Activity and inducibility of drug-metabolizing enzymes in immortalized hepatocyte-like cells (mhPKT) derived from a L-PK/Tag1 transgenic mouse

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Can hepatoma cell lines be redifferentiated to be used in drug metabolism studies?

Knowledge of metabolism, enzymes so far involved, and potential enzyme-inhibiting or enzyme-inducing properties of new compounds is a key issue in drug development. Primary cultured hepatocytes, cytochrome P450 (CYP)-engineered cells and hepatoma cell lines are currently being used for this purpose, but only primary cultures can produce a metabolic profile of a drug similar to that found in vivo and can respond to inducers. Because of their limited accessibility, alternatives to replace human hepatocytes are currently being explored, including the immortalisation of hepatocytes by using different strategies (i.e. SV40 T-large antigen, conditionally immortalised hepatocytes, transfection with c-myc, cH-ras, N-ras oncogenes, transgenic animals over-expressing growth factors or oncogenes and cre-lox recombination/excision). However, none of the resulting cells has the desirable phenotypic characteristics to replace primary cultures in drug metabolisms studies. We investigated why these differentiated human hepatomas do not express CYP genes and found that the levels of certain key transcription factors clearly differ from those found in hepatocytes. It was then conceivable that re-expression of one (or more) of these transcription factors could lead to an efficient transcription of CYP genes. The feasibility of this hypothesis was demonstrated by genetic engineering of Hep G2 cells with liver-enriched transcription factors followed by the analysis of the expression of the most relevant human CYPs.

Hepatocyte cell lines: their use, scope and limitations in drug metabolism studies

Gaining knowledge on the metabolism of a drug, the enzymes involved and its inhibition or induction potential is a necessary step in pharmaceutical development of new compounds. Primary human hepatocytes are considered a cellular model of reference, as they express the majority of drug-metabolising enzymes, respond to enzyme inducers and are capable of generating in vitro a metabolic profile similar to what is found in vivo. However, hepatocytes show phenotypic instability and have a restricted accessibility. Different alternatives have been explored in the past recent years to overcome the limitations of primary hepatocytes. These include immortalisation of adult or fetal human hepatic cells by means of transforming tumour virus genes, oncogenes, conditionally immortalised hepatocytes, and cell fusion. New strategies are currently being used to upregulate the expression of drug-metabolising enzymes in cell lines or to derive hepatocytes from progenitor cells. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism studies as well as the state-of-the-art of the strategies pursued in order to generate metabolically competent hepatic cell lines.

Pregnane X receptor is a target of farnesoid X receptor

The pregnane X receptor (PXR) is an essential component of the body's detoxification system. PXR is activated by a broad spectrum of xenobiotics and endobiotics, including bile acids and their precursors. Bile acids in high concentrations are toxic; therefore, their synthesis is tightly regulated by the farnesoid X receptor, and their catabolism involves several enzymes regulated by PXR. Here we demonstrate that the expression of PXR is regulated by farnesoid X receptor. Feeding mice with cholic acid or the synthetic farnesoid X receptor (FXR) agonist GW4064 resulted in a robust PXR induction. This effect was abolished in FXR knock-out mice. Long time bile acid treatment resulted in an increase of PXR target genes in wild type mice. A region containing four FXR binding sites (IR1) was identified in the mouse Pxr gene. This region was able to trigger an 8-fold induction after GW4064 treatment in transactivation studies. Deletion or mutation of single IR1 sites caused a weakened response. The importance of each individual IR1 element was assessed by cloning a triple or a single copy and was tested in transactivation studies. Two elements were able to trigger a strong response, one a moderate response, and one no response to GW4064 treatment. Mobility shift assays demonstrated that the two stronger responding elements were able to bind FXR protein. This result was confirmed by chromatin immunoprecipitation. These results strongly suggest that PXR is regulated by FXR. Bile acids activate FXR, which blocks synthesis of bile acids and also leads to the transcriptional activation of PXR, promoting breakdown of bile acids. The combination of the two mechanisms leads to an efficient protection of the liver against bile acid induced toxicity.

Characterization of cytochrome P450 expression in murine embryonic stem cell-derived hepatic tissue system

An in vitro system for liver organogenesis from murine embryonic stem (ES) cells has been recently established. This system is expected to be applied to the development of a new drug metabolism assay system that uses ES cells as a substitute for animal experiments. The objective of this study was to elucidate the drug metabolism profiles of the murine ES cell-derived hepatic tissue system compared with those of primary cultures of murine adult and fetal hepatocytes. The expression of the genes of the cytochrome P450 (P450) family, such as Cyp2a5, Cyp2b10, Cyp2c29, Cyp2d9, Cyp3a11, and Cyp7a1, was observed in the murine ES cell-derived hepatic tissue system at 16 days and 18 days after plating (A16 and A18). To investigate the activities of these P450 family enzymes in the murine ES cell-derived hepatic tissue system at A16 and A18, testosterone metabolism in this system was analyzed. Testosterone was hydroxylated to 6beta-hydroxytestosterone (6beta-OHT), 16alpha-OHT, 2alpha-OHT, and 2beta-OHT in this system, and was not hydroxylated to 15alpha-OHT, 7alpha-OHT, and 16beta-OHT. This metabolism profile was similar to that of fetal hepatocytes and different from that of adult hepatocytes. Furthermore, pretreatment with phenobarbital resulted in a 2.5- and 2.6-fold increase in the production of 6beta-OHT and 16beta-OHT. Thus, evidence for drug metabolic activities in relation to P450s has been demonstrated in this system. These results in this system would be a stepping stone of the research on the development and differentiation to adult liver.

Characterization of cytochrome P450 expression in murine embryonic stem cell-derived hepatic tissue system

An in vitro system for liver organogenesis from murine embryonic stem (ES) cells has been recently established. This system is expected to be applied to the development of a new drug metabolism assay system that uses ES cells as a substitute for animal experiments. The objective of this study was to elucidate the drug metabolism profiles of the murine ES cell-derived hepatic tissue system compared with those of primary cultures of murine adult and fetal hepatocytes. The expression of the genes of the cytochrome P450 (P450) family, such as Cyp2a5, Cyp2b10, Cyp2c29, Cyp2d9, Cyp3a11, and Cyp7a1, was observed in the murine ES cell-derived hepatic tissue system at 16 days and 18 days after plating (A16 and A18). To investigate the activities of these P450 family enzymes in the murine ES cell-derived hepatic tissue system at A16 and A18, testosterone metabolism in this system was analyzed. Testosterone was hydroxylated to 6beta-hydroxytestosterone (6beta-OHT), 16alpha-OHT, 2alpha-OHT, and 2beta-OHT in this system, and was not hydroxylated to 15alpha-OHT, 7alpha-OHT, and 16beta-OHT. This metabolism profile was similar to that of fetal hepatocytes and different from that of adult hepatocytes. Furthermore, pretreatment with phenobarbital resulted in a 2.5- and 2.6-fold increase in the production of 6beta-OHT and 16beta-OHT. Thus, evidence for drug metabolic activities in relation to P450s has been demonstrated in this system. These results in this system would be a stepping stone of the research on the development and differentiation to adult liver.

New hepatocytes for toxicology?

In a recent article M.G. Sacco and co-workers described the establishment of immortalized untransformed transgenic hepatocyte (MMH-GH) cell lines, obtained from a cross between the AT/cytoMet and Hsp70/hGH transgenic mice. This strategy proved to be successful because the MMH-GH showed stability in culture and sensitivity to chemical exposure. Based on these results, the MMH-GH cell lines could prove to be a valid alternative cell-based assay for use in toxicological studies.

LIVER-SPECIFIC PHYSIOLOGY OF IMMORTAL, FUNCTIONALLY DIFFERENTIATED HEPATOCYTES AND OF DEFICIENT HEPATOCYTE-LIKE VARIANTS

Five different immortalized transgenic hepatocyte cell lines derived from mice were investigated with respect to their potential to maintain the physiological properties of primary hepatocytes using chemically defined medium. This research completes a previous study by Klocke and coworkers in 2002, using gene expression analysis of the same cell lines by the respective physiological analysis for investigating the hepatocyte-like function. Three transgenic cell lines harboring a fusion gene derivative (construct 202) consisting of the complete SV40 early region, including the coding sequences for the transforming large and small tumor antigens, placed under the control of the murine metallothioneine 1-promotor/enhancer element, showed a hepatocyte-like function and physiology. They grew as a monolayer with a polygonal cell shape, consumed lactate, and secreted albumin at a cell-specific rate of 1.5 pg/h, which is in the range of primary hepatocytes. In addition, the potential of detoxifying ammonium could be maintained. Ammonium was metabolized and urea was produced and released into the medium. A complete urea cycle could be determined. A cell line established from neonatal transgenic mice and expressing a secretory variant of the human epidermal growth factor (IgEGF) under the control of the albumin promoter was characterized by an incomplete urea cycle. Another cell line isolated from the liver of homozygote neonatal p53-knockout mice showed no hepatocyte-specific functions but only properties of continuous cell lines. Specific nucleoside triphosphate (NTP) and uridine (U) ratios were used to characterize the differentiation status of the particular cell lines. A low NTP-U value was found for the three cell lines containing construct 202, which was identical to that observed for primary hepatocytes. In contrast, the cell line harvested from the liver of homozygote neonatal p53-knockout mice presented a NTP-U ratio characteristic for continuous cell lines. This study demonstrates that the four transgenic and the p53-knockout hepatocyte-derived cell lines can be used as models for investigating the conservation of tissue-specific functions in immortalized cells.

Constitutive and inducible expression of CYP enzymes in immortal hepatocytes derived from SV40 transgenic mice

1. The expression of liver-specific transcription factors and cytochrome P450 (CYP) enzymes have been studied in three new hepatocyte-like cell lines derived from SV Delta 202 transgenic mice: AMH-Delta 202 (adult mouse hepatocytes), TAMH-Delta 202 (tumour-derived adult mouse hepatocytes) and NMH-Delta 202 (newborn mouse hepatocytes). 2. mRNA levels of liver-enriched transcription factors such as D-element binding protein (DBP), liver-enriched transcription activating protein (LAP) and the hepatic nuclear factors (HNF) 1, 2 and 3 in all Delta 202 transgenic hepatocyte lines were similar to those in the wild-type liver and in primary mouse hepatocytes. 3. Analysis of basal CYP activities and testosterone metabolism revealed that Delta 202 cells showed higher similarities to mouse hepatocytes than Hepa 1c1c7 hepatoma cells. All three Delta 202 cell lines exhibited substantial active CYP1A1/2, CYP2A4/5 and CYP3A11 activities and lower levels of CYP2B, CYP2C and CYP2E1 activities. 4. The Delta 202 cells also responded to model inducers. 3-Methylcholanthrene induced CYP1A1/2 (7-ethoxyresorufin O-deethylation); phenobarbital induced CYP2B (7-benzoxyresorufin O-debenzylation), CYP2A4/5 (testosterone 7alpha -hydroxylation) and CYP3A11 (testosterone 6beta -hydroxylation); and rifampicin and dexamethasone induced CYP3A11 activities in the three Delta 202 cell lines, whereas only AMH-Delta 202 cells reproduced to a limited extent the response of CYP2E1 to ethanol observed in hepatocytes. 5. The results suggest that generation of hepatocyte lines from transgenic animals constitutes a successful approach to obtain in vitro models alternative to primary hepatocytes for drug metabolism and CYP inducibility studies.

Establishment and characterization of immortal hepatocytes derived from various transgenic mouse lines

The potential of three genetic changes introduced into mice by the transgenic or knockout technology aimed at immortalizing hepatocytes in vitro and concomitantly preserving their differentiated hepatic functions was analyzed. Six hepatocyte lines were isolated from neonatal and adult transgenic mice expressing either IgEGF (a secretable variant of hEGF) or SV40 T antigen in the liver and from neonatal and adult p53 knockout (KO) mice and have been subcultured >150 times in serum-free, arginine-deficient medium. Only in SV40 T antigen transgenic lines profiles of mRNAs encoding serum proteins, transcription factors, and liver-specific enzymes were similar to those found in livers and primary hepatocytes. Accordingly, these cells displayed basal and inducible expression of CYP proteins as well as testosterone metabolizing activities. Thus, either knockout of the p53 gene or expression of SV40 T antigen or of IgEGF imparts immortality to hepatocytes in vitro, but only SV40 T antigen expression is compatible with the concomitant long-term preservation of differentiated liver functions.

Characterization of transgenic mouse strains using six human hepatic cytochrome P450 probe substrates

1. Transgenic mice were evaluated with six human cytochrome P450 (CYP) selective probe substrates, as little is known about their metabolism in the mouse. Mouse strains characterized include C57BL/SJL, FVB/N, mdr 1a/1b (-/-), ob/ob and ACCA. 2. Human CYP probe substrates used for characterization of mouse CYP activities included bufuralol, testosterone, dextromethorphan, phenacetin, diclofenac and S-mephenytoin. Activities were compared with those obtained in human liver microsomes and in human recombinant enzyme preparations. All transgenic mouse strains showed similar apparent K(m) with bufuralol, testosterone and dextromethorphan which compared favourably with those observed in human liver microsomes. 3. K(m) for phenacetin O-deethylase and S-mephenytoin 4'-hydroxylation were more variable across strains and in some cases demonstrated biphasic kinetics. Phenacetin O-deethylase activity was low in all mouse strains except FVB/N and mdr 1a/1b (-/-). Diclofenac 4-hydroxylation did not occur to any significant extent in the five strains of mouse evaluated here. 4. The findings suggest the validity of using five of the probes for transgenic mouse hepatic CYP characterization and gross comparison with data generated with human CYP.

Immortalized kidney epithelial cells as tools for hormonally regulated ion transport studies

The development of transgenic mice carrying the simian virus-40 large T antigen gene or the temperature-sensitive simian virus-40 large T antigen gene, either alone or placed under the control of the 5'-regulatory regions of tissue-specific or ubiquitous genes, has permitted the production of differentiated, polarized kidney epithelial cells. This review covers the immortalized cell lines issued from the various parts of the renal tubule and, in particular, the recently established collecting duct cell lines that have been used as ex-vivo cell models to analyze the regulation of ion transport processes by hormones.

A mouse Sertoli cell line expressing anti-Müllerian hormone and its type II receptor

Anti-Müllerian hormone (AMH) induces the regression of Müllerian ducts in the male foetus; it is secreted by prepubertal testicular Sertoli cells and repressed at puberty. Using an AMH promoter/Simian virus 40 (SV40) oncogene fusion gene, we generated transgenic mouse lines exhibiting heritable Sertoli cell tumorigenesis. One cell line, derived from an adult male, expressed mRNAs characteristic of mature Sertoli cells, but no AMH. Two other cell lines were obtained from pretumoral testes at 6.5 days. One was cloned to yield SMAT1, whose expression pattern was characteristic of prepubertal Sertoli cells, namely no transferrin and high SF-I and AMH expression. SMAT1 also secretes AMH protein into the culture medium and expresses the AMH receptor. To the best of our knowledge, this is the first Sertoli cell line stably expressing AMH and its receptor. Our results show that, in targeted oncogenesis, the timing of cell line derivation plays a critical role even when using a developmentally regulated promoter.