Glutathione transferases in hepatocyte-like cells derived from human embryonic stem cells

"iPSC-derived liver organoids and inherited bleeding disorders: Potential and future perspectives"

The bleeding phenotype of hereditary coagulation disorders is caused by the low or undetectable activity of the proteins involved in hemostasis, due to a broad spectrum of genetic alterations. Most of the affected coagulation factors are produced in the liver. Therefore, two-dimensional (2D) cultures of primary human hepatocytes and recombinant overexpression of the factors in non-human cell lines have been primarily used to mimic disease pathogenesis and as a model for innovative therapeutic strategies. However, neither human nor animal cells fully represent the hepatocellular biology and do not harbor the exact genetic background of the patient. As a result, the inability of the current in vitro models in recapitulating the in vivo situation has limited the studies of these inherited coagulation disorders. Induced Pluripotent Stem Cell (iPSC) technology offers a possible solution to overcome these limitations by reprogramming patient somatic cells into an embryonic-like pluripotent state, thus giving the possibility of generating an unlimited number of liver cells needed for modeling or therapeutic purposes. By combining this potential and the recent advances in the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 technology, it allows for the generation of autologous and gene corrected liver cells in the form of three-dimensional (3D) liver organoids. The organoids recapitulate cellular composition and organization of the liver, providing a more physiological model to study the biology of coagulation proteins and modeling hereditary coagulation disorders. This advanced methodology can pave the way for the development of cell-based therapeutic approaches to treat inherited coagulation disorders. In this review we will explore the use of liver organoids as a state-of-the-art methodology for modeling coagulation factors disorders and the possibilities of using organoid technology to treat the disease.

In Vitro Differentiation of Human Amniotic Epithelial Cells into Hepatocyte-like Cells

Human amniotic epithelial cells (hAECs) represent an interesting clinical alternative to human embryonic (hESCs) and induced pluripotent (hiPSCs) stem cells in regenerative medicine. The potential of hAECs can be enhanced ex vivo by their partial pre-differentiation. The aim of this study was to evaluate the effectiveness of 18-day differentiation of hAECs into endodermal cells, hepatic precursor cells, and cells showing functional features of hepatocytes using culture media supplemented with high (100 ng/mL) concentrations of EGF or HGF. The cells obtained after differentiation showed changes in morphology and increased expression of AFP, ALB, CYP3A4, CYP3A7, and GSTP1 genes. HGF was more effective than EGF in increasing the expression of liver-specific genes in hAECs. However, EGF stimulated the differentiation process more efficiently and yielded more hepatocyte-like cells capable of synthesizing α-fetoprotein during differentiation. Additionally, after 18 days, GST transferases, albumin, and CYP P450s, which proved their partial functionality, were expressed. In summary, HGF and EGF at a dose of 100 ng/mL can be successfully used to obtain hepatocyte-like cells between days 7 and 18 of hAEC differentiation. However, the effectiveness of this process is lower compared with hiPSC differentiation; therefore, optimization of the composition of the medium requires further research.

Development of a Scalable Three-Dimensional Culture of Human Induced Pluripotent Stem Cells-Derived Liver Organoids

Human induced pluripotent stem cells (hiPSCs) represent a powerful tool for the generation of specialized cells to be used in regenerative medicine as well as hepatocellular repopulation tool to treat liver metabolic diseases such as nonalcoholic steatohepatitis (NASH). Here we describe a strategy to obtain fully functional liver organoids from hiPSCs in a scalable manner. Our approach uses a two-step process, with a first step involving the scalable formation of homogeneous and uniform-sized human embryoid bodies (hEBs), followed by the application of a four-step liver differentiation protocol for the derivation of liver organoids that possess all the features of primary human hepatocytes. This chapter will also illustrate the characterization of the liver organoids by directed biomolecular techniques.

Generation of Hepatocyte Organoids from Human iPS Cells

Human-induced pluripotent stem cells (hiPSCs) constitute a great source to generate specialized cells that can be employed in cell replacement therapy for a number of degenerative diseases. In this chapter, I describe a strategy to mass-produce fully functional hepatocyte organoids using hiPSCs interlaced with human adipose microvascular endothelial cells (HAMEC). Our unique technology employs a two-step strategy, consisting of the scalable generation of nearly spherical uniform-sized human embryoid bodies (hEBs), and the subsequent employment of a four-step hepatocyte differentiation approach for the generation of hepatocyte organoids that display all the characteristics of human primary hepatocytes. In this chapter, we also describe methodologies to characterize the hepatocyte organoids by using different biomolecular assays.

Role of human glutathione transferases in biotransformation of the nitric oxide prodrug JS-K

Nitric oxide (NO) plays a prominent physiological role as a low-molecular-mass signal molecule involved in diverse biological functions. Great attention has been directed to pharmacologically modulating the release of NO for various therapeutic applications. We have focused on O2-(2,4-dinitrophenyl) 1-[(4-ethoxycarbonyl)piperazin-1-yl]diazen-1-ium-1,2-diolate (JS-K) as an example of diazeniumdiolate prodrugs with potential for cancer chemotherapy. JS-K is reportedly activated by glutathione conjugation by glutathione transferase (GST), but the scope of activities among the numerous members of the GSTome is unknown. We demonstrate that all human GSTs tested except GST T1-1 are active with JS-K as a substrate, but their specific activities are notably spanning a > 100-fold range. The most effective enzyme was the mu class member GST M2-2 with a specific activity of 273 ± 5 µmol min-1 mg-1 and the kinetic parameters Km 63 µM, kcat 353 s-1, kcat/Km 6 × 106 M-1 s-1. The abundance of the GSTs as an ensemble and their high catalytic efficiency indicate that release of NO occurs rapidly in normal tissues such that this influence must be considered in clarification of the tumor-killing effect of JS-K.

Generation of fully functional hepatocyte-like organoids from human induced pluripotent stem cells mixed with Endothelial Cells

Despite advances in stem cell research, cell transplantation therapy for liver failure is impeded by a shortage of human primary hepatocytes (HPH), along with current differentiation protocol limitations. Several studies have examined the concept of co-culture of human induced pluripotent cells (hiPSCs) with various types of supporting non-parenchymal cells to attain a higher differentiation yield and to improve hepatocyte-like cell functions both in vitro and in vivo. Co-culturing hiPSCs with human endothelial cells (hECs) is a relatively new technique that requires more detailed studies. Using our 3D human embryoid bodies (hEBs) formation technology, we interlaced Human Adipose Microvascular Endothelial Cells (HAMEC) with hiPSCs, leading to a higher differentiation yield and notable improvements across a wide range of hepatic functions. We conducted a comprehensive gene and protein secretion analysis of our HLCs coagulation factors profile, showing promising results in comparison with HPH. Furthermore, a stage-specific glycomic analysis revealed that the differentiated hepatocyte-like clusters (HLCs) resemble the glycan features of a mature tissue rather than cells in culture. We tested our HLCs in animal models, where the presence of HAMEC in the clusters showed a consistently better performance compared to the hiPSCs only group in regard to persistent albumin secretion post-transplantation.

Stem cell-derived models to improve mechanistic understanding and prediction of human drug-induced liver injury

Current preclinical drug testing does not predict some forms of adverse drug reactions in humans. Efforts at improving predictability of drug-induced tissue injury in humans include using stem cell technology to generate human cells for screening for adverse effects of drugs in humans. The advent of induced pluripotent stem cells means that it may ultimately be possible to develop personalized toxicology to determine interindividual susceptibility to adverse drug reactions. However, the complexity of idiosyncratic drug-induced liver injury means that no current single-cell model, whether of primary liver tissue origin, from liver cell lines, or derived from stem cells, adequately emulates what is believed to occur during human drug-induced liver injury. Nevertheless, a single-cell model of a human hepatocyte which emulates key features of a hepatocyte is likely to be valuable in assessing potential chemical risk; furthermore, understanding how to generate a relevant hepatocyte will also be critical to efforts to build complex multicellular models of the liver. Currently, hepatocyte-like cells differentiated from stem cells still fall short of recapitulating the full mature hepatocellular phenotype. Therefore, we convened a number of experts from the areas of preclinical and clinical hepatotoxicity and safety assessment, from industry, academia, and regulatory bodies, to specifically explore the application of stem cells in hepatotoxicity safety assessment and to make recommendations for the way forward. In this short review, we particularly discuss the importance of benchmarking stem cell-derived hepatocyte-like cells to their terminally differentiated human counterparts using defined phenotyping, to make sure the cells are relevant and comparable between labs, and outline why this process is essential before the cells are introduced into chemical safety assessment. (Hepatology 2017;65:710-721).

One Standardized Differentiation Procedure Robustly Generates Homogenous Hepatocyte Cultures Displaying Metabolic Diversity from a Large Panel of Human Pluripotent Stem Cells

Human hepatocytes display substantial functional inter-individual variation regarding drug metabolizing functions. In order to investigate if this diversity is mirrored in hepatocytes derived from different human pluripotent stem cell (hPSC) lines, we evaluated 25 hPSC lines originating from 24 different donors for hepatic differentiation and functionality. Homogenous hepatocyte cultures could be derived from all hPSC lines using one standardized differentiation procedure. To the best of our knowledge this is the first report of a standardized hepatic differentiation procedure that is generally applicable across a large panel of hPSC lines without any adaptations to individual lines. Importantly, with regard to functional aspects, such as Cytochrome P450 activities, we observed that hepatocytes derived from different hPSC lines displayed inter-individual variation characteristic for primary hepatocytes obtained from different donors, while these activities were highly reproducible between repeated experiments using the same line. Taken together, these data demonstrate the emerging possibility to compile panels of hPSC-derived hepatocytes of particular phenotypes/genotypes relevant for drug metabolism and toxicity studies. Moreover, these findings are of significance for applications within the regenerative medicine field, since our stringent differentiation procedure allows the derivation of homogenous hepatocyte cultures from multiple donors which is a prerequisite for the realization of future personalized stem cell based therapies.

Scalable Differentiation of Human iPSCs in a Multicellular Spheroid-based 3D Culture into Hepatocyte-like Cells through Direct Wnt/β-catenin Pathway Inhibition

Treatment of acute liver failure by cell transplantation is hindered by a shortage of human hepatocytes. Current protocols for hepatic differentiation of human induced pluripotent stem cells (hiPSCs) result in low yields, cellular heterogeneity, and limited scalability. In the present study, we have developed a novel multicellular spheroid-based hepatic differentiation protocol starting from embryoid bodies of hiPSCs (hiPSC-EBs) for robust mass production of human hepatocyte-like cells (HLCs) using two novel inhibitors of the Wnt pathway. The resultant hiPSC-EB-HLCs expressed liver-specific genes, secreted hepatic proteins such as Albumin, Alpha Fetoprotein, and Fibrinogen, metabolized ammonia, and displayed cytochrome P450 activities and functional activities typical of mature primary hepatocytes, such as LDL storage and uptake, ICG uptake and release, and glycogen storage. Cell transplantation of hiPSC-EB-HLC in a rat model of acute liver failure significantly prolonged the mean survival time and resolved the liver injury when compared to the no-transplantation control animals. The transplanted hiPSC-EB-HLCs secreted human albumin into the host plasma throughout the examination period (2 weeks). Transplantation successfully bridged the animals through the critical period for survival after acute liver failure, providing promising clues of integration and full in vivo functionality of these cells after treatment with WIF-1 and DKK-1.

Gene expression pattern of some classes of cytochrome P-450 and glutathione S-transferase enzymes in differentiated hepatocytes-like cells from menstrual blood stem cells

Recently, valuable characteristics of menstrual blood stem cells (MenSCs) have impelled scientists to take its advantages for cell therapy of different diseases including liver disorders. In this study, we examined messenger RNA (mRNA) expression levels of phases I and II drug metabolizing enzymes including glutathione S-transferase (GST) and cytochrome P-450 (CYP) in differentiated hepatocyte-like cells from MenSCs. The isolated MenSCs were characterized and differentiated into hepatocyte-like cells using hepatocyte growth factor (HGF) and oncostatin M (OSM) in combination with other components in serum-free culture media. After primary characterization of hepatocyte markers, mRNA expression of GSTA1, GSTA2, GSTP1, CYP3A4, and CYP7A1 was assessed in differentiated cells in reference to undifferentiated cells using real-time PCR. Based on immunofluorescent staining and real-time PCR data, the differentiated MenSCs could express functional hepatocyte markers at mRNA and/or protein levels suggesting development of hepatocyte-like cells from MenSCs. Moreover, the expression levels of GSTA1, GSTA2, and CYP3A4 mRNA were upregulated in differentiated cells compared to undifferentiated cells. The expression of CYP7A1 gene was also remarkable on the last day of differentiation process. However, the expression level of GSTP1 did not exhibit statistically significant change during differentiation (P = 0.6). Based on accumulative data, MenSCs could be viewed as an accessible population of stem cells with differentiation ability into drug-metabolizing hepatocyte-like cells.

MicroRNA-122: a novel hepatocyte-enriched in vitro marker of drug-induced cellular toxicity

Emerging hepatic models for the study of drug-induced toxicity include pluripotent stem cell-derived hepatocyte-like cells (HLCs) and complex hepatocyte-non-parenchymal cellular coculture to mimic the complex multicellular interactions that recapitulate the niche environment in the human liver. However, a specific marker of hepatocyte perturbation, required to discriminate hepatocyte damage from non-specific cellular toxicity contributed by non-hepatocyte cell types or immature differentiated cells is currently lacking, as the cytotoxicity assays routinely used in in vitro toxicology research depend on intracellular molecules which are ubiquitously present in all eukaryotic cell types. In this study, we demonstrate that microRNA-122 (miR-122) detection in cell culture media can be used as a hepatocyte-enriched in vitro marker of drug-induced toxicity in homogeneous cultures of hepatic cells, and a cell-specific marker of toxicity of hepatic cells in heterogeneous cultures such as HLCs generated from various differentiation protocols and pluripotent stem cell lines, where conventional cytotoxicity assays using generic cellular markers may not be appropriate. We show that the sensitivity of the miR-122 cytotoxicity assay is similar to conventional assays that measure lactate dehydrogenase activity and intracellular adenosine triphosphate when applied in hepatic models with high levels of intracellular miR-122, and can be multiplexed with other assays. MiR-122 as a biomarker also has the potential to bridge results in in vitro experiments to in vivo animal models and human samples using the same assay, and to link findings from clinical studies in determining the relevance of in vitro models being developed for the study of drug-induced liver injury.

Stem cells and stem cell-derived tissues and their use in safety assessment

Toxicology has long relied on animal models in a tedious approach to understanding risk of exposure to an uncharacterized molecule. Stem cell-derived tissues can be made in high purity, quality, and quantity to enable a new approach to this problem. Currently, stem cell-derived tissues are primarily "generic" genetic backgrounds; the future will see the integration of various genetic backgrounds and complex three-dimensional models to create truly unique in vitro organoids. This minireview focuses on the state of the art of a number of stem cell-derived tissues and details their application in toxicology.

Stem cell-derived hepatocytes as a predictive model for drug-induced liver injury: are we there yet?

Amongst the different types of adverse drug reactions, drug-induced liver injury is the most prominent cause of patient morbidity and mortality. However, the current available hepatic model systems developed for evaluating safety have limited utility and relevance as they do not fully recapitulate a fully functional hepatocyte, and do not sufficiently represent the genetic polymorphisms present in the population. The rapidly advancing research in stem cells raises the possibility of using human pluripotent stem cells in bridging this gap. The generation of human induced pluripotent stem cells via reprogramming of mature human somatic cells may also allow for disease modelling in vitro for the purposes of assessing drug safety and toxicology. This would also allow for better understanding of disease processes and thus facilitate in the potential identification of novel therapeutic targets. This review will focus on the current state of effort to derive hepatocytes from human pluripotent stem cells for potential use in hepatotoxicity evaluation and aims to provide an insight as to where the future of the field may lie.

Drug metabolizing enzyme and transporter protein profiles of hepatocytes derived from human embryonic and induced pluripotent stem cells

Human embryonic and induced pluripotent stem cell-derived hepatocytes (hESC-Hep and hiPSC-Hep) have the potential to provide relevant human in vitro model systems for toxicity testing and drug discovery studies. In this study, the expression and function of important drug metabolizing cytochrome P450 (CYP) enzymes and transporter proteins in hESC-Hep and hiPSC-Hep were compared to cryopreserved human primary hepatocytes (hphep) and HepG2 cells. Overall, CYP activities in hESC-Hep and hiPSC-Hep were much lower than in hphep cultured for 4 h, but CYP1A and 3A activities were comparable to levels in hphep cultured for 48h (CYP1A: 35% and 26% of 48 h hphep, respectively; CYP3A: 80% and 440% of 48 h hphep, respectively). Importantly, in hESC-Hep and hiPSC-Hep, CYP activities were stable or increasing for at least one week in culture which was in contrast to the rapid loss of CYP activities in cultured hphep between 4 and 48 h after plating. With regard to transporters, in hESC-Hep and hiPSC-Hep, pronounced NTCP activity (17% and 29% of 4 h hphep, respectively) and moderate BSEP activity (6% and 8% of 4 h hphep, respectively) were observed. Analyses of mRNA expression and immunocytochemistry supported the observed CYP and transporter activities and showed expression of additional CYPs and transporters. In conclusion, the stable expression and function of CYPs and transporters in hESC-Hep and hiPSC-Hep for at least one week opens up the possibility to reproducibly perform long term and extensive studies, e.g. chronic toxicity testing, in a stem cell-derived hepatic system.

Embryonic stem cells in safety pharmacology and toxicology

Embryonic stem (ES) cells undergo self-renewal and are pluripotent, i.e., they can give rise to all the types of specialised cells in the body. Scientific knowledge on ES cells is increasing rapidly, leading to opportunities for establishment of ES cell-based in vitro tests for drug discovery, preclinical safety pharmacology and toxicology. The main properties of ES cells making them useful in in vitro assays are that they have a normal diploid karyotype and can provide a large number of cells for high-throughput assays. Human ES cells additionally have the potential to provide solutions to problems related to interspecies differences and methods for screening for human polymorphisms, thus supporting robust human hazard identification and optimised drug discovery strategies. Importantly, ES cell based assays could be potential tools to reduce and perhaps replace, animal experiments. This chapter will describe ongoing research in the use of ES cells in toxicology and safety pharmacology, focusing on the major areas of progress, namely, embryotoxicology, cardiotoxicology and hepatoxicology.

Human pluripotent stem cells for modeling toxicity

The development of xenobiotics, driven by the demand for therapeutic, domestic and industrial uses continues to grow. However, along with this increasing demand is the risk of xenobiotic-induced toxicity. Currently, safety screening of xenobiotics uses a plethora of animal and in vitro model systems which have over the decades proven useful during compound development and for application in mechanistic studies of xenobiotic-induced toxicity. However, these assessments have proven to be animal-intensive and costly. More importantly, the prevalence of xenobiotic-induced toxicity is still significantly high, causing patient morbidity and mortality, and a costly impediment during drug development. This suggests that the current models for drug safety screening are not reliable in toxicity prediction, and the results not easily translatable to the clinic due to insensitive assays that do not recapitulate fully the complex phenotype of a functional cell type in vivo. Recent advances in the field of stem cell research have potentially allowed for a readily available source of metabolically competent cells for toxicity studies, derived using human pluripotent stem cells harnessed from embryos or reprogrammed from mature somatic cells. Pluripotent stem cell-derived cell types also allow for potential disease modeling in vitro for the purposes of drug toxicology and safety pharmacology, making this model possibly more predictive of drug toxicity compared with existing models. This article will review the advances and challenges of using human pluripotent stem cells for modeling metabolism and toxicity, and offer some perspectives as to where its future may lie.

Toward Preclinical Predictive Drug Testing for Metabolism and Hepatotoxicity by Using In Vitro Models Derived from Human Embryonic Stem Cells and Human Cell Lines — A Report on the Vitrocellomics EU-project

Drug-induced liver injury is a common reason for drug attrition in late clinical phases, and even for post-launch withdrawals. As a consequence, there is a broad consensus in the pharmaceutical industry, and within regulatory authorities, that a significant improvement of the current in vitro test methodologies for accurate assessment and prediction of such adverse effects is needed. For this purpose, appropriate in vivo-like hepatic in vitro models are necessary, in addition to novel sources of human hepatocytes. In this report, we describe recent and ongoing research toward the use of human embryonic stem cell (hESC)-derived hepatic cells, in conjunction with new and improved test methods, for evaluating drug metabolism and hepatotoxicity. Recent progress on the directed differentiation of human embryonic stem cells to the functional hepatic phenotype is reported, as well as the development and adaptation of bioreactors and toxicity assay technologies for the testing of hepatic cells. The aim of achieving a testing platform for metabolism and hepatotoxicity assessment, based on hESC-derived hepatic cells, has advanced markedly in the last 2–3 years. However, great challenges still remain, before such new test systems could be routinely used by the industry. In particular, we give an overview of results from the Vitrocellomics project (EU Framework 6) and discuss these in relation to the current state-of-the-art and the remaining difficulties, with suggestions on how to proceed before such in vitro systems can be implemented in industrial discovery and development settings and in regulatory acceptance.

Improved preclinical safety assessment using micro-BAL devices: the potential impact on human discovery and drug attrition

Hepatotoxicity is often unpredictable in the early phase of drug discovery and leads to drug attrition in preclinical and clinical development. Here, we discuss the conventional preclinical liver models that do not mimic in vivo livers. We focus on key components such as new sources of hepatocyte-derived human stem cells, enhanced direct oxygenation, defined biocompatibility nanoscaffolds, organotypical cellular models, dynamic culture, and metabolite status inside and outside the cell for effective configuration for the development of a bioartificial liver (BAL) device to mimic the in vivo liver microenvironment. The potential for development of BAL devices could open up new avenues in: (i) hepatotoxicity assessment for selecting drug candidates during preclinical screening; and (ii) therapeutic approaches for liver cell therapy at the clinical stage.

Present state and future perspectives of using pluripotent stem cells in toxicology research

The use of novel drugs and chemicals requires reliable data on their potential toxic effects on humans. Current test systems are mainly based on animals or in vitro–cultured animal-derived cells and do not or not sufficiently mirror the situation in humans. Therefore, in vitro models based on human pluripotent stem cells (hPSCs) have become an attractive alternative. The article summarizes the characteristics of pluripotent stem cells, including embryonic carcinoma and embryonic germ cells, and discusses the potential of pluripotent stem cells for safety pharmacology and toxicology. Special attention is directed to the potential application of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) for the assessment of developmental toxicology as well as cardio- and hepatotoxicology. With respect to embryotoxicology, recent achievements of the embryonic stem cell test (EST) are described and current limitations as well as prospects of embryotoxicity studies using pluripotent stem cells are discussed. Furthermore, recent efforts to establish hPSC-based cell models for testing cardio- and hepatotoxicity are presented. In this context, methods for differentiation and selection of cardiac and hepatic cells from hPSCs are summarized, requirements and implications with respect to the use of these cells in safety pharmacology and toxicology are presented, and future challenges and perspectives of using hPSCs are discussed.

Preferential glutathione conjugation of a reverse diol epoxide compared to a bay region diol epoxide of phenanthrene in human hepatocytes: relevance to molecular epidemiology studies of glutathione-s-transferase polymorphisms and cancer

Bay region diol epoxides are recognized ultimate carcinogens of polycyclic aromatic hydrocarbons (PAH), and in vitro studies have demonstrated that they can be detoxified by conjugation with glutathione, leading to the widely investigated hypothesis that individuals with low activity forms of glutathione-S-transferases are at higher risk of PAH induced cancer, a hypothesis that has found at most weak support in molecular epidemiology studies. A weakness in this hypothesis was that the mercapturic acids resulting from the conjugation of PAH bay region diol epoxides had never been identified in human urine. We recently analyzed smokers' urine for mercapturic acids derived from phenanthrene, the simplest PAH with a bay region. The only phenanthrene diol epoxide-derived mercapturic acid in smokers' urine was produced from the reverse diol epoxide, anti-phenanthrene-3,4-diol-1,2-epoxide (11), not the bay region diol epoxide, anti-phenanthrene-1,2-diol-3,4-epoxide (10), which does not support the hypothesis noted above. In this study, we extended these results by examining the conjugation of phenanthrene metabolites with glutathione in human hepatocytes. We identified the mercapturic acid N-acetyl-S-(r-4,t-2,3-trihydroxy-1,2,3,4-tetrahydro-c-1-phenanthryl)-L-cysteine (14a), (0.33-35.9 pmol/mL at 10 microM 8, 24 h incubation, N = 10) in all incubations with phenanthrene-3,4-diol (8) and the corresponding diol epoxide 11, but no mercapturic acids were detected in incubations with phenanthrene-1,2-diol (7), and only trace amounts were observed in incubations with the corresponding bay region diol epoxide 10. Taken together with our previous results, these studies clearly demonstrate that glutathione conjugation of a reverse diol epoxide of phenanthrene is favored over conjugation of a bay region diol epoxide. Since reverse diol epoxides of PAH are generally weakly or nonmutagenic/carcinogenic, these results, if generalizable to other PAH, do not support the widely held assumption that glutathione-S-transferases are important in the detoxification of PAH in humans.

Transcriptional profiling of human embryonic stem cells differentiating to definitive and primitive endoderm and further toward the hepatic lineage

Human embryonic stem cells (hESC) can differentiate into a variety of specialized cell types, and they constitute a useful model system to study embryonic development in vitro. In order to fully utilize the potential of these cells, the mechanisms that regulate the developmental processes of specific lineage differentiation need to be better defined. The aim of this study was to explore the molecular program involved in the differentiation of hESC toward definitive endoderm (DE) and further into the hepatic lineage, and to compare that with primitive endoderm (PrE) differentiation. To that end, we applied two protocols: a specific DE differentiation protocol and an intrinsic differentiation protocol that mainly mediates PrE formation. We collected hESC, hESC-derived DE, DE-derived hepatocyte-progenitors (DE-Prog), DE-derived hepatocyte-like cells (DE-Hep), and the corresponding PrE derivatives. The samples were analyzed using microarrays, and we identified sets of genes that were exclusively up-regulated in DE derivatives (compared to PrE derivatives) at discrete developmental stages. We also investigated known protein interactions among the set of up-regulated genes in DE-Hep. The results demonstrate important differences between DE and PrE differentiation on the transcriptional level. In particular, our results identify a unique molecular program, exclusively activated during development of DE and the subsequent differentiation of DE toward the hepatic lineage. We identified key genes and pathways of potential importance for future efforts to improve hepatic differentiation from hESC. These results reveal new opportunities for rational design of specific interventions with the purpose of generating enriched populations of DE derivatives, including functional hepatocytes.

Human pluripotent stem cells in drug discovery and predictive toxicology

Human pluripotent stem cells are a biological resource most commonly considered for their potential in cell therapy or, as it is now called, ‘regenerative medicine’. However, in the near future, their most important application for human health may well be totally different, as they are more and more envisioned as opening new routes for pharmacological research. Pluripotent stem cells indeed possess the main attributes that make them theoretically fully equipped for the development of cell-based assays in the fields of drug discovery and predictive toxicology. These cells are characterized by: (i) an unlimited self-renewal capacity, which make them an inexhaustible source of cells; (ii) the potential to differentiate into any cell phenotype of the body at any stage of differentiation, with probably the notable exception, however, of the most mature forms of many lineages; and (iii) the ability to express genotypes of interest via the selection of donors, whether they be of embryonic origin, through pre-implantation genetic diagnosis, or adults, by genetic reprogramming of somatic cells, so-called iPSCs (induced pluripotent stem cells). In the present review, we provide diverse illustrations of the use of pluripotent stem cells in drug discovery and predictive toxicology, using either human embryonic stem cell lines or iPSC lines.

Differentiation and characterization of metabolically functioning hepatocytes from human embryonic stem cells

Human embryonic stem cells (hESCs) may provide a cell source for functional hepatocytes for clinical applications and drug development. Initially, the hESC population was enriched to be more than 85% definitive endoderm (DE) as assessed by the expression of CXCR4, SOX17, and FOXA2. We then successfully converted DE into hepatic progenitors with 93% of the cells being positive for alpha-feto protein within 9 days. The percentage of albumin positive cells gradually increased to 90% at days 20-22 after differentiation. Moreover, our hESC-derived hepatocytes (hEH) developed a complete biotransformation system including phase I and II metabolizing enyzmes and phase III transporters. Nuclear receptors, which are critical in regulating the expression of metabolizing enzymes, were also expressed by our hEH. Using ultraperformance liquid chromatography-tandem mass spectrometry technology, we identified seven metabolic pathways of the drug bufuralol including four newly-reported ones in our hEH, which are the same as those in freshly isolated human primary hepatocytes (hPH). In addition, the results of the metabolism of four drugs indicate that our hEH have the capacity to metabolize these drugs at levels that are comparable to hPH. In conclusion, we have generated a relatively homogenous population of hepatocytes from hESCs, which appear to have complete metabolic function that is comparable to primary liver cells. These results represent a significant step towards the efficient differentiation of mature hepatocytes for cell-based therapeutics as well as for pharmacology and toxicology studies.

Generation of hepatocytes from human embryonic stem cells

Human embryonic stem cells (HESCs) are pluripotent cells having a self-renewal capacity. These unique characteristics of HESCs allow them to be an unlimited source of cells that was shown to differentiate into many cell types, among them hepatocytes. The creation of hepatocytes in culture will allow us to further understand the mechanisms involved in the embryogenesis of hepatocytes in humans and to study pathologies related to aberrant differentiation of these cells. The resultant hepatocytes may serve as a source of cells for transplantation and as cells for toxicological studies and drug screening. In the past 10 years, since the derivation of HESCs, various protocols for the differentiation of HESCs to hepatic-like cells were published. In this chapter we detail our protocol for differentiating HESCs into hepatic-like cells through embryoid bodies. We further describe the method for the genetic labeling of the hepatic-like cells derived from the HESCs and their isolation by fluorescence-activated cell sorter. We also summarize the published protocols for differentiation of HESCs into hepatic-like cells.

[Embryonic stem cells in pharmacology]

Because of their self-renewal and pluripotency properties, human embryonic stem cells (hES) receive a marked attention from scientists and clinicians for regenerative medicine. The most recent application of hES cells may however reside in their use as a tool in drug development. The currently available cellular models for preclinical testing consist in primary and immortalized cells that display limitations in terms of available amount and likeliness to their in vivo counterparts, respectively. hES cells have the potential to revolutionize drug discovery by providing a physiological model for any human cell type in the desired amount for the earliest steps of drug development, notably for pharmacological, metabolic and toxicity evaluation. This new generation of model may contribute to reduce, refine or replace animal testing and decrease drug attrition.

Stem cell-derived hepatocytes and their use in toxicology

Better prediction of safety risk and understanding of mechanism of action of drug candidates remain a major challenge in order to prevent late stage attrition. Continuous efforts are made to improve and develop new models, especially in some areas such as hepatotoxicity. Besides primary hepatocytes and transformed liver cell lines, stem cells either isolated from embryos or adult tissues or obtained by reprogramming somatic cells are emerging as a new potential source of unlimited numbers of hepatocytes. Presently, only hepatocyte-like cells expressing low levels of liver-specific markers, especially drug metabolizing and detoxifying enzymes, are usually obtained, making them still unsuitable as metabolically competent cells for toxicity studies. The only exceptions are some hepatoma cell lines, particularly the HepaRG cell line that can differentiate from a bipotent progenitor stage to attain the functional capacity of normal adult hepatocytes in primary culture without losing the indefinite growth property of transformed cells. Since the research field on stem cells is growing fast marked advances might be expected in the next future.

The possible use of stem cells in regenerative medicine: dream or reality?

Stem cells are one of the most fascinating areas in regenerative medicine today. They play a crucial role in the development and regeneration of human life and are defined as cells that continuously reproduce themselves while maintaining the ability to differentiate into various cell types. Stem cells are found at all developmental stages, from embryonic stem cells that differentiate into all cell types found in the human body to adult stem cells that are responsible for tissue regeneration. The general opinion postulates that clinical therapies based on the properties of stem cells may have the potential to change the treatment of degenerative diseases or important traumatic injuries in the "near" future. We here briefly review the literature in particularly for the liver, heart, kidney, cartilage, and bone regeneration.

Differential expression of glutathione S-transferases P1-1 and A1-1 at protein and mRNA levels in hepatocytes derived from human bone marrow mesenchymal stem cells

The aim of this study was to find out the profile of cellular glutathione (GSH) and GSH S-transferase (GST) in hepatocytes differentiated from adult mesenchymal stem cells (MSC). For this purpose, we have derived functionally active hepatocyte-like cells from normal human multipotent adult MSC. Then the differentiated cells were characterized by specific hepatic markers. The cellular GSH and GST catalytic activity toward 1-chloro-2,4-dinitrobenzene (CDNB) were determined in hepatocyte-like cells differentiated from MSC compared with undifferentiated MSC. Reverse transcription polymerase chain reaction (RT-PCR) and immunoblotting techniques were used to study GST-P1-1 and GST-A1-1 expression in differentiated and undifferentiated cells. The results showed that there is more than threefold increase in GST catalytic activity in hepatocytes recovered by day 14 of differentiation. GST-P1-1 mRNA expression was detected in both differentiated hepatocyte-like cells and their undifferentiated progenitors. Under similar conditions, only differentiated hepatocyte-like cells expressed GST-A1-1 mRNA. These results were further confirmed by showing that the undifferentiated cells expressed both GST-A and GST-P proteins. Unlike GST, the level of cellular GSH was declined (approximately 20%) in hepatocytes derived from MSC as compared to that of undifferentiated cells. These data may suggest that hepatogenic differentiation of human bone marrow MSC is accompanied with the regulation of factors participating in GSH conjugation pathway.

Efficient differentiation of functional hepatocytes from human embryonic stem cells

Differentiation of human embryonic stem cells (hESCs) to specific functional cell types can be achieved using methods that mimic in vivo embryonic developmental programs. Current protocols for generating hepatocytes from hESCs are hampered by inefficient differentiation procedures that lead to low yields and large cellular heterogeneity. We report here a robust and highly efficient process for the generation of high-purity (70%) hepatocyte cultures from hESCs that parallels sequential hepatic development in vivo. Highly enriched populations of definitive endoderm were generated from hESCs and then induced to differentiate along the hepatic lineage by the sequential addition of inducing factors implicated in physiological hepatogenesis. The differentiation process was largely uniform, with cell cultures progressively expressing increasing numbers of hepatic lineage markers, including GATA4, HNF4alpha, alpha-fetoprotein, CD26, albumin, alpha-1-antitrypsin, Cyp7A1, and Cyp3A4. The hepatocytes exhibited functional hepatic characteristics, such as glycogen storage, indocyanine green uptake and release, and albumin secretion. In a mouse model of acute liver injury, the hESC-derived definitive endoderm differentiated into hepatocytes and repopulated the damaged liver. The methodology described here represents a significant step toward the efficient generation of hepatocytes for use in regenerative medicine and drug discovery.

The application of human embryonic stem cell technologies to drug discovery

The isolation of human embryonic stem cells about a decade ago marked the birth of a new era in biomedical research. These pluripotent stem cells possess unique properties that make them exceptionally useful in a range of applications. Discussions about human stem cells are most often focused around the area of regenerative medicine and indeed, the possibility to apply these cells in cell replacement therapies is highly attractive. More imminent, however, is the employment of stem cell technologies for drug discovery and development. Novel improved in vitro models based on physiologically relevant human cells will result in better precision and more cost-effective assays ultimately leading to lower attrition rates and safe new drugs.

Human embryonic stem cells: current technologies and emerging industrial applications

The efficiency and accuracy of the drug development process is severely restricted by the lack of functional human cell systems. However, the successful derivation of pluripotent human embryonic stem (hES) cell lines in the late 1990s is expected to revolutionize biomedical research in many areas. Due to their growth capacity and unique developmental potential to differentiate into almost any cell type of the human body, hES cells have opened novel avenues both in basic and applied research as well as for therapeutic applications. In this review we describe, from an industrial perspective, the basic science that underlies the hES cell technology and discuss the current and future prospects for hES cells in novel and improved stem cell based applications for drug discovery, toxicity testing as well as regenerative medicine.

Can human embryonic stem cells contribute to the discovery of safer and more effective drugs?

Few scientific achievements have received such irresistible attention from scientists, clinicians, and the general public as the ability of human embryonic stem (hES) cells to differentiate into functional cell types for regenerative medicine. The most immediate benefit of neurons, cardiomyocytes, and insulin-secreting cells derived from hES cells, however, may reside in their application in drug discovery and toxicology. The availability of renewable human cells with functional similarities to their in vivo counterparts is the first landmark for a new generation of cell-based assays. The development of cell-based assays using human cells that are physiological targets of drug activity will increase the robustness of target validation and efficacy, high-throughput screening (HTS), structure-activity relationship (SAR), and should introduce safer drugs into clinical trials and the marketplace. The pluripotency of embryonic stem cells, that is, the capacity to generate multiple cell types, is a novel path for the discovery of 'regenerative drugs', the pursuit of small molecules that promote tissue repair (neurogenesis, cardiogenesis) or proliferation of resident stem cells in different organs, thus creating drugs that work by a novel mechanism.

Expression of drug metabolizing enzymes in hepatocyte-like cells derived from human embryonic stem cells

Human embryonic stem cells (hESC) offer a potential unlimited source for functional human hepatocytes, since they can differentiate into hepatocyte-like cells displaying a characteristic hepatic morphology and expressing several hepatic markers. Such cells could be used for, e.g. studies of drug metabolism and hepatotoxicity, which however would require a significant expression of drug metabolising enzymes. Thus, we have investigated the expression of cytochrome P450s (CYPs), UDP-glucuronosyltransferases (UGTs), drug transporters, transcription factors and other liver specific genes in hepatocyte-like cells derived from hESC using a simple direct differentiation protocol. The mRNA and protein expression of several important CYPs were determined using low density arrays, real time PCR and Western blotting. Significant CYP expression on the mRNA level was detected in hepatocyte-like cells derived from one out of two different hESC lines tested, which was much higher than in undifferentiated hESC and generally higher than in HepG2 cells. CYP1A2, CYP3A4/7 and low levels of CYP1A1 and CYP2C8/9/19 protein were detected in both lines. The mRNAs for a variety of CYPs and liver specific factors were shown to be inducible in both cell lines, and this was reflected in induced levels of CYP1A2 and CYP3A4/7 protein. This first report on expression of all major CYPs in hepatocyte-like cells derived from hESC represents an important step towards functional hepatocytes, but efforts to further differentiate the cells using optimized protocols are needed before they exhibit similar levels of drug metabolizing enzymes as primary human hepatocytes and liver.