Use of hepatocyte cultures for the study of hepatotoxic compounds

Obesity III: Obesogen assays: Limitations, strengths, and new directions

There is increasing evidence of a role for environmental contaminants in disrupting metabolic health in both humans and animals. Despite a growing need for well-understood models for evaluating adipogenic and potential obesogenic contaminants, there has been a reliance on decades-old in vitro models that have not been appropriately managed by cell line providers. There has been a quick rise in available in vitro models in the last ten years, including commercial availability of human mesenchymal stem cell and preadipocyte models; these models require more comprehensive validation but demonstrate real promise in improved translation to human metabolic health. There is also progress in developing three-dimensional and co-culture techniques that allow for the interrogation of a more physiologically relevant state. While diverse rodent models exist for evaluating putative obesogenic and/or adipogenic chemicals in a physiologically relevant context, increasing capabilities have been identified for alternative model organisms such as Drosophila, C. elegans, zebrafish, and medaka in metabolic health testing. These models have several appreciable advantages, including most notably their size, rapid development, large brood sizes, and ease of high-resolution lipid accumulation imaging throughout the organisms. They are anticipated to expand the capabilities of metabolic health research, particularly when coupled with emerging obesogen evaluation techniques as described herein.

Quantitation of Protein Adducts of Aristolochic Acid I by Liquid Chromatography-Tandem Mass Spectrometry: A Novel Method for Biomonitoring Aristolochic Acid Exposure

Emerging evidence suggests that chronic exposure to aristolochic acids (AAs) is one of the etiological pathways leading to chronic kidney disease (CKD). Due to the traditional practice of herbal medicine and AA-containing plants being used extensively as medicinal herbs, over 100 million East Asians are estimated to be at risk of AA poisoning. Given that the chronic nephrotoxicity of AAs only manifests itself after decades of exposure, early diagnosis of AA exposure could allow for timely intervention and disease risk reduction. However, an early detection method is not yet available, and diagnosis can only be established at the end stage of CKD. The goal of this study was to develop a highly sensitive and selective method to quantitate protein adducts of aristolochic acid I (AAI) as a biomarker of AA exposure. The method entails the release of protein-bound aristolactam I (ALI) by heat-assisted alkaline hydrolysis, extraction of ALI, addition of internal standard, and quantitation by liquid chromatography-tandem mass spectrometric analysis. Accuracy and precision of the method were critically evaluated using a synthetic ALI-containing glutathione adduct. The validated method was subsequently used to detect dose-dependent formation of ALI-protein adducts in human serum albumin exposed to AAI and in proteins isolated from the tissues and sera of AAI-exposed rats. Our time-dependent study showed that ALI-protein adducts remained detectable in rats even at 28 days postdosing. It is anticipated that the developed method will fill the technical gap in diagnosing AA intoxication and facilitate the biomonitoring of human exposures to AAs.

Obeticholic acid, a selective farnesoid X receptor agonist, regulates bile acid homeostasis in sandwich-cultured human hepatocytes

Farnesoid X receptor (FXR) is a master regulator of bile acid homeostasis through transcriptional regulation of genes involved in bile acid synthesis and cellular membrane transport. Impairment of bile acid efflux due to cholangiopathies results in chronic cholestasis leading to abnormal elevation of intrahepatic and systemic bile acid levels. Obeticholic acid (OCA) is a potent and selective FXR agonist that is 100‐fold more potent than the endogenous ligand chenodeoxycholic acid (CDCA). The effects of OCA on genes involved in bile acid homeostasis were investigated using sandwich‐cultured human hepatocytes. Gene expression was determined by measuring mRNA levels. OCA dose‐dependently increased fibroblast growth factor‐19 (FGF‐19) and small heterodimer partner (SHP) which, in turn, suppress mRNA levels of cholesterol 7‐alpha‐hydroxylase (CYP7A1), the rate‐limiting enzyme for de novo synthesis of bile acids. Consistent with CYP7A1 suppression, total bile acid content was decreased by OCA (1 μmol/L) to 42.7 ± 20.5% relative to control. In addition to suppressing de novo bile acids synthesis, OCA significantly increased the mRNA levels of transporters involved in bile acid homeostasis. The bile salt excretory pump (BSEP), a canalicular efflux transporter, increased by 6.4 ± 0.8‐fold, and the basolateral efflux heterodimer transporters, organic solute transporter α (OST_α) and OST_β increased by 6.4 ± 0.2‐fold and 42.9 ± 7.9‐fold, respectively. The upregulation of BSEP and OST_α and OST_β, by OCA reduced the intracellular concentrations of d₈‐TCA, a model bile acid, to 39.6 ± 8.9% relative to control. These data demonstrate that OCA does suppress bile acid synthesis and reduce hepatocellular bile acid levels, supporting the use of OCA to treat bile acid‐induced toxicity observed in cholestatic diseases.

Metabolism of the Tobacco Carcinogen 2-Amino-9H-pyrido[2,3-b]indole (AαC) in Primary Human Hepatocytes

2-Amino-9H-pyrido[2,3-b]indole (AαC) is the most abundant carcinogenic heterocyclic aromatic amine (HAA) formed in mainstream tobacco smoke. AαC is a liver carcinogen in rodents, but its carcinogenic potential in humans is not known. To obtain a better understanding of the genotoxicity of AαC in humans, we have investigated its metabolism and its ability to form DNA adducts in human hepatocytes. Primary human hepatocytes were treated with AαC at doses ranging from 0.1-50 μM, and the metabolites were characterized by ultra-performance LC/ion trap multistage mass spectrometry (UPLC/MSn). Six major metabolites were identified: a ring-oxidized doubly conjugated metabolite, N2-acetyl-2-amino-9H-pyrido[2,3-b]indole-6-yl-oxo-(β-d-glucuronic acid) (N2-acetyl-AαC-6-O-Gluc); two ring-oxidized glucuronide (Gluc) conjugates: 2-amino-9H-pyrido[2,3-b]indol-3-yl-oxo-(β-d-glucuronic acid) (AαC-3-O-Gluc) and 2-amino-9H-pyrido[2,3-b]indol-6-yl-oxo-(β-d-glucuronic acid) (AαC-6-O-Gluc); two sulfate conjugates, 2-amino-9H-pyrido[2,3-b]indol-3-yl sulfate (AαC-3-O-SO3H) and 2-amino-9H-pyrido[2,3-b]indol-6-yl sulfate (AαC-6-O-SO3H); and the Gluc conjugate, N2-(β-d-glucosidurony1)-2-amino-9H-pyrido[2,3-b]indole (AαC-N2-Gluc). In addition, four minor metabolites were identified: N2-acetyl-9H-pyrido[2,3-b]indol-3-yl sulfate (N2-acetyl-AαC-3-O-SO3H), N2-acetyl-9H-pyrido[2,3-b]indol-6-yl sulfate (N2-acetyl-AαC-6-O-SO3H), N2-acetyl-2-amino-9H-pyrido[2,3-b]indol-3-yl-oxo-(β-d-glucuronic acid) (N2-acetyl-AαC-3-O-Gluc), and O-(β-d-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (AαC-HN2-O-Gluc). The latter metabolite, AαC-HN2-O-Gluc is a reactive intermediate that binds to DNA to form the covalent adduct N-(2'-deoxyguanosin-8-yl)-2-amino-9H-pyrido[2,3-b]indole (dG-C8-AαC). Preincubation of hepatocytes with furafylline, a selective mechanism-based inhibitor of P450 1A2, resulted in a strong decrease in the formation of AαC-HN2-O-Gluc and a concomitant decrease in DNA adduct formation. Our findings describe the major pathways of metabolism of AαC in primary human hepatocytes and reveal the importance of N-acetylation and glucuronidation in metabolism of AαC. P450 1A2 is a major isoform involved in the bioactivation of AαC to form the reactive AαC-HN2-O-Gluc conjugate and AαC-DNA adducts.

Biomonitoring Human Albumin Adducts: The Past, the Present, and the Future

Serum albumin (Alb) is the most abundant protein in blood plasma. Alb reacts with many carcinogens and/or their electrophilic metabolites. Studies conducted over 20 years ago showed that Alb forms adducts with the human carcinogens aflatoxin B1 and benzene, which were successfully used as biomarkers in molecular epidemiology studies designed to address the role of these chemicals in cancer risk. Alb forms adducts with many therapeutic drugs or their reactive metabolites such as β-lactam antibiotics, acetylsalicylic acid, acetaminophen, nonsteroidal anti-inflammatory drugs, chemotherapeutic agents, and antiretroviral therapy drugs. The identification and characterization of the adduct structures formed with Alb have served to understand the generation of reactive metabolites and to predict idiosyncratic drug reactions and toxicities. The reaction of candidate drugs with Alb is now exploited as part of the battery of screening tools to assess the potential toxicities of drugs. The use of gas chromatography-mass spectrometry, liquid chromatography, or liquid chromatography-mass spectrometry (LC-MS) enabled the identification and quantification of multiple types of Alb xenobiotic adducts in animals and humans during the past three decades. In this perspective, we highlight the history of Alb as a target protein for adduction to environmental and dietary genotoxicants, pesticides, and herbicides, common classes of medicinal drugs, and endogenous electrophiles, and the emerging analytical mass spectrometry technologies to identify Alb-toxicant adducts in humans.

Microengineered cell and tissue systems for drug screening and toxicology applications: Evolution of in-vitro liver technologies

The liver performs many key functions, the most prominent of which is serving as the metabolic hub of the body. For this reason, the liver is the focal point of many investigations aimed at understanding an organism's toxicological response to endogenous and exogenous challenges. Because so many drug failures have involved direct liver toxicity or other organ toxicity from liver generated metabolites, the pharmaceutical industry has constantly sought superior, predictive in-vitro models that can more quickly and efficiently identify problematic drug candidates before they incur major development costs, and certainly before they are released to the public. In this broad review, we present a survey and critical comparison of in-vitro liver technologies along a broad spectrum, but focus on the current renewed push to develop "organs-on-a-chip". One prominent set of conclusions from this review is that while a large body of recent work has steered the field towards an ever more comprehensive understanding of what is needed, the field remains in great need of several key advances, including establishment of standard characterization methods, enhanced technologies that mimic the in-vivo cellular environment, and better computational approaches to bridge the gap between the in-vitro and in-vivo results.

Long-term coculture strategies for primary hepatocytes and liver sinusoidal endothelial cells

Hepatocytes and their in vitro models are essential tools for preclinical screening studies for drugs that affect the liver. Most of the current models primarily focus on hepatocytes alone and lack the contribution of non-parenchymal cells (NPCs), which are significant through both molecular and the response of the NPCs themselves. Models that incorporate NPCs alongside hepatocytes hold the power to enable more realistic recapitulation and elucidation of cell interactions and cumulative drug response. Hepatocytes and liver sinusoidal endothelial cells (LSECs) account for ∼ 80% of the liver mass where the LSECs line the walls of blood vessels, and act as a barrier between hepatocytes and blood. Culturing LSECs with hepatocytes to generate multicellular physiologically relevant in vitro liver models has been a major hurdle since LSECs lose their phenotype rapidly after isolation. To this end, we describe the application of collagen gel (1) in a sandwich and (2) as an intervening extracellular matrix layer to coculture hepatocytes with LSECs for extended periods. These coculture configurations provide environments wherein hepatocyte and LSECs, through cell-cell contacts and/or secretion factors, lead to enhanced function and stability of the cocultures. Our results show that in these configurations, hepatocytes and LSECs maintained their phenotypes when cultured together as a mixture, and showed stable secretion and metabolic activity for up to 4 weeks. Immunostaining for sinusoidal endothelial 1 (SE-1) antibody demonstrated retention of LSEC phenotype during the culture period. In addition, LSECs cultured alone maintained high viability and SE-1 expression when cultured within a collagen sandwich configuration up to 4 weeks. Albumin production of the cocultures was 10-15 times higher when LSECs were cultured as a bottom layer (with an intervening collagen layer) and as a mixture in a sandwich configuration, and native CYP 1A1/2 activity was at least 20 times higher than monoculture controls. Together, these data suggest that collagen gel-based hepatocyte-LSEC cocultures are highly suitable models for stabilization and long-term culture of both cell types. In summary, these results indicate that collagen gel-based hepatocyte-LSEC coculture models are promising for in vitro toxicity testing, and liver model development studies.

MicroRNA regulation of DNA repair gene expression in 4-aminobiphenyl-treated HepG2 cells

We examined the role of miRNAs in DNA damage response in HepG2 cells following exposure to 4-aminobiphenyl (4-ABP). The arylamine 4-ABP is a human carcinogen. Using the Comet assay, we showed that 4-ABP (18.75-300μM) induces DNA damage in HepG2 cells after 24h. DNA damage signaling pathway-based PCR arrays were used to investigate expression changes in genes involved in DNA damage response. Results showed down-regulation of 16 DNA repair-related genes in 4-ABP-treated cells. Among them, the expression of selected six genes (UNG, LIG1, EXO1, XRCC2, PCNA, and FANCG) from different DNA repair pathways was decreased with quantitative real-time PCR (qRT-PCR). In parallel, using the miRNA array, we reported that the expression of 27 miRNAs in 4-ABP-treated cells was at least 3-fold higher than that in the control group. Of these differential 27 miRNAs, the most significant expression of miRNA-513a-5p and miRNA-630 was further validated by qRT-PCR, and was predicted to be implicated in the deregulation of FANCG and RAD18 genes, respectively, via bioinformatic analysis. Both FANCG and RAD18 proteins were found to be down-regulated in 4-ABP-treated cells. In addition, overexpression and knockdown of miRNA-513a-5p and miRNA-630 reduced and increased the expression of FANCG and RAD18 proteins, respectively. Based on the above results, we indicated that miRNA-513a-5p and miRNA-630 could play a role in the suppression of DNA repair genes, and eventually lead to DNA damage.

A novel ultrathin collagen nanolayer assembly for 3-D microtissue engineering: Layer-by-layer collagen deposition for long-term stable microfluidic hepatocyte culture

The creation of stable hepatocyte cultures using cell-matrix interactions has proven difficult in microdevices due to dimensional constraints limiting the utility of classic tissue culture techniques that involve the use of hydrogels such as the collagen "double gel" or "overlay". To translate the collagen overlay technique into microdevices, we modified collagen using succinylation and methylation reactions to create polyanionic and polycationic collagen solutions, and deposited them layer-by-layer to create ultrathin collagen nanolayers on hepatocytes. These ultrathin collagen layers covered hepatocytes in microdevices and 1) maintained cell morphology, viability, and polarity, 2) induced bile canalicular formation and actin reorganization, and 3) maintained albumin and urea secretions and CYP activity similar to those observed in hepatocytes in collagen double gel hepatocytes in plate cultures. Beyond the immediate applications of this technique to create stable, in vitro microfluidic hepatocyte cultures for drug toxicity testing, this technique is generally applicable as a thin biomaterial for other 3D microtissues.

DNA adducts of the tobacco carcinogens 2-amino-9H-pyrido[2,3-b]indole and 4-aminobiphenyl are formed at environmental exposure levels and persist in human hepatocytes

Aromatic amines and structurally related heterocyclic aromatic amines (HAAs) are produced during the combustion of tobacco or during the high-temperature cooking of meat. Exposure to some of these chemicals may contribute to the etiology of several common types of human cancers. 2-Amino-9H-pyrido[2,3-b]indole (AαC) is the most abundant HAA formed in mainstream tobacco smoke: it arises in amounts that are 25-100 times greater than the levels of the arylamine, 4-aminobiphenyl (4-ABP), a human carcinogen. 2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) is a prevalent HAA formed in cooked meats. AαC and MeIQx are rodent carcinogens; however, their carcinogenic potency in humans is unknown. A preliminary assessment of the carcinogenic potential of these HAAs in humans was conducted by examining the capacity of primary human hepatocytes to form DNA adducts of AαC and MeIQx, in comparison to 4-ABP, followed by the kinetics of DNA adduct removal by cellular enzyme repair systems. The principal DNA adducts formed were N-(deoxyguanosin-8-yl) (dG-C8) adducts. Comparable levels of DNA adducts were formed with AαC and 4-ABP, whereas adduct formation was ∼5-fold lower for MeIQx. dG-C8-AαC and dG-C8-4-ABP were formed at comparable levels in a concentration-dependent manner in human hepatocytes treated with procarcinogens over a 10,000-fold concentration range (1 nM-10 μM). Pretreatment of hepatocytes with furafylline, a selective inhibitor of cytochrome P450 1A2, resulted in a strong diminution of DNA adducts signifying that P450 1A2 is a major P450 isoform involved in bioactivation of these procarcinogens. The kinetics of adduct removal varied for each hepatocyte donor. Approximately half of the DNA adducts were removed within 24 h of treatment; however, the remaining lesions persisted over 5 days. The high levels of AαC present in tobacco smoke and its propensity to form persistent DNA adducts in human hepatocytes suggest that AαC can contribute to DNA damage and the risk of hepatocellular cancer in smokers.

Metabolic characterization of primary rat hepatocytes cultivated in parallel microfluidic biochips

The functionality of primary rat hepatocytes was assessed in an Integrated Dynamic Cell Cultures in Microsystem (IDCCM) device. We characterized the hepatocytes over 96 h of culture and evaluated the impact of dynamic cell culture on their viability, inducibility, and metabolic activity. Reverse Transcription quantitative Polymerase Chain Reaction (RTqPCR) was performed on selected genes: liver transcription factors (HNF4α and CEBP), nuclear receptors sensitive to xenobiotics (AhR, PXR, CAR, and FXR), cytochromes P450 (CYPs) (1A2, 3A2, 3A23/3A1, 7A1, 2B1, 2C6, 2C, 2D1, 2D2, and 2E1), phase II metabolism enzymes (GSTA2, SULT1A1, and UGT1A6), ABC transporters (ABCB1b and ABCC2), and oxidative stress related enzymes (HMOX1 and NQO1). Microperfused-cultured hepatocytes remained viable and differentiated with in vivo-like phenotype and genotype. In contrast with postadhesion gene levels, the first 48 h of perfusion enhanced the expression of xenosensors and their target CYPs. Furthermore, CYP3A1, CYP2B1, GSTA2, SULT1A1, UGT1A1, ABCB1b, and ABCC2 were upregulated in IDCCM and reached above postextraction levels all along the duration of culture. Metabolic activities were also confirmed with the detection of metabolism rate and induced mRNAs after exposure to several inducers: 3-methylcholanthrene, caffeine, phenacetin, paracetamol,, and midazolam. Finally, this metabolic characterization confirms that IDCCM is able to maintain rat hepatocytes functions to investigate drug metabolism.

Evaluation of drug-induced tissue injury by measuring alanine aminotransferase (ALT) activity in silkworm hemolymph

Background

Our previous studies suggest silkworms can be used as model animals instead of mammals in pharmacologic studies to develop novel therapeutic medicines. We examined the usefulness of the silkworm larvae Bombyx mori as an animal model for evaluating tissue injury induced by various cytotoxic drugs. Drugs that induce hepatotoxic effects in mammals were injected into the silkworm hemocoel, and alanine aminotransferase (ALT) activity was measured in the hemolymph 1 day later.

Results

Injection of CCl₄ into the hemocoel led to an increase in ALT activity. The increase in ALT activity was attenuated by pretreatment with N-acetyl-_L-cysteine. Injection of benzoic acid derivatives, ferric sulfate, sodium valproate, tetracycline, amiodarone hydrochloride, methyldopa, ketoconazole, pemoline (Betanamin), N-nitroso-fenfluramine, and _D-galactosamine also increased ALT activity.

Conclusions

These findings indicate that silkworms are useful for evaluating the effects of chemicals that induce tissue injury in mammals.

DNA adduct formation of 4-aminobiphenyl and heterocyclic aromatic amines in human hepatocytes

DNA adduct formation of the aromatic amine, 4-aminobiphenyl (4-ABP), a known human carcinogen present in tobacco smoke, and the heterocyclic aromatic amines (HAAs), 2-amino-9H-pyrido[2,3-b]indole (AαC), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), and 2-amino-3,8-dimethylmidazo[4,5-f]quinoxaline (MeIQx), potential human carcinogens, which are also present in tobacco smoke or formed during the high-temperature cooking of meats, was investigated in freshly cultured human hepatocytes. The carcinogens (10 μM) were incubated with hepatocytes derived from eight different donors for time periods up to 24 h. The DNA adducts were quantified by liquid chromatography-electrospray ionization mass spectrometry with a linear quadrupole ion trap mass spectrometer. The principal DNA adducts formed for all of the carcinogens were N-(deoxyguanosin-8-yl) (dG-C8) adducts. The levels of adducts ranged from 3.4 to 140 adducts per 10(7) DNA bases. The highest level of adduct formation occurred with AαC, followed by 4-ABP, then by PhIP, MeIQx, and IQ. Human hepatocytes formed dG-C8-HAA-adducts at levels that were up to 100-fold greater than the amounts of adducts produced in rat hepatocytes. In contrast to HAA adducts, the levels of dG-C8-4-ABP adduct formation were similar in human and rat hepatocytes. These DNA binding data demonstrate that the rat, an animal model that is used for carcinogenesis bioassays, significantly underestimates the potential hepatic genotoxicity of HAAs in humans. The high level of DNA adducts formed by AαC, a carcinogen produced in tobacco smoke at levels that are up to 100-fold higher than the amounts of 4-ABP, is noteworthy. The possible causal role of AαC in tobacco-associated cancers warrants investigation.

Metabolic control through hepatocyte and adipose tissue cross-talk in a multicompartmental modular bioreactor

Physiological processes involve a complex network of signaling molecules that act through paracrinal or endocrinal pathways; however, traditional in vitro models cannot mimic these interactions because of the lack of a dynamic cross-talk between cells belonging to different tissues. The multicompartmental modular bioreactor is a novel cell culture system where hepatocytes and adipose tissue are shown to interact in a more physiological manner. In the multicompartmental modular bioreactor, cells and tissues can be cultured in a common medium, which flows through the system acting as the bloodstream. Primary rat hepatocytes and adipose tissue were cultured separately and together in conventional conditions and in the bioreactor. Urea synthesis, albumin secretion, glycerol, free fatty acid, and glucose concentrations were analyzed and compared. The dynamic connected culture of adipose tissue and hepatocytes led to a significant enhancement of hepatic function in terms of increase of albumin and urea production with respect to conventional cultures. Interestingly, the glycerol gradually released from adipose tissue was buffered in the dynamic connected culture, manifesting a homeostatic-like control. These data show that the dynamic culture not only improves hepatocyte function, but also allows a cross-talk between tissues, leading to enhanced metabolic regulation in vitro.

Validation of reference gene stability for APAP hepatotoxicity studies in different in vitro systems and identification of novel potential toxicity biomarkers

Liver cell lines and primary hepatocytes are becoming increasingly valuable for in vitro toxicogenomic studies, with RT-qPCR enabling the analysis of gene expression profiles following exposure to potential hepatotoxicants. Supporting the accurate normalisation of RT-qPCR data requires the identification of reference genes which have stable expression during in vitro toxicology studies. Therefore, we performed a comprehensive analysis of reference gene stability in two routinely used cell types, (HepG2 cells and primary rat hepatocytes), and two in vitro culture systems, (2D monolayer and 3D scaffolds). A robust reference gene validation strategy was performed, consisting of geNorm analysis, to test for pair wise variation in gene expression, and statistical analysis using analysis of variance. This strategy identified stable reference genes with respect to acetaminophen treatment and time in HepG2 cells (GAPDH and PPIA), and with respect to acetaminophen treatment and culture condition in primary hepatocytes (18S rRNA and α-tubulin). Following the selection of reference genes, the novel target genes E2F7 and IL-11RA were identified as potential toxicity biomarkers for acetaminophen treatment. We conclude that accurate quantification of gene expression requires the use of a validated normalisation strategy for each species and experimental system employed.

Xenobiotic-metabolizing enzyme and transporter gene expression in primary cultures of human hepatocytes modulated by ToxCast chemicals

Primary human hepatocyte cultures are useful in vitro model systems of human liver because when cultured under appropriate conditions the hepatocytes retain liver-like functionality such as metabolism, transport, and cell signaling. This model system was used to characterize the concentration- and time-response of the 320 ToxCast chemicals for changes in expression of genes regulated by nuclear receptors. Fourteen gene targets were monitored in quantitative nuclease protection assays: six representative cytochromes P-450, four hepatic transporters, three Phase II conjugating enzymes, and one endogenous metabolism gene involved in cholesterol synthesis. These gene targets are sentinels of five major signaling pathways: AhR, CAR, PXR, FXR, and PPARalpha. Besides gene expression, the relative potency and efficacy for these chemicals to modulate cellular health and enzymatic activity were assessed. Results demonstrated that the culture system was an effective model of chemical-induced responses by prototypical inducers such as phenobarbital and rifampicin. Gene expression results identified various ToxCast chemicals that were potent or efficacious inducers of one or more of the 14 genes, and by inference the 5 nuclear receptor signaling pathways. Significant relative risk associations with rodent in vivo chronic toxicity effects are reported for the five major receptor pathways. These gene expression data are being incorporated into the larger ToxCast predictive modeling effort.

The use of hepatocytes to investigate drug toxicity

The liver is very active in metabolizing foreign compounds and the major target for toxicity caused by drugs. Hepatotoxicity may be the result of the drug itself or, more frequently, a result of the bioactivation process and the production of reactive metabolites. Prioritization of compounds based on human hepatotoxicity potential is currently a key unmet need in drug discovery, as it can become a major problem for several lead compounds in later stages of the drug discovery pipeline. Therefore, evaluation of potential hepatotoxicity represents a critical step in the development of new drugs. Cultured hepatocytes are increasingly used by the pharmaceutical industry for the screening of hepatotoxic potential of new molecules. Hepatocytes in culture retain hepatic key functions and constitute a valuable tool to identify chemically induced cellular damage. Their use has notably contributed to the understanding of mechanisms responsible for hepatotoxicity (disruption of cellular energy status, alteration of Ca(2+) homeostasis, inhibition of transport systems, metabolic activation, oxidative stress, covalent binding, etc.). Assessment of current cytotoxicity and hepatic-specific biochemical effects is limited by the inability to measure a wide spectrum of potential mechanistic changes involved in the drug-induced toxic injury. A convenient selection of endpoints allows a multiparametric evaluation of drug toxicity. In this regard, cytomic, proteomic, toxicogenomic and metabonomic approaches help to define patterns of hepatotoxicity for early identification of potential adverse effects of the drug to the liver.

In vitro assessment of P450 induction potential of novel chemopreventive agents SR13668, 9-cis-UAB30, and pentamethychromanol in primary cultures of human hepatocytes

Several compounds, including 2,10-dicarbethoxy-6-methoxy-5,7-dihydroindolo[2,3-b]carbazole (SR13668), (2E,4E,6Z,8E)-8-(3',4'-dihydro-1'(2'H)-napthalen-1'-ylidene)-3,7-dimethyl-2,4,6-octatrienoic acid (9-cis-UAB30), and 2,2,5,7,8-pentamethyl-6-chromanol (PMCol), were selected as promising chemopreventive agents and have entered preclinical trials for cancer prevention. The potential for adverse drug events resulting from interactions with other administered drugs, food components, or food additives presents an important question. Among the most important drug-drug interactions (DDI) is the potential of a new chemical entity (NCE) to induce cytochrome P450 enzymes (P450). Drug induction of P450 enzymes can lead to adverse drug interactions by increasing the metabolism of other drugs that are substrates for the induced isoform. Currently, sandwich cultured primary human hepatocytes are the standard for predicting human P450 enzyme induction in vitro as these cells retain the ability to respond to prototypical P450 inducers with the same specificity and potency exhibited in vivo. Therefore, a select panel of inducible P450 target genes (CYP1A2, CYP2B6, and CYP3A4) and their induction activity (measured by LC-MS/MS of respective marker substrate metabolites) were monitored in cultured hepatocytes following treatment with SR13668, 9-cis-UAB30, or PMCol to predict clinically significant drug-induced expression. The concentration ranges of the NCE used were selected to maximize the clinical relevance of these results. All responses were evaluated according to major prototypical P450 inducers (i.e., 3-methylcholanthrene, 3-MC; phenobarbital, PB; rifampicin, RIF) and increases > or = 40% of the respective positive control(s) were considered an indication of demonstrable induction. Herein, we report that there is low potential for DDI with SR13668 and PMCol due to enzyme induction of CYP1A2, CYP2B6, and CYP3A4 expression at the concentrations examined. Similarly, the study results suggested that 9-cis-UAB30 has low potential to induce CYP1A2 and CYP3A4 expression at the concentrations examined. However, 9-cis-UAB30 was shown to significantly induce CYP2B6 enzyme activity at 10 microM suggesting the potential for DDI as a result.

Primary hepatocytes: current understanding of the regulation of metabolic enzymes and transporter proteins, and pharmaceutical practice for the use of hepatocytes in metabolism, enzyme induction, transporter, clearance, and hepatotoxicity studies

This review brings you up-to-date with the hepatocyte research on: 1) in vitro-in vivo correlations of metabolism and clearance; 2) CYP enzyme induction, regulation, and cross-talk using human hepatocytes and hepatocyte-like cell lines; 3) the function and regulation of hepatic transporters and models used to elucidate their role in drug clearance; 4) mechanisms and examples of idiosyncratic and intrinsic hepatotoxicity; and 5) alternative cell systems to primary human hepatocytes. We also report pharmaceutical perspectives of these topics and compare methods and interpretations for the drug development process.

N-Acetylcysteine Partially Reverses Oxidative Stress and Apoptosis Exacerbated by Mg-Deficiency Culturing Conditions in Primary Cultures of Rat and Human Hepatocytes

OBJECTIVE

The effects of magnesium (Mg) deficiency on the rate of oxidative stress and apoptosis in primary cultures of human hepatocytes were compared to cultured rat hepatocytes. The possible reversion by N-acetylcysteine (NAC) in Mg-deficient culturing conditions was evaluated.

METHODS

Incubations were conducted for up to 72 h in media containing a deficient (0-0.4 mM) or a physiological (0.8 mM) Mg concentration, and in the presence or absence of NAC after 24 h of culture in these Mg concentration conditions.

RESULTS

We obtained similar profiles in terms of apoptosis and oxidative stress in primary cultures of human hepatocytes, as compared to rat hepatocytes, i.e. a Mg concentration-dependent effect on the caspase-3 activity and GSH levels after 72 h of culture, caspase-3 activity being highest and GSH levels being lowest in Mg-free cultures. The addition of NAC to culture media after the first 24 h of culture increased GSH concentrations. This was accompanied in Mg-deficient cultures by a decrease in both the caspase-3 activity and the lipid peroxidation. However, when culturing hepatocytes with physiological Mg concentrations, an increase in both caspase-3 activity and lipid peroxidation was observed.

CONCLUSIONS

Our results indicate that Mg deficiency exacerbates the rate of apoptosis in cultured hepatocytes, associated with an increase in oxidative stress, the sensitivity of human hepatocytes being equivalent to that of rat hepatocytes. They also indicate a dual role of NAC and/or GSH, i.e. protective for hepatocytes placed in a Mg-deficient environment, while deleterious for hepatocytes placed in a Mg-physiological environment.

New technologies and screening strategies for hepatotoxicity: use of in vitro models

Hepatotoxicity remains a significant cause for drug failures during clinical trials. This is due, in part, to the idiosyncratic nature of toxicity in humans and inherent physiological differences between humans and preclinical species leading to limited correct prediction of adverse responses in humans. To address this issue, robust screening assays are being developed, which have heightened predictive capacity for human hepatotoxicity, and may be utilized throughout the discovery and development phases in conjunction with traditional in vivo methods, for decision making during drug selection and risk assessment. This manuscript describes an example application of in vitro-based strategies using human hepatocyte cultures in lead optimization screening in conjunction with ADME profiling, for evaluation of compound-associated CYP450 induction potential, and the identification of potentially useful biomarkers as predictors of hepatotoxicity for use in vitro, and in preclinical species and humans.

Expression and induction potential of cytochromes P450 in human cryopreserved hepatocytes

Fresh human hepatocytes are still considered as the "gold standard" to screen in vitro for cytochrome P450 (P450) induction. However, sparse availability of good quality human liver tissue for research purposes and the demand for standardized cell populations, together with the need for proper storage of the cells not immediately required, have resulted in the development of cryopreservation techniques that provide adequate viability and plateability of hepatocytes after thawing. This study aimed at validating cryopreserved human hepatocytes as a model to investigate P450 induction. Cryopreserved cells from four different donors were plated and cultured for 48 h, followed by incubation in the presence of typical P450 inducers. During the experiments, quality of the cultured cells was monitored both physiologically and morphologically. Concomitantly, the activity of CYP1A2, 2B6, 2C9, 2E1, and 3A4 was measured together with their mRNA and protein expression. Determination of CYP1A2, 2B6, 2C9, 2E1, and 3A4 activity in control versus prototypical inducer-treated hepatocytes revealed a maximal significant mean 11.6-, 2.8-, 1.9-, 1.5-, and 9.0-fold induction over their basal expression, respectively. Protein expression analysis of these P450s confirmed these results. Moreover, a mean 44.9-, 3.5-, 3.2-, and 13.8-fold induction of CYP1A2, 2B6, 2C9, and 3A4 mRNA was observed. Our data demonstrate that cryopreserved human hepatocytes are a valuable tool to study the induction of CYP1A2, 2B6, 2C9, 2E1, and 3A4.

AMP-activated Protein Kinase Mediates Phenobarbital Induction of CYP2B Gene Expression in Hepatocytes and a Newly Derived Human Hepatoma Cell Line

Phenobarbital (PB) administration is known to trigger pleiotropic responses, including liver hypertrophy, tumor promotion, and induction of genes encoding drug-metabolizing enzymes. The induction of human CYP2B6 and the rat (CYP2B1) and mouse (Cyp2b10) homologues by PB is mediated by the nuclear receptor constitutive androstane receptor (CAR). The study of CYP2B gene regulation and CAR activity by PB has been difficult due to the lack of a cellular model. In this study, we describe a novel differentiated human hepatoma cell line (WGA), derived from HepG2, which expresses CYP2B6 and CAR. WGA cells represent a powerful system to study the regulation of CYP2B6 gene expression by PB. There is evidence that CAR activity is regulated by phosphorylation and that regulation of some CYP genes depends on the nutritional status of cells. The AMP-activated protein kinase (AMPK) functions as an energy sensor and is activated when cells experience energy-depleting stresses. In this report, we show that addition of 5-amino-imidazole carboxamide riboside, an AMPK activator, to WGA and human hepatocytes induces CYP2B6 gene expression. Expression of a constitutively active form of AMPK mimics the PB induction of CYP2B6 and CYP2B1 gene expression. Conversely, the expression of a dominant negative form of AMPK inhibits the induction of these genes by PB. Finally, we demonstrate, for the first time, that AMPK activity increases in cells cultured with PB. Our data strongly support a role for AMPK in the PB induction of CYP2B gene expression and provide new insights into the regulation of gene expression by barbiturate drugs.

New modified polyetheretherketone membrane for liver cell culture in biohybrid systems: adhesion and specific functions of isolated hepatocytes

There has been growing interest in innovative materials with physico-chemical properties that provide improved blood/cell compatibility. We propose new polymeric membranes made of modified polyetheretherketone (PEEK-WC) as materials with potential for use in biohybrid devices. PEEK-WC exhibits high chemical, thermal stability and mechanical resistance. Owing to its lack of crystallinity this polymer can be used for preparing membranes with cheap and flexible methods. We compared the properties of PEEK-WC membranes to polyurethane membranes prepared using the same phase inverse technique and commercial membranes. The physico-chemical properties of the membranes were characterised by contact angle measurements. The different parameters acid (gamma+), base (gamma-) and Lifshitz-van der Waals (gammaLW) of the surface free energy were calculated according to Good-van Oss's model. We evaluated the cytocompatibility of PEEK-WC membranes by culturing hepatocytes isolated from rat liver. Cell adhesion and metabolic behaviour in terms of ammonia elimination, urea synthesis and protein synthesis were evaluated during the first days of culture. Liver cells adhered and formed three-dimensional aggregates on the most tested membranes. PEEK-WC membranes promoted hepatocyte adhesion most effectively. Urea synthesis, ammonia elimination and protein synthesis improved significantly when cells adhered to PEEK-WC membrane. The considerable metabolic activities of cells cultured on this membrane confirmed the good structural and physico-chemical properties of the PEEK-WC membrane that could be a promising biomaterial for cell culture in biohybrid devices.

Determination of cytochrome P450 1A2 and cytochrome P450 3A4 induction in cryopreserved human hepatocytes

Freshly prepared human hepatocytes are considered as the 'gold standard' for in vitro testing of drug candidates. However, several disadvantages are associated with the use of this model system. The availability of hepatocytes is often low and consequently the planning of the experiments rendered difficult. In addition, the quality of the available cells is in some cases poor. As an alternative, cryopreserved human hepatocytes were validated as a model to study cytochrome P450 1A2 (CYP1A2) and cytochrome P450 3A4 (CYP3A4) induction. In a single blinded experiment, hepatocytes from three separate lots were incubated with three concentrations of different compounds, and compared to non-treated cells and cells incubated with omeprazole or rifampicin. CYP1A2 and CYP3A4 induction was determined by measuring 7-ethoxyresorufin-O-deethylation activity and 6beta-hydroxytestosterone formation, respectively. CYP1A2 and CYP3A4 mRNA and protein expression were analyzed by TaqMan QRT-PCR and immunodetection. Cells responded well to the prototypical inducers with a mean 38.8- and 6.2-fold induction of CYP1A2 and CYP3A4 activity, respectively. Similar as with fresh human hepatocytes, high batch-to-batch variation of CYP1A2 and CYP3A4 induction was observed. Except for 1 and 10 microM rosiglitazone, the glitazones did not significantly affect CYP1A2. A similar result was observed for CYP3A4 activity although CYP3A4 mRNA and protein expression were dose-dependently upregulated. In conclusion, cryopreserved human hepatocytes may be a good alternative to fresh hepatocytes to study CYP1A and 3A induction.

The effect of hypothermia on primary porcine hepatocyte metabolism monitored by (1H) nuclear magnetic resonance spectroscopy

OBJECTIVE

The aim of our study was to use (1H) nuclear magnetic resonance (NMR) spectroscopy as a tool to assess metabolic functions of hepatocytes and to monitor major metabolic pathways of these cells during culture following hypothermic preservation.

METHODS

After isolation, 2 x 10(7) primary porcine hepatocytes were preserved at 4 degrees C in supplemented Leibovitz L-15 medium for 48 h. Viability was assessed at isolation, 24 and 48 h. At isolation and at 48 h cells were plated and cultured with serum free supplemented Williams E medium. 1H NMR spectroscopy was used to assess indices of glucose metabolism, ammonia clearance indices and ketone bodies precursors at 48 h post-plating. Peak integration was applied with sodium 3-(trimethylsilyl-2,2,3,3-2H4)-1-propionate as an internal standard to obtain quantitative results.

RESULTS

Results were obtained from six isolations. Viability was 78.1 +/- 1.2% at isolation, 69 +/- 3.4% at 24 h and 58.9 +/- 3.8% at 48 h of hypothermia. Plating efficiency was 87 +/- 4% for freshly isolated cells and 33.6 +/- 7.6% for hypothermically preserved cells. Glucose consumption was comparable in both groups. Hypothermically preserved cells consumed more threonine, produced more pyruvate and alanine but less lactate. They also produced less acetate and consumed less tyrosine. Glutamate and glutamine concentrations were similar under both conditions.

CONCLUSION

1H NMR spectroscopy is a valid method for assessing metabolic pathways of cultured primary porcine hepatocytes. Although hypothermically preserved cells had a reduced plating efficiency, they were still metabolically active. Thus, hypothermia can be used as a temporary preservation technique for primary porcine hepatocytes.

Human hepatocyte culture systems for the in vitro evaluation of cytochrome P450 expression and regulation

Primary cultures of human hepatocytes have been used extensively by both academic and industrial laboratories for evaluating the hepatic disposition of drugs and other xenobiotics. Their primary utility has been for assessing the induction potential of new chemical entities (NCEs) and they continue to serve as the gold standard. Primary considerations for conducting in vitro drug testing utilizing cultures of human hepatocytes, such as the effects of culture and study conditions, are discussed. The maintenance of normal cellular physiology and intercellular contacts in vitro is of particular importance for optimal phenotypic gene expression and response to drugs and other xenobiotics. Significant advances in our understanding of cytochrome P450 (CYP450) enzyme regulation have been made with the recent identification of the nuclear receptors mediating the induction of CYP2B and CYP3A enzymes. In particular, the activation of pregnane X receptor (PXR) by prototypical inducers of CYP3A has been found to correlate well with the species-specific modulation of CYP3A by various drugs and other xenobiotics. Concomitant with the discovery of PXR has been the identification of compounds that may act synergistically or antagonistically on multiple receptors (e.g., co-repressors and/or co-activators of the receptor) introducing novel mechanisms of drug-drug interactions. Differential expression of the individual isoforms of the major CYP450 enzymes over time in culture suggest that this model system is not reflective of in vivo profiles and, therefore, may be limited in its application for drug metabolism studies. Overall, primary cultures of human hepatocytes can serve as a sensitive and selective model for predicting the regulation of CYP450 modulation by drugs and other xenobiotics. Considerations and recommendations for standardizing testing conditions and choosing relevant endpoint(s) are presented.

Characterization and inhibition by a wide range of xenobiotics of organic anion excretion by primary human hepatocytes

Organic anion secretion by human hepatocytes was characterized using primary liver parenchymal cell cultures and the anionic fluorescent dye carboxy-2',7'-dichlorofluorescein (CF). Probenecid, a well-known common blocker of the membrane transport process for anions, was shown to increase CF accumulation in primary human hepatocytes by inhibiting cellular CF efflux in a dose-dependent manner, thereby establishing the presence of an efflux system for organic anions in cultured hepatocytes. Outwardly directed transport of CF from hepatocytes was found to be temperature-dependent; it was not altered by changes in the ionic composition of the incubation medium used in efflux experiments. In addition to probenecid, various structurally and functionally unrelated xenobiotics such as glibenclamide, rifampicin, vinblastine, MK-571, indomethacin, and cyclosporin A were shown to inhibit secretion of CF by primary human hepatocytes, thus suggesting that organic anion excretion by human liver may be impaired by various drugs. Northern blot and Western blot analyses of the expression of multidrug resistance proteins (MRP), such as MRP1 and MRP2, which are known to mediate cellular outwardly directed transport of organic anions indicated that MRP2 was present at substantial levels in cultured human hepatocytes as well as in their in vivo counterparts, whereas MRP1 expression was only barely detectable. These results therefore suggest that MRP2, unlike MRP1, may contribute to the organic anion efflux system displayed by primary human hepatocytes and inhibited by a wide range of xenobiotics.

Heavy metal cytotoxicity studied by electron probe X-ray microanalysis of cultured rat hepatocytes

Cytotoxicity of the heavy metals gold, mercury, thallium and lead was studied by measuring the intracellular element distribution of cultured rat hepatocytes by energy dispersive electron probe X-ray microanalysis of freeze-dried cryosections in a scanning transmission electron microscope. Exposure of the cells to aqueous solutions containing heavy metal ions in concentrations reaching a critical concentration caused increase of intracellular sodium and chloride content accompanied or followed by decrease of intracellular potassium content. Thus, the intracellular potassium/sodium ratio drastically decreased from control values of approximately 10 to values below 1 before changes of cell morphology became visible. In experiments with gold or mercury the decrease of the potassium/sodium ratio was preceded by transient cytoplasmic increase of sulfur and phosphorus. Heavy metal concentrations exceeding the critical concentration also caused an increase of cytoplasmic calcium concentration and finally decay of the cell structure. Cytotoxicity of heavy metals was found to increase in the order Pb, Au, Tl, Hg. Cytotoxic effects by Au, Tl or Hg in moderate concentrations were reduced by simultaneous addition of Zn or Pb to the culture medium. The results obtained prove electron probe X-ray microanalysis of cryosections as a sensitive probe of cell viability.

Drug-induced liver diseases

Drug-induced liver injuries make up a persisting and challenging problem for physicians, health agencies and pharmaceutical firms. The clinical expression is polymorphous, acute hepatitis being predominant. The diagnosis is frequently difficult because of the absence of specific signs in most cases and mainly relies on the exclusion of other causes. The diagnosis should be particularly evoked in patients over 50 yr who are taking many drugs, after viral infections have been ruled out. Acute hepatocellular hepatitis is particularly severe because of the risk of fulminant hepatitis or of a more insidious course leading to cirrhosis. Cross hepatotoxicity can sometimes occur. One should avoid re-administration of not only the causative agents but also of other drugs belonging to the same family or having a related chemical structure. The prediction of the hepatotoxicity of new drugs must be improved. Investigations would be particularly useful for drugs having critical chemical structures and belonging to families with an established history of hepatotoxicity.

Modulation of murine phenobarbital-inducible CYP2A5, CYP2B10 and CYP1A enzymes by inhibitors of protein kinases and phosphatases

Phenobarbital causes a multitude of effects in hepatocytes, including increased cell proliferation, inhibition of apoptosis and upregulation of xenobiotic and endobiotic metabolizing enzymes. In this study, the involvement of several protein kinase and phosphatase pathways on constitutive and phenobarbital-induced activities of CYP2A5, CYP2B10 and CYP1A1/2 in primary mouse hepatocytes was determined using well-defined chemical modulators of intracellular protein phosphorylation and desphosphorylation events. A 48-h treatment of the hepatocytes with 2-aminopurine, a nonspecific serine/threonine kinase inhibitor, elicited dose-dependent increases in both basal and phenobarbital-induced CYP2A5 catalytic activity (assayed as coumarin 7-hydroxylation), the maximal induction being 60-fold greater than the control value upon cotreatment with 1.5 mM phenobarbital and 10 mM 2-aminopurine. In contrast, phenobarbital induction of CYP2B10 (pentoxyresorufin O-deethylase) and CYP1A1/2 (ethoxyresorufin O-deethylase) activities were blocked by 2-aminopurine. Increases in CYP2A5 activity were also observed after exposure of the hepatocytes to other protein kinase inhibitors affecting the cell cycle, i.e. roscovitine, K-252a and rapamycin. Inhibitors of protein kinases A and C, as well as tyrosine kinases, did not appreciably affect CYP2A5 activity levels. The serine/threonine phosphatase inhibitors tautomycin, calyculin A and okadaic acid all reduced both basal and phenobarbital-induced CYP2A5, CYP2B10 and CYP1A1/2 activities. These results further strengthen the concept that hepatic CYP2A5 is regulated in a unique way compared with CYP2B10 and CYP1A.