Rat hepatocyte suspensions as a suitable in vitro model for studying the biotransformation of histone deacetylase inhibitors

This paper focuses on the use of liver-derived in vitro systems for biotransformation studies during early drug development, as exemplified by the two molecules recently studied in our laboratory: Trichostatin A (TSA) and its structural analogue 5-(4-dimethylaminobenzoyl)aminovaleric acid hydroxamide (4-Me2N-BAVAH). Phase I biotransformation of TSA, a histone deacetylase inhibitor with promising antifibrotic and antitumoural properties, was investigated in liver microsomal (rat and human) and in hepatocyte (rat) suspensions. Within 40 minutes, 50 microM of TSA was completely metabolised by 2 x 10(6) hepatocytes/ml. Reduction of the hydroxamic acid function to its corresponding amide and N-demethylation were the two major phase I biotransformation pathways, while hydrolysis products of TSA were minor metabolites. Lower concentrations of TSA (5 microM and 25 microM) were N-demethylated faster. Liver microsomes, however, metabolised TSA incompletely with the formation of two major metabolites, N-mono- and N-didemethylated TSA. Unlike TSA, 4-Me2N-BAVAH (50 microM) could still be detected after 3 hours of incubation with 2 x 10(6) rat hepatocytes/ml suspension. Hydrolysis and reduction of the hydroxamic acid function to its corresponding acid and amide, respectively, were shown to be the major phase I biotransformation pathways. Lower concentrations of 4-Me2N-BAVAH were hydrolysed more readily. 4-Me2N-BAVAH and its metabolites were less subjected to N-demethylation than TSA.

Proliferation of epidermal growth factor-stimulated hepatocytes in a hormonally defined serum-free medium

The present study shows that adult rat hepatocytes in primary culture, which normally exhibit a restricted capacity to proliferate, can proceed through the cell cycle when cultured in a mixture of minimal essential medium (MEM) and Medium 199 (MEM-M199; 3:1, v/v), containing epidermal growth factor (EGF; 50 ng/ml), low glucose (0.75 g/l) and low levels of inorganic salts, amino acids and vitamins. Under these conditions, hepatocytes flatten and cell extensions appear. In contrast, Dulbecco's modified Eagle's medium (DMEM) containing high glucose (4.5g/l) levels enriched with inorganic salts, amino acids and vitamins favours maintenance of differentiated functional hepatocyte capacities (albumin secretion), but does not allow proliferation or cell spreading. Cultivation of hepatocytes in MEM-M199 (3:1, v/v) results in the onset of DNA synthesis at 48 hours of culture and a concomitant induction of cyclin D1 protein. Under these conditions, cells successively progress through the mitogen-dependent restriction point in mid-late G1 phase, G1/S transition and S phase, as evidenced by Western blot analysis of the markers cyclins E and A and cyclin dependent kinase (CDK)2 and CDK1, respectively. Progression through the cell cycle is accompanied by a decrease in albumin secretion, indicating a decline in differentiated capacities. This study demonstrates that hepatocytes cultured in a mixture of MEM-M199 (3:1) provide a useful in vitro model for studying the regulation of hepatocyte proliferation.

Inhibition of NF-kappaB activation by the histone deacetylase inhibitor 4-Me2N-BAVAH induces an early G1 cell cycle arrest in primary hepatocytes

OBJECTIVE

Benzoylaminoalkanohydroxamic acids, including 5-(4-dimethylaminobenzoyl)aminovaleric acid hydroxamide (4-Me(2)N-BAVAH), are structural analogues of Trichostatin A, a naturally occurring histone deacetylase inhibitor (HDACi). 4-Me(2)N-BAVAH has been shown to induce histone hyperacetylation and to inhibit proliferation in Friend erythroleukaemia cells in vitro. However, the molecular mechanisms have remained unidentified.

MATERIALS AND METHODS

In this study, we evaluated the effects of 4-Me(2)N-BAVAH on proliferation in non-malignant cells, namely epidermal growth factor-stimulated primary rat hepatocytes.

RESULTS AND CONCLUSION

We have found that 4-Me(2)N-BAVAH inhibits HDAC activity at non-cytotoxic concentrations and prevents cells from responding to the mitogenic stimuli of epidermal growth factor. This results in an early G(1) cell cycle arrest that is independent of p21 activity, but instead can be attributed to inhibition of cyclin D1 transcription through a mechanism involving inhibition of nuclear factor-kappaB activation. In addition, 4-Me(2)N-BAVAH delays the onset of spontaneous apoptosis in primary rat hepatocyte cultures as evidenced by down-regulation of the pro-apoptotic proteins Bid and Bax, and inhibition of caspase-3 activation.

The Pharmaceutical Potential of Histone Deacetylase Inhibitors

Protein acetylation, catalyzed by the opposing activities of histone deacetylases (HDAC) and histone acetyltransferases, is now recognized to be an important epigenetic modulator of gene transcriptional activity and cell function. As a result of the intense search for HDAC inhibitors (HDACi) during the past fifteen years, a large number of structurally divergent classes with variable potencies and isoenzyme selectivities have been identified. They occupy an important and promising position in a number of therapeutic areas. Several HDACi are under clinical evaluation as tumor cell-selective chemotherapeutics, and show great promise for the treatment of inflammatory disorders, neurodegenerative diseases, protozoal and latent viral infections, and (fibro)proliferative disorders. Recently, it was discovered that they might be used as enhancers of differentiation in stem cell therapy, and as medium supplements that stabilize the phenotype of primary cells in culture. Next to biological activity, the pharmaceutical potential of a compound is also dependent on the adequate translation of in vitro potency into in vivo efficacy whilst maintaining an acceptable safety profile. Therefore, this review will not only address the formerly mentioned applications, but will also deal with the pharmacokinetic and toxicological properties of currently available HDACi. Several compounds exert potent activities in vitro, but have been shown to be of limited therapeutic value due to rapid biotransformation, and thus poor in vivo bioavailability. The first attempts to improve the metabolic properties of HDACi have been made and will be discussed. In contrast to conventional chemotherapeutics, HDACi exert no drastic side effects at therapeutically effective doses. Although a bulk effect on histone acetylation is observed, HDACi display a remarkable tumor cell-selective toxicity. The mechanisms underlying these cell type-dependent differences in sensitivity to HDACi-mediated effects, however, remain largely elusive.

A Metabolic Screening Study of Trichostatin A (TSA) and TSA-Like Histone Deacetylase Inhibitors in Rat and Human Primary Hepatocyte Cultures

Hydroxamic acid (HA)-based histone deacetylase (HDAC) inhibitors, with trichostatin A (TSA) as the reference compound, are potential antitumoral drugs and show promise in the creation of long-term primary cell cultures. However, their metabolic properties have barely been investigated. TSA is rapidly inactivated in rodents both in vitro and in vivo. We previously found that 5-(4-dimethylaminobenzoyl)aminovaleric acid hydroxyamide or 4-Me2N-BAVAH (compound 1) is metabolically more stable upon incubation with rat hepatocyte suspensions. In this study, we show that human hepatocytes also metabolize TSA more rapidly than compound 1 and that similar pathways are involved. Furthermore, structural analogs of compound 1 (compounds 2-9) are reported to have the same favorable metabolic properties. Removal of the dimethylamino substituent of compound 1 creates a very stable but 50% less potent inhibitor. Chain lengthening (4 to 5 carbon spacer) slightly improves both potency and metabolic stability, favoring HA reduction to hydrolysis. On the other hand, Calpha-unsaturation and spacer methylation not only reduce HDAC inhibition but also increase the rate of metabolic inactivation approximately 2-fold, mainly through HA reduction. However, in rat hepatocyte monolayer cultures, compound 1 is shown to be extensively metabolized by phase II conjugation. In conclusion, this study suggests that simple structural modifications of amide-linked TSA analogs can improve their phase I metabolic stability in both rat and human hepatocyte suspensions. Phase II glucuronidation, however, can compensate for their lower phase I metabolism in rat hepatocyte monolayers and could play a yet unidentified role in the determination of their in vivo clearance.

Trichostatin A, a critical factor in maintaining the functional differentiation of primary cultured rat hepatocytes

Histone deacetylase inhibitors (HDI) have been shown to increase differentiation-related gene expression in several tumor-derived cell lines by hyperacetylating core histones. Effects of HDI on primary cultured cells, however, have hardly been investigated. In the present study, the ability of trichostatin A (TSA), a prototype hydroxamate HDI, to counteract the loss of liver-specific functions in primary rat hepatocyte cultures has been investigated. Upon exposure to TSA, it was found that the cell viability of the cultured hepatocytes and their albumin secretion as a function of culture time were increased. TSA-treated hepatocytes also better maintained cytochrome P450 (CYP)-mediated phase I biotransformation capacity, whereas the activity of phase II glutathione S-transferases (GST) was not affected. Western blot and qRT-PCR analysis of CYP1A1, CYP2B1 and CYP3A11 protein and mRNA levels, respectively, further revealed that TSA acts at the transcriptional level. In addition, protein expression levels of the liver-enriched transcription factors (LETFs) hepatic nuclear factor 4 alpha (HNF4alpha) and CCAAT/enhancer binding protein alpha (C/EBPalpha) were accordingly increased by TSA throughout culture time. In conclusion, these findings indicate that TSA plays a major role in the preservation of the differentiated hepatic phenotype in culture. It is suggested that the effects of TSA on CYP gene expression are mediated via controlling the expression of LETFs.

Molecular mechanisms underlying the dedifferentiation process of isolated hepatocytes and their cultures

Primary hepatocytes and their cultures are a simple but versatile, well-controlled, and relatively easy to handle in vitro system that is well-accepted for investigating xenobiotic biotransformation, enzyme induction and inhibition, and (biotransformation-mediated) hepatotoxicity. In addition, hepatocyte cultures have proven to be valuable tools in the study of liver physiology, viral hepatitis, and liver regeneration and are proposed as an alternative to orthotopic liver transplantation. It has been observed, however, that a number of liver-specific functions are progressively lost with time when hepatocytes are isolated and cultivated. These phenotypic changes are primarily the result of fundamental changes in gene expression concomitant with a diminished transcription of the relevant liver-specific genes, and can be interpreted as a 'dedifferentiation' of the isolated hepatocytes. Ischemia-reperfusion stress induced during the isolation process, disruption of the normal tissue architecture, as well as an adaptation to the in vitro environment are underlying factors and will be extensively discussed. A detailed description of the regulation of the hepatocyte phenotype in vivo in the first section of this review will help to understand the effect of these factors on hepatocyte gene expression. Although different approaches, mainly mimicking the in vivo hepatocyte environment, have been succesfully used to prevent or slow down the dedifferentiation of primary hepatocytes in monolayer culture, the ideal hepatocyte-based culture model, characterized by a long-term expression of hepatocyte-specific functions comparable to the in vivo level, does not exist at the moment. Consequently, alternative strategies should focus on the isolation procedure, during which dedifferentiation is already initiated. In addition, identification of the conditions needed for the full in vitro maturation of hepatic progenitor cells to quiescent, functional hepatocyte-like cells opens promising perspectives.

Rat hepatocyte cultures: conventional monolayer and cocultures with rat liver epithelial cells

Primary cultures of hepatocytes are useful tools for both short- and long-term pharmacotoxicological research. Under conventional conditions, isolated hepatocytes form a monolayer and survive for about 1 wk but lose some liver-specific functions, including xenobiotic biotransformation. In comparison with the conventional monolayer culture model, cocultures with rat liver epithelial cells (RLECs) have an extended lifespan and better maintain their drug-metabolizing capacity, owing to the presence of cell-cell interactions. In this chapter, techniques for setting up conventional monolayer cultures and cocultures of hepatocytes with RLECs (including isolation, culture, and cryopreservation of RLECs) are described in detail. In addition, comments derived from our own experience are given for successfully culturing primary hepatocytes.

Rat hepatocyte cultures: collagen gel sandwich and immobilization cultures

Mimicking the in vivo microenvironment is one of the current strategies to maintain liver-specific functionality in primary cultured hepatocytes for long periods. Freshly isolated hepatocytes entrapped in collagen gel type I (collagen gel immobilization culture) or sandwiched between two layers of hydrated collagen type I (collagen gel sandwich culture) are known to display liver-specific functions (e.g., biotransformation capacity) for more than 6 wk. We describe how to set up both types of organotypical hepatocyte culture systems. Besides a detailed protocol, we give some practical tips, taken from our own experience with long-term hepatocyte culture.

Hepatocytes in suspension

Isolated hepatocytes are a physiologically relevant in vitro model exhibiting intact subcellular organelles, xenobiotic transport, and integrated phase I and phase II biotransformation. They represent the "gold standard" for investigating xenobiotic biotransformation and metabolic bioactivation. When used in suspension, they provide an easy-to-handle and relatively cheap in vitro system that can be used for up to 4 h. The use of animal- and human-derived hepatocytes allows interspecies comparisons of metabolic properties. In contrast with microsomes, which are easily prepared from human liver tissue and can be stored in liquid nitrogen with minimal loss of functionality, cryopreservation of isolated human hepatocytes has been shown to be more difficult: after thawing losses of cell viability and biotransformation capacity occur. We provide general recommendations for the appropriate use of hepatocytes in suspension for pharmaco-toxicological studies. We also provide protocols for the cryopreservation of freshly isolated hepatocytes and their handling on thawing.

Isolation of rat hepatocytes

In vitro models, based on liver cells or tissues, are indispensable in the early preclinical phase of drug development. An important breakthrough in establishing cell models has been the successful high-yield preparation of intact hepatocytes. In this chapter, the practical aspects of the two-step collagenase perfusion method, modified from the original procedure of Seglen, are outlined. Although applicable to the liver of various species, including human, the practical aspects of the method are explained here for rat liver. Critical parameters for the successful isolation of primary rat hepatocytes are highlighted and a troubleshooting guide is provided. In addition, a new development based on the inhibition of histone deacetylase activity is presented. This approach allows inhibition of cell-cycle reentry during hepatocyte isolation, a process known to underlie the dedifferentiation process of cultured hepatocytes.

Differential effects of histone deacetylase inhibitors in tumor and normal cells-what is the toxicological relevance?

Histone deacetylase (HDAC) inhibitors target key steps of tumor development: They inhibit proliferation, induce differentiation and/or apoptosis, and exhibit potent antimetastatic and antiangiogenic properties in transformed cells in vitro and in vivo. Preliminary studies in animal models have revealed a relatively high tumor selectivity of HDAC inhibitors, strenghtening their promising potential in cancer chemotherapy. Until now, preclinical in vitro research has almost exclusively been performed in cancer cell lines and oncogene-transformed cells. However, as cell proliferation and apoptosis are essential for normal tissue and organ homeostasis, it is important to investigate how HDAC inhibitors influence the regulation of and interplay between proliferation, differentiation, and apoptosis in primary cells as well. This review highlights the discrepancies in molecular events triggered by trichostatin A, the reference compound of hydroxamic acid-containing HDAC inhibitors, in hepatoma cells and primary hepatocytes (which are key targets for drug-induced toxicity). The implications of these differential outcomes in both cell types are discussed with respect to both toxicology and drug development. In view of the future use of HDAC inhibitors as cytostatic drugs, it is highly recommended to include both tumor cells and their healthy counterparts in preclinical developmental studies. Screening the toxicological properties of compounds early in their development process, using a battery of different cell types, will enable researchers to discard those compounds bearing undesirable adverse activity before entering into expensive clinical trials. This will not only reduce the risk for harmful exposure of patients but also save time and money.

Trichostatin A-like hydroxamate histone deacetylase inhibitors as therapeutic agents: toxicological point of view

Modulation of chromatin structure through histone acetylation/deacetylation is known to be one of the major mechanisms involved in the regulation of gene expression. Two opposing enzyme activities determine the acetylation state of histones: histone acetyltransferases (HATs) and histone deacetylases (HDACs), respectively acetylating or deacetylating the epsilon-amino groups of lysine residues located in the amino-terminal tails of the histones. In general, transcriptionally active chromatin is associated with hyperacetylated histones, whilst silenced chromatin is linked to hypoacetylated histones. A number of structurally divergent classes of HDAC inhibitors have been identified. They have been shown to induce cell cycle arrest, terminal differentiation and/or apoptosis in various cancer cell lines and inhibit tumor growth in animals. In particular, the reversible HDAC inhibitor Trichostatin A (TSA) and its hydroxamate analogues can effectively and selectively induce tumor growth arrest at very low concentrations (nano- to micromolar range). They form a group of so-called promising antitumor agents of which some are currently under clinical trial. Since the selection of a molecule for further drug development requires a balance of biological potency, safety and pharmacokinetics, it is of paramount importance to elucidate the pharmacokinetic and toxicological properties of these HDAC inhibitors before they can be considered as potential new drugs. Primary hepatocytes and their cultures are well-differentiated in vitro models and can be used to study simultaneously the biological effects of HDAC inhibitors and their biotransformation. The present review provides a state-of-the-art of our current knowledge of the pharmacological and toxicological effects on proliferating cells of TSA and its hydroxamate-based structural analogues. Besides a theoretical basis, an overview of the experimental results, obtained by the authors using primary rat hepatocytes as an in vitro model, is given.

Trichostatin A induces differential cell cycle arrests but does not induce apoptosis in primary cultures of mitogen-stimulated rat hepatocytes

BACKGROUND/AIMS

The effects of Trichostatin A (TSA), a drug candidate for cancer therapy, on proliferation and survival of primary hepatocytes, the major site of xenobiotic biotransformation and primary target of drug-induced toxicity, were investigated.

METHODS

DNA replication was measured using [methyl-3H]-thymidine incorporation. Cell cycle markers were analyzed by Western and Northern blottings. Necrosis and apoptosis were monitored by LDH release, caspase-3-activation, respectively.

RESULTS

We identified two distinct cell cycle arrests, prior DNA replication, in two experimental conditions. First, perfusion of the liver in presence of TSA, prevented c-jun and cyclin D1 induction, characteristic for G1 entry and progression through late G1, respectively. Secondly, TSA treatment of isolated hepatocytes, located in early G1, led to an early S-phase arrest evidenced by the absence of the S/G2/M marker, CDK1. TSA upregulated the expression of the anti-apoptotic protein Bcl(xL) and did not increase caspase-3-activity and LDH release.

CONCLUSIONS

TSA inhibits hepatocyte proliferation at different steps of the cell cycle. Our data suggest that this inhibition may involve downregulation of distinct subsets of genes. TSA does not induce apoptosis in primary hepatocytes, in contrast to what has been observed in hepatoma cells. This finding supports its use in the treatment of proliferative disorders.

Amide analogues of TSA: synthesis, binding mode analysis and HDAC inhibition

The synthesis of new amide type histone deacetylase inhibitors is described, having an (R)-methyl substituent and a diene or saturated structure of the chain linking the hydroxamic acid and dimethylaminobenzoyl groups. The saturated compound shows stronger HDAC inhibition than the unsaturated analogue. Molecular modeling suggests that the flexibility of the linker chain is important for an optimal orientation of the dimethylaminobenzoyl group in the enzyme.

Major phase I biotransformation pathways of Trichostatin a in rat hepatocytes and in rat and human liver microsomes

Phase I biotransformation of Trichostatin A (TSA), a histone deacetylase inhibitor with promising antifibrotic and antitumoral properties, was investigated in rat and human liver microsomes and in suspensions of rat hepatocytes. TSA (50 micro M) was readily and completely metabolized by rat hepatocytes in suspension (2 x 10(6) cells/ml), whereafter its phase I metabolites were separated by high-performance liquid chromatography and detected with simultaneous UV and electrospray ionization mass spectrometry (ESI-MS). ESI tandem mass spectrometry (ESI-MS/MS) was used to identify the metabolites. Two major phase I biotransformation pathways in rat hepatocytes were shown to be N-demethylation and reduction of the hydroxamic acid function to its corresponding amide. N-monodemethylated TSA and TSA amide were preferentially formed during the first 20 min of exposure, and N-monodemethylated TSA amide appeared as the main metabolite after a 30 min incubation period. At this time, virtually all TSA had been metabolized. Trichostatic acid, N-monodemethylated Trichostatic acid, and N-didemethylated TSA were identified as minor metabolites. Longer incubation led to the formation of N-didemethylated TSA amide as the main metabolite. Lower concentrations of TSA (5 and 25 micro M) formed relatively higher amounts of N-demethylated, nonreduced metabolites. Incubations of TSA with rat and human microsomal suspensions, however, led to an incomplete biotransformation with the formation of two major metabolites, N-mono- and N-didemethylated TSA. Traces of Trichostatic acid, TSA amide, N-mono- and N-didemethylated TSA amide were also detected. This study is the first to show that TSA undergoes intensive phase I biotransformation in rat hepatocytes. This has important consequences for its potential development as a drug, since rapid biotransformation resulting in a short exposure to the pharmacologically active parent compound, and a complex mixture of metabolites is usually not desired. Further biotransformation studies of TSA and structural analogs with antitumoral and antifibrotic properties need to be performed in cultured intact hepatocytes, in particular since one of the major phase I biotransformation pathways is catalyzed by nonmicrosomal enzymes.

Effect of oxygen concentration on the expression of glutathione S-transferase activity in periportal and perivenous rat hepatocyte cultures

Cultures of perivenous (PV) and periportal (PP) hepatocytes could provide suitable in vitro models for studying the zone-specific hepatotoxic potential of xenobiotics. However, it is not known whether cultured PP and PV hepatocytes keep their phenotypes when the microcirculation of the liver changes. This question has been studied by culturing rat hepatocytes at 13 and 4% (v/v) O(2), respectively, mimicking the acinar oxygen gradient. PP and PV adult rat hepatocytes were isolated by digitonin-collagenase in situ perfusion and cultured on plastic Falcon and gas-permeable Petriperm dishes in Williams' E medium and kept at 13 and 4% (v/v) O(2), respectively. Cultures at 20% (v/v) O(2) on plastic dishes served as a control. Two types of cultures were studied, namely conventional cultures either unsupplemented or supplemented with 30 mM pyruvate. The activities of glutamine synthetase (GS) and glutathione S-transferase (GST) were measured in freshly isolated PP and PV hepatocytes and all cultures. The heterogeneous expression of GS (PV>PP), observed in freshly isolated hepatocytes, was kept for at least 4 days in culture. Total, Mu and Alpha class GST activities were predominantly expressed in PV freshly isolated cells. However, no beneficial effect could be observed in culture by exposing the cells to their specific in vivo oxygen concentration. The best maintenance of GST PV predominance in culture was observed in Petriperm dishes at 20% (v/v) O(2), as well in pyruvate-supplemented as unsupplemented cultures. PV GST predominance was thus kept in particular when the highest oxygen concentration was used and made available to the cells through the gas-permeable membranes. The results on GS PV predominance support these findings.