The use of cultured hepatocytes to investigate the mechanisms of drug hepatotoxicity

Depolarization of mitochondrial membrane potential is the initial event in non-nucleoside reverse transcriptase inhibitor efavirenz induced cytotoxicity

Efavirenz is a non-nucleoside reverse transcriptase inhibitor (NNRTI) and an active constituent of the highly active antiretroviral therapy regime. It has significantly contributed in control and management of human immunodeficiency virus propagation. However, EFV administration has also led to severe adverse effects, several reports highlighted the role of EFV in mitochondrial dysfunction and toxicity but the molecular mechanism has been poorly understood. In present study, human hepatoma cells Huh 7.5 were treated with clinically relevant concentrations of EFV and parameters like cytotoxicity, mitochondrial transmembrane potential, mitochondrial morphology, cytochrome c release, mitochondria-mediated apoptosis, mtDNA and mtRNA levels and EFV distribution into mitochondrial compartment were evaluated to understand sequence of events leading to cell death in EFV-treated cells. EFV at its clinically relevant concentration was significantly toxic after 48 and 72 h of treatments. EFV-mediated toxicity is initiated with the permeabilization of mitochondrial outer membrane and change in mitochondrial membrane potential (Δψm) which triggers a series of events like cytochrome c release, alteration in mitochondrial morphology and mitochondria-mediated apoptosis. Total mitochondrial content is reduced after 48 h of EFV treatment at IC₅₀ concentration which is also reflected in reduced mitochondrial DNA and RNA levels. After detecting EFV in mitochondrial compartment after 12 h of incubation with EFV, we hypothesize that EFV being a lipophilic molecule is internalized into the mitochondrial compartment causing depolarization of Δψm which subsequently leads to a cascade of events causing cell death.

Transcriptome of extracellular vesicles released by hepatocytes

The discovery that the cells communicate through emission of vesicles has opened new opportunities for better understanding of physiological and pathological mechanisms. This discovery also provides a novel source for non-invasive disease biomarker research. Our group has previously reported that hepatocytes release extracellular vesicles with protein content reflecting the cell-type of origin. Here, we show that the extracellular vesicles released by hepatocytes also carry RNA. We report the messenger RNA composition of extracellular vesicles released in two non-tumoral hepatic models: primary culture of rat hepatocytes and a progenitor cell line obtained from a mouse foetal liver. We describe different subpopulations of extracellular vesicles with different densities and protein and RNA content. We also show that the RNA cargo of extracellular vesicles released by primary hepatocytes can be transferred to rat liver stellate-like cells and promote their activation. Finally, we provide in vitro and in vivo evidence that liver-damaging drugs galactosamine, acetaminophen, and diclofenac modify the RNA content of these vesicles. To summarize, we show that the extracellular vesicles secreted by hepatocytes contain various RNAs. These vesicles, likely to be involved in the activation of stellate cells, might become a new source for non-invasive identification of the liver toxicity markers.

General review on in vitro hepatocyte models and their applications

In vitro hepatocyte models represent very useful systems in both fundamental research and various application areas. Primary hepatocytes appear as the closest model for the liver in vivo. However, they are phenotypically unstable, have a limited life span and in addition, exhibit large interdonor variability when of human origin. Hepatoma cell lines appear as an alternative but only the HepaRG cell line exhibits various functions, including major cytochrome P450 activities, at levels close to those found in primary hepatocytes. In vitro hepatocyte models have brought a substantial contribution to the understanding of the biochemistry, physiology, and cell biology of the normal and diseased liver and in various application domains such as xenobiotic metabolism and toxicity, virology, parasitology, and more generally cell therapies. In the future, new well-differentiated hepatocyte cell lines derived from tumors or from either embryonic or adult stem cells might be expected and although hepatocytes will continue to be used in various fields, these in vitro liver models should allow marked advances, especially in cell-based therapies and predictive and mechanistic hepatotoxicity of new drugs and other chemicals. All models will benefit from new developments in throughput screening based on cell chips coupled with high-content imaging and in toxicogenomics technologies.

Measuring and modeling of binary mixture effects of pharmaceuticals and nickel on cell viability/cytotoxicity in the human hepatoma derived cell line HepG2

The interaction of drugs and non-therapeutic xenobiotics constitutes a central role in human health risk assessment. Still, available data are rare. Two different models have been established to predict mixture toxicity from single dose data, namely, the concentration addition (CA) and independent action (IA) model. However, chemicals can also act synergistic or antagonistic or in dose level deviation, or in a dose ratio dependent deviation. In the present study we used the MIXTOX model (EU project ENV4-CT97-0507), which incorporates these algorithms, to assess effects of the binary mixtures in the human hepatoma cell line HepG2. These cells possess a liver-like enzyme pattern and a variety of xenobiotic-metabolizing enzymes (phases I and II). We tested binary mixtures of the metal nickel, the anti-inflammatory drug diclofenac, and the antibiotic agent irgasan and compared the experimental data to the mathematical models. Cell viability was determined by three different methods the MTT-, AlamarBlue(R) and NRU assay. The compounds were tested separately and in combinations. We could show that the metal nickel is the dominant component in the mixture, affecting an antagonism at low-dose levels and a synergism at high-dose levels in combination with diclofenac or irgasan, when using the NRU and the AlamarBlue assay. The dose-response surface of irgasan and diclofenac indicated a concentration addition. The experimental data could be described by the algorithms with a regression of up to 90%, revealing the HepG2 cell line and the MIXTOX model as valuable tool for risk assessment of binary mixtures for cytotoxic endpoints. However the model failed to predict a specific mode of action, the CYP1A1 enzyme activity.

Characterization of increased drug metabolism activity in dimethyl sulfoxide (DMSO)-treated Huh7 hepatoma cells

The objective of this study was to characterize Huh7 cells' baseline capacity to metabolize drugs and to investigate whether the drug metabolism was enhanced upon treatment with dimethyl sulfoxide (DMSO). The messenger RNA (mRNA) levels of major Phase I and Phase II enzymes were determined by quantitative real-time-polymerase chain reaction (RT-PCR), and activities of major drug-metabolizing enzymes were examined using probe drugs by analysing relevant metabolite production rates. The expression levels of drug-metabolizing enzymes in control Huh7 cells were generally very low, but DMSO treatment dramatically increased the mRNA levels of most drug-metabolizing enzymes as well as other liver-specific proteins. Importantly, functionality assays confirmed concomitant increases in drug-metabolizing enzyme activity. Additionally, treatment of the Huh7 cells with 3-methylcholanthrene induced cytochrome P450 (CYP) 1A1 expression. The results indicate that DMSO treatment of Huh7 cells profoundly enhances their differentiation state, thus improving the usefulness of this common cell line as an in vitro hepatocyte model.

Validation of an in vitro model for assessment of androstenedione hepatotoxicity using the rat liver cell line clone-9

Androstenedione, a naturally occurring steroid hormone, has been used to enhance athletic performance. Little is known, however, about its hepatotoxicity. Clone-9 cells, a non-transformed epithelial cell line that was originally isolated from normal liver of a 4-week old Sprague-Dawley rat, were used as an in vitro model to assess the hepatotoxic potential of androstenedione. The cultures were treated with androstenedione for 24 h at 37 degrees C in 5% CO(2) at concentrations of 0-100 microg ml(-1). After the treatment period, the cells and the culture supernatants were assayed for markers of cytotoxicity which included: release of liver enzymes, cell viability, cellular double-stranded DNA content, oxidative stress, steatosis, cellular ATP content, caspase-3 activity, the mitochondrial permeability transition and induction of cytochrome P450 activity. Significant concentration-dependent differences from control were observed in some endpoints at medium concentrations of 10 microg ml(-1) and above. These in vitro findings were compared with comparable endpoints obtained from an in vivo study of androstenedione toxicity in female Sprague-Dawley rats. Of the eight endpoints that could be compared between the two studies, only three (lipid accumulation, ATP depletion and P450 activity) appeared to be concordant. This suggests that, under the experimental conditions used, the clone-9 cells were not a good model for androstenedione hepatotoxicity.

Hepatotoxic effect of cyclosporin A in the mitochondrial respiratory chain

Cyclosporin A (CyA), a potent immunosuppressant, was used to determine the hepatotoxic effect in long-term treatments. Male Wistar rats were used in these experiments. They were given CyA chronically at doses used in patients for 120 days, and at doses of 5, 10, 15 and 20 mg kg(-1) day(-1). These doses amount to CyA values in blood of 200 +/- 24, 314 +/- 40, 445 +/- 33 and 598 +/- 53 ng ml(-1), respectively. A significant increase in glutamate dehydrogenase (GLDH) was found in the groups treated with 15 and 20 mg kg(-1) day(-1), which would point to mitochondria as the potential target of the toxic action of CyA. The mitochondrial respiratory chain of rat livers was studied in enzyme complexes I and II. Enzyme complex I was determined by spectrophotometry at 340 nm using NADH oxidase with the respirable substrate 10 mm NADH; enzyme complex II was determined by monitoring succinate dehydrogenase by oxymetry using the respirable substrate 10 mm succinate. The results show the inhibition of NADH oxidase in the groups treated with 10, 15 and 20 mg kg(-1) day(-1), an effect dependent both on time and on CyA concentration. Enzyme complex II showed a decrease in oxygen consumption. These findings were confirmed by histological studies (hematoxylin-eosin technique).

CONCLUSIONS

Long-term treatment with CyA at doses of 15 and 20 mg kg(-1) day(-1), amounting to concentrations in blood of 445 +/- 33 and 598 +/- 53 ng ml(-1), causes alterations in the mitochondria, revealed by the increase in serum GLDH and by the functional alteration of enzyme complexes I and II of the mitochondrial respiratory chain.

Selection of new chemical entities with decreased potential for adverse drug reactions

Adverse drug reactions, such as hepatotoxicity, blood dyscrasias and hypersensitivity are a major obstacle for the use and the development of new medicines. Many forms of organ-directed toxicity can arise from the bioactivation of drugs to so-called chemically reactive metabolites, which can modify tissue macromolecules. It is well established that the toxicities of model hepatotoxins, such as acetaminophen, furosemide, bromobenzene and methapyrilene can be correlated with the generation of chemically reactive metabolites, which can be detected by measurement of the irreversible binding of radiolabelled material to hepatic protein and/or the detection of stable phase II metabolites such as glutathione conjugates. The basic chemistry of the reaction of such metabolites with model nucleophiles is relatively well understood. A major challenge is to define how certain reactive intermediates may chemically modify critical proteins and how modification of specific amino acids may alter protein function which in turn may affect cell signalling, regulation, defence, function and viability. This in turn will determine whether or not bioactivation will result in a particular form of drug-induced injury. It is now clear that even relatively simple reactive intermediates can react in a discriminative manner with particular cellular proteins and even with specific amino acids within those proteins. Therefore both non-covalent, as well as covalent bonds will be important determinants of the target protein for a particular reactive metabolite. Mammalian cells have evolved numerous defence systems against reactive intermediates. Sensitive redox proteins such as Nrf-2 recognize oxidative stress and electrophilic agents. This is achieved by chemical modification of cysteine groups within keap-1, which normally forms an inactive heterodimer with Nrf-2. Modification of keap-1 releases Nrf-2 that translocates to the nucleus and effects gene transcription of a number of genes involved in the detoxication of chemically reactive metabolites. Diminution of protein function can occur by either covalent modification of nucleophilic amino acids (e.g. cysteine, lysine, histidine etc.) or oxidation of thiols, which can be reversible or irreversible. In the case of acetaminophen, more than 30 target proteins have been identified and for several of them, corresponding alterations in protein function have been defined in the context of tissue necrosis. Alternatively, protein modification may induce signalling systems which initiate cell death, an immune response or to an altered tissue genotype.

Prooxidant activity and toxicity of nordihydroguaiaretic acid in clone-9 rat hepatocyte cultures

Nordihydroguaiaretic acid (NDGA) is a polyphenol. It is present at high concentrations in the leaves of the evergreen desert shrub, Larrea tridentate (Creosote bush), which has a long history of medicinal use traditionally by the native Americans and Mexicans. It is generally believed that the antioxidant properties of NDGA are responsible for the medicinal value of this desert shrub. The clone-9 rat hepatocyte cultures were used as an in vitro model to assess the hepatotoxic potential of NDGA and to determine whether it exhibits any prooxidant activity. The hepatocyte cultures were treated with NDGA for 2 h at 37 degrees C at concentrations of 0-100 microM. After the treatment period the cells, the culture supernatants and cell lysates were assayed for evaluation of prooxidant activity and toxicity of NDGA. Oxidative stress level and oxidative cell injury as measured by the peroxidation of membrane lipids and DNA double-strand breaks were used to index prooxidant activity. Cytotoxicity as measured by the leakage of the liver enzyme lactate dehydrogenase (LDH) into the culture medium, mitochondrial function and extent of cell proliferation were used as the endpoints of toxicity. Significant concentration-dependent differences were observed in these biomarkers over the concentration range examined demonstrating the prooxidant activity and toxicity of NDGA in clone-9 rat hepatocyte cultures.

Toxicity of used orthodontic archwires assessed by three-dimensional cell culture

The aim of the present study was to determine whether used orthodontic wires made of different materials cause toxicity and loss of viability on three-dimensional (3D) cell cultures. Three types of orthodontic wires, stainless steel, Nitinol, and TMA (n = 9) which had been used clinically in fixed appliances for a period of 1 month, were retrieved at random from five patients. Both upper and lower archwires were collected and subjected to two different protocols: to assess toxicity, two pieces of each wire were placed on 3D cell cultures (reconstituted human epithelium); to investigate the possibility of cell damage, the 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl tetrazolium bromide (MTT) assay was used and haematoxylin and eosin staining was performed to evaluate morphological changes. Copper wire served as the control to determine the morphology of severe toxicity, and native cell cultures and silk were used as the negative controls. Morphological evaluation of the native cell cultures revealed no toxic reactions. The ranking, from mild to severe toxicity was as follows: stainless steel < Nitinol = TMA. There were no significant differences between TMA and Nitinol. The MTT assay revealed the following mean percentage values for viability: native cell line (negative control), 100; stainless steel, 102.25; TMA, 87.4; Nitinol, 85.3; and copper wire (positive control) 57.2. Histological evaluation of the 3D cell cultures showed no severe toxicity or loss of viability for any of the wires. However, relative comparison between the different wires revealed that stainless steel induced less toxicity/loss of viability compared with TMA and Nitinol wire.

A new bioassay including a small scale hepatocyte bioreactor for hepato-mediated toxicity testing in a target cell line

New approaches for in vitro testing of hepato-mediated toxicity are undertaken to offer alternatives to in vivo animal testing. The described bioassay for hepato-mediated toxicity testing is based on a small scale hepatocyte-bioreactor with pig hepatocytes connected to a silicon sensor based microphysiometer system for monitoring of the extracellular acidification rate (EAR) of cells and the microphysiometer alone. EAR represents the metabolic activity of tested cells (hepatocytes and ZR 751 cells) under the influence of perfused media, compared to controls, which were set to 100%. Cyclophosphamide (CYCL), whose cytostatic effect is dependent on CYP 450 biotransformation was used as a model substrate. CYCL showed decrease of EAR in hepatocytes, but not in ZR 751 cells. Bioreactor supernatant including CYCL was pumped into the microphysiometer and EARs of the target ZR 751 cell line were recorded. After 7 h of bioreactor supernatant perfusion the ZR 751 cell line showed an EAR decrease of 18.68% +/- 10.18, as compared to controls (bioreactor supernatant from the identical set-up without CYCL). Thus the presented model of hepato-activated toxicity showed an EAR decrease in the ZR 751 cell line that reflected the toxic activation of CYCL by the bioreactor. This new bioassay serves as an example of future applications for hepatocyte bioreactors in automated toxicity testing devices, e.g. in preclinical drug studies or evaluation of hepato-mediated toxicity, not depending on cell destruction or further assays.

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.

Role of cytochrome P450 3A in the metabolism of mefloquine in human and animal hepatocytes

We studied mefloquine metabolism in cells and microsomes isolated from human and animal (monkey, dog, rat) livers. In both hepatocytes and microsomes, mefloquine underwent conversion to two major metabolites, carboxymefloquine and hydroxymefloquine. In human cells and microsomes these metabolites only were formed, as already demonstrated in vivo, while in other species several unidentified metabolites were also detected. After a 48 hr incubation with human and rat hepatocytes, metabolites accounted for 55-65% of the initial drug concentration, whereas in monkey and dog hepatocytes, mefloquine was entirely metabolized after 15 and 39 hrs, respectively. The consumption of mefloquine was less extensive in microsomes, and unchanged drug represented 60% (monkey) to 85-100% (human, dog, rat) of the total radioactivity after 5 hr incubations. The involvement of the cytochrome P450 3A subfamily in mefloquine biotransformation was suggested by several lines of evidence. Firstly, mefloquine metabolism was strongly increased in hepatic microsomes from dexamethasone-pretreated rats, and also in human and rat hepatocytes after prior treatment with a cytochrome P450 3A inducer. Secondly, mefloquine biotransformation in rifampycin-induced human hepatocytes was inhibited in a concentration-dependent manner by the cytochrome P450 3A inhibitor ketoconazole and thirdly, a strong correlation was found between erythromycin-N-demethylase activity (mediated by cytochrome P450 3A) and mefloquine metabolism in human microsomes (r=0.81, P < 0.05, N=13). Collectively, these findings concerning the role of cytochrome P450 3A in mefloquine metabolism may have important in vivo consequences especially with regard to the choice of agents used in multidrug antimalarial regimens.

Efficient adenoviral vector transduction and expression of functional human factor VIII in cultured primary human hepatocytes

Hemophilia A is a severe bleeding disorder caused by a deficiency in blood coagulation factor VIII (FVIII). Adenoviral vectors containing a potent human FVIII expression cassette encoding a truncated FVIII cDNA were developed that mediated sustained FVIII expression in normal and haemophiliac mice and complete phenotypic correction of the bleeding disorder in haemophiliac mice and dogs (Connelly and Kaleko, Haemophilia, 1998; 4: 380-8). Here, we evaluated two E1/E2a/E3-deleted adenoviral vectors encoding human FVIII, one containing the full-length cDNA and the second containing a truncated cDNA lacking the B-domain. Viral vectors encoding the human full-length FVIII cDNA have not been described previously. Hepatocyte transduction was efficient and dose dependent, ranging from 50% to 100%. High levels of functional FVIII were secreted from transduced cells at amounts up to 6000 mU-1 10(6)cells-1 60 h. B-domain deleted FVIII was expressed at levels at least 8-fold higher than the full-length FVIII protein, whereas FVIII RNA levels were similar with both vectors. These data provide the first demonstration of FVIII adenoviral vector function in primary human cells and verify the potential clinical utility of adenoviral vectors for the treatment of haemophilia A.