel contraction. Alternatively, the mass transfer resistance induced by the combined effect of collagen concentration, gel contraction and cell density could be a side effect to reduce cell activities. The findings with gel entrapment culture of hepatocytes would be also informative for the other cell culture targeting pathological studies and tissue engineering.

Inter-laboratory evaluation of the response of primary human hepatocyte cultures to model CYP inducers - a European Centre for Validation of Alternative Methods (ECVAM) - funded pre-validation study

The aim of the current work was to harmonise protocols between three laboratories by performing independent isolations and cultures of human hepatocytes and to assess their responses to prototypical cytochrome P450 (CYP) enzyme inducers, beta-naphthoflavone (BNF), rifampicin (RIF) or phenobarbital (PB). The magnitudes of the induction responses were CYP and donor-dependent but there was a good reproducibility between laboratories. CYP1A2 activity was evident in all cultures treated with BNF but not RIF or PB. Likewise, CYP3A4/5 activity was induced to the same extent by RIF and PB, while BNF did not affect this CYP in any of the cultures tested. All three compounds caused a concentration-dependent increase in CYP2B6 in cultures from 2 of the 3 laboratories and the response to PB was at least twice that of the other two inducers. In conclusion, the harmonised protocols used to study the response of primary cultures of human hepatocytes to prototypical inducers are transferable, reproducible within a given laboratory and between laboratories. The results obtained will support setting up a definitive validation study of the harmonised protocols.

Cell-based models to study hepatic drug metabolism and enzyme induction in humans

Cell-based in vitro models are invaluable tools in elucidating the pharmacokinetic profile of a drug candidate during its drug discovery and development process. As biotransformation is one of the key determinants of a drug's disposition in the body, many in vitro models to study drug metabolism have been established, and others are still being developed and validated. This review is aimed at providing the reader with a concise overview of the characteristics and optimal application of established and emerging in vitro cell-based models to study human drug metabolism and induction of drug metabolising enzymes in the liver. The strengths and weaknesses of liver-derived models, such as primary hepatocytes, either freshly isolated or cryopreserved, and from adult or fetal donors, precision-cut liver slices, and cell lines, including immortalised cells, reporter cell lines, hepatocarcinoma-derived cell lines and recombinant cell lines, are discussed. Relevant cell culture configuration aspects as well as other models such as stem cell-derived hepatocyte-like cells and humanised animal models are also reviewed. The status of model development, their acceptance by health authorities and recommendations for the most appropriate use of the models are presented.

Toxicity testing: creating a revolution based on new technologies

Biotechnology is evolving at a tremendous rate. Although drug discovery is now heavily focused on high throughput and miniaturized screening, the application of these advances to the toxicological assessment of chemicals and chemical products has been slow. Nevertheless, the impending surge in demands for the regulatory toxicity testing of chemicals provides the impetus for the incorporation of novel methodologies into hazard identification and risk assessment. Here, we review the current and likely future value of these new technologies in relation to toxicological evaluation and the protection of human health.

Studies on the induction of rat hepatic CYP1A, CYP2B, CYP3A and CYP4A subfamily form mRNAs in vivo and in vitro using precision-cut rat liver slices

1. Real-time quantitative reverse transcription-polymerase chain reaction methodology (TaqMan(R)) was used to examine the induction of some selected rat hepatic cyto-chrome P450 (CYP) forms in vivo and in vitro using cultured precision-cut liver slices. 2. TaqMan primers and probe sets were developed for rat CYP1A1, CYP1A2, CYP2B1, CYP2B1/2, CYP3A1, CYP3A2 and CYP4A1 mRNAs. 3. To characterize the responsiveness of the rat CYP mRNA TaqMan primers and probe sets, rats were treated in vivo with a single intraperitoneal dose of 500 mg kg(-1) Aroclor 1254 (ARO) and with four daily oral doses of either 50 mg kg(-1) day(-1) dexamethasone (DEX) or 75 mg kg(-1) day(-1) methylclofenapate (MCP). Treatment with ARO produced 22 600-, 5480-, 648-, 52-, 47- and 9-fold increases in levels of CYP1A1, CYP2B1, CYP2B1/2, CYP1A2, CYP3A1 and CYP3A2 mRNA, respectively. DEX treatment produced 97-, 24-, 8- and 4-fold increases, respectively, in CYP3A1, CYP2B1, CYP2B1/2 and CYP3A2 mRNA levels, and MCP produced 339-, 126- and 25-fold increases, respectively, in CYP4A1, CYP2B1 and CYP2B1/2 mRNA levels. All three CYP inducers also increased microsomal CYP content and produced corresponding increases in CYP1A, CYP2B, CYP3A and CYP4A form marker enzyme activities. 4. Rat liver slices were cultured for 6 and 24 h in medium containing 0.1 micro M insulin and 0.1 micro M DEX, and also for 24 h in medium containing only 0.1 micro M insulin (DEX-free medium). Liver slices were cultured in control medium or in medium containing either 10 micro M beta-naphthoflavone (BNF), 10 micro g ml(-1) ARO, 500 micro M sodium phenobarbitone (NaPB), 20 micro M pregnenolone-16alpha -carbonitrile (PCN), 50 micro M Wy-14,643 (WY) or 50 micro M MCP. 5. With the exception of the effect of BNF on CYP1A1 mRNA levels, the induction of all the CYP mRNAs studied was greater after 24- than after 6-h treatment. Generally, the magnitude of induction of CYP mRNA levels was greater after 24 h in liver slices cultured in DEX-free than in DEX-supplemented medium. 6. Treatment of liver slices with BNF and ARO for 24 h in DEX-free medium produced 21- and 35-fold increases, respectively, and 38- and 37-fold increases, respectively, in CYP1A1 and CYP1A2 mRNA levels. NaPB, PCN, WY and MCP did not increase either CYP1A1 or CYP1A2 mRNA levels. 7. After 24 h, levels of CYP2B1/2 mRNA were increased 18-, 20-, 9-, 16- and 13-fold by treatment with ARO, NaPB, PCN, WY and MCP, respectively. PCN also produced 56- and 4-fold increases, respectively, in CYP3A1 and CYP3A2 mRNA levels. 8. Treatment with WY and MCP for 24 h produced 437- and 186-fold increases, respectively, in levels of CYP4A1 mRNA. None of the other CYP inducers studied had any effect on CYP4A1 mRNA levels. 9. The results demonstrate the utility of cultured precision-cut liver slices as an in vitro model system to evaluate the effects of xenobiotics on rat CYP1A, CYP2B, CYP3A and CYP4A form mRNA levels.

Induction of cytochrome P450 2B6 and 3A4 expression by phenobarbital and cyclophosphamide in cultured human liver slices

PURPOSE

To examine the potential of cultured human liver slices to predict cytochrome P450 (CYP) inducibility, regarding global and zonal CYP expression, together with drug-induced histologic changes.

METHODS

We first assessed whether CYP2B6, 3A4, and 2C9 expression was maintained in cultured liver slices. Cultured hepatocytes were used as the reference culture system. Then we tested the effects of phenobarbital and cyclophosphamide on CYP expression in both models.

RESULTS

Morphologic features are preserved in slices. Basal CYP expression declines with time in culture in both models. Slices display the same region specificity of CYP2B6, 2C9, and 3A4 expression as intact liver. CYP2B6 and 3A4 mRNA, apoprotein, and enzyme-related activities were induced by phenobarbital and cyclophosphamide, whereas CYP2C9 apoprotein was not. Their immunoreactivities were also increased, while their zonal distribution was preserved on slice tissue sections. Microsomal enzyme induction was confirmed by histology.

CONCLUSIONS

Cultured human liver slices are an attractive alternative to hepatocyte culture for the prediction of human CYP isoenzyme induction by xenobiotics.

Effects of medium composition on the morphology and function of rat hepatocytes cultured as spheroids and monolayers

Primary hepatocytes cultured as monolayers or as spheroids were studied to compare the effects of four different culture media (Williams' E, Chee's, Sigma Hepatocyte, and HepatoZYME medium). Rat hepatocytes were cultured as conventional monolayers for 3 d or as spheroids for 2 wk. For spheroid formation a method was emplOyed that combined the use of a nonadherent substratum with rotation of cultures. Hepatocyte integrity and morphology were assessed by light and electron microscopy and by reduced glutathione content. Hepatocyte function was measured by albumin secretion and 7-ethoxycoumarin metabolism. Chee's medium was found to be optimal for maintenance of hepatocyte viability and function in monolayers, but it failed to support spheroid formation. For spheroid formation and for the maintenance of spheroid morphology and function, Sigma HM was found to be optimal. These results demonstrate that the medium requirements of hepatocytes differ markedly depending on the culture model employed. Spheroid culture allowed better preservation of morphology and function of hepatocytes compared with conventional monolayer culture. Hepatocytes in spheroids formed bile canaliculi. and expressed an actin distribution resembling that found in hepatocytes in vivo. Albumin secretion was maintained at the same level as that found during the first d in primary culture, and 7-ethoxycoumarin metabolism was maintained over 2 wk in culture at approximately 30% of the levels found in freshly isolated hepatocytes. The improved morphology and function of hepatocyte cultures as spheroids may provide a more appropriate in vitro model for certain applications where the maintenance of liver-specific functions in long-term culture is crucial.

Effects of medium composition on the morphology and function of rat hepatocytes cultured as spheroids and monolayers

Primary hepatocytes cultured as monolayers or as spheroids were studied to compare the effects of four different culture media (Williams' E, Chee's, Sigma Hepatocyte, and HepatoZYME medium). Rat hepatocytes were cultured as conventional monolayers for 3 d or as spheroids for 2 wk. For spheroid formation a method was emplOyed that combined the use of a nonadherent substratum with rotation of cultures. Hepatocyte integrity and morphology were assessed by light and electron microscopy and by reduced glutathione content. Hepatocyte function was measured by albumin secretion and 7-ethoxycoumarin metabolism. Chee's medium was found to be optimal for maintenance of hepatocyte viability and function in monolayers, but it failed to support spheroid formation. For spheroid formation and for the maintenance of spheroid morphology and function, Sigma HM was found to be optimal. These results demonstrate that the medium requirements of hepatocytes differ markedly depending on the culture model employed. Spheroid culture allowed better preservation of morphology and function of hepatocytes compared with conventional monolayer culture. Hepatocytes in spheroids formed bile canaliculi. and expressed an actin distribution resembling that found in hepatocytes in vivo. Albumin secretion was maintained at the same level as that found during the first d in primary culture, and 7-ethoxycoumarin metabolism was maintained over 2 wk in culture at approximately 30% of the levels found in freshly isolated hepatocytes. The improved morphology and function of hepatocyte cultures as spheroids may provide a more appropriate in vitro model for certain applications where the maintenance of liver-specific functions in long-term culture is crucial.