Expedited growth factor-mediated specification of human embryonic stem cells toward the hepatic lineage

Hepatic differentiation of human pluripotent stem cells by developmental stage-related metabolomics products

Endogenous cell signals regulate tissue homeostasis and are significant for directing the fate of stem cells. During liver development, cytokines released from various cell types are critical for stem/progenitor cell differentiation and lineage expansions. To determine mechanisms in these stage-specific lineage interactions, we modeled potential effects of soluble signals derived from immortalized human fetal liver parenchymal cells on stem cells, including embryonic and induced pluripotent stem cells. For identifying lineage conversion and maturation, we utilized conventional assays of cell morphology, gene expression analysis and lineage markers. Molecular pathway analysis used functional genomics approaches. Metabolic properties were analyzed to determine the extent of hepatic differentiation. Cell transplantation studies were performed in mice with drug-induced acute liver failure to elicit benefits in hepatic support and tissue regeneration. These studies showed signals emanating from fetal liver cells induced hepatic differentiation in stem cells. Gene expression profiling and comparison of regulatory networks in immature and mature hepatocytes revealed stem cell-derived hepatocytes represented early fetal-like stage. Unexpectedly, differentiation-inducing soluble signals constituted metabolomics products and not proteins. In stem cells exposed to signals from fetal cells, mechanistic gene networks of upstream regulators decreased pluripotency, while simultaneously inducing mesenchymal and epithelial properties. The extent of metabolic and synthetic functions in stem cell-derived hepatocytes was sufficient for providing hepatic support along with promotion of tissue repair to rescue mice in acute liver failure. During this rescue, paracrine factors from transplanted cells contributed in stimulating liver regeneration. We concluded that hepatic differentiation of pluripotent stem cells with metabolomics products will be significant for developing therapies. The differentiation mechanisms involving metabolomics products could have an impact on advancing recruitment of stem/progenitor cells during tissue homeostasis.

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).

In vitro culture of isolated primary hepatocytes and stem cell-derived hepatocyte-like cells for liver regeneration

Various liver diseases result in terminal hepatic failure, and liver transplantation, cell transplantation and artificial liver support systems are emerging as effective therapies for severe hepatic disease. However, all of these treatments are limited by organ or cell resources, so developing a sufficient number of functional hepatocytes for liver regeneration is a priority. Liver regeneration is a complex process regulated by growth factors (GFs), cytokines, transcription factors (TFs), hormones, oxidative stress products, metabolic networks, and microRNA. It is well-known that the function of isolated primary hepatocytes is hard to maintain; when cultured in vitro, these cells readily undergo dedifferentiation, causing them to lose hepatocyte function. For this reason, most studies focus on inducing stem cells, such as embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), hepatic progenitor cells (HPCs), and mesenchymal stem cells (MSCs), to differentiate into hepatocyte-like cells (HLCs) in vitro. In this review, we mainly focus on the nature of the liver regeneration process and discuss how to maintain and enhance in vitro hepatic function of isolated primary hepatocytes or stem cell-derived HLCs for liver regeneration. In this way, hepatocytes or HLCs may be applied for clinical use for the treatment of terminal liver diseases and may prolong the survival time of patients in the near future.

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.

The human constitutive androstane receptor promotes the differentiation and maturation of hepatic-like cells

Expression of the constitutive androstane receptor (CAR, NR1I3) is enriched in the mature mammalian liver and increasingly recognized for its prominent role in regulating a myriad of processes including biotransformation, chemical transport, energy metabolism and lipid homeostasis. Previously, we demonstrated that CAR levels were markedly enhanced during the differentiation of hepatic-like cells derived from hESCs, prompting the hypothesis that CAR contributes a key functional role in directing human hepatogenesis. Here we demonstrate that over-expression of CAR in human embryonic stem cells (ESCs), transduced by a lentiviral vector, accelerates the maturation of hepatic-like cells, with CAR over-expressing cells exhibiting a 2.5-fold increase in albumin secretion by day 20 in culture differentiation, and significantly enhanced levels of mRNA expression of several liver-selective markers, including hepatic transcription factors, plasma proteins, biotransformation enzymes, and metabolic enzymes. CAR over-expressing cells also exhibited enhanced CITCO-inducible CYP3A7 enzymatic activity. Knockdown of CAR via siRNA attenuated the differentiation-dependent expression programs. In contrast, expression levels of the pregnane X receptor (PXR), a nuclear receptor most similar to CAR in primary sequence, were negligible in human fetal liver tissues or in the differentiating hESCs, and stable over-expression of PXR in hepatic-induced hESCs failed to enhance expression of hepatic phenotype markers. Together, these results define a novel role for human CAR in hepatic lineage commitment.

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.

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.

Differentiation of human embryonic stem cells along a hepatic lineage

The limited availability of hepatic tissue suitable for the treatment of liver disease and drug discovery research advances the generation of hepatic-like cells from alternative sources as a valuable approach. In this investigation we exploited a unique hepatic differentiation approach to generate hepatocyte-like cells from human embryonic stem cells (hESCs). hESCs were cultured for 10-20 days on collagen substrate in highly defined and serum free hepatocyte media. The resulting cell populations exhibited hepatic cell-like morphology and were characterized with a variety of biological endpoint analyses. Real-time PCR analysis demonstrated that mRNA expression of the 'stemness' marker genes NANOG and alkaline phosphatase in the differentiated cells was significantly reduced, findings that were functionally validated using alkaline phosphatase activity detection measures. Immunofluorescence studies revealed attenuated levels of the 'stemness' markers OCT4, SOX2, SSEA-3, TRA-1-60, and TRA-1-81 in the hepatic-like cell population. The hepatic character of the cells was evaluated additionally by real-time PCR analyses that demonstrated increased mRNA expression of the hepatic transcription factors FOXA1, C/EBPα, and HNF1α, the nuclear receptors CAR, RXRα, PPARα, and HNF4α, the liver-generated plasma proteins α-fetoprotein, transthyretin, transferrin, and albumin, the protease inhibitor α-1-antitrypsin, metabolic enzymes HMGCS2, PEPCK, and biotransformation enzymes CYP3A7, CYP3A4, CYP3A5, and CYP2E1. Indocyanine green uptake albumin secretion and glycogen storage capacity further confirmed acquisition of hepatic function. These studies define an expeditious methodology that facilitates the differentiation of hESCs along a hepatic lineage and provide a framework for their subsequent use in pharmacological and toxicological research applications requiring a renewable supply of human hepatocytes.

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.

Generating hepatic cell lineages from pluripotent stem cells for drug toxicity screening

Hepatotoxicity is an enormous and increasing problem for the pharmaceutical industry. Early detection of problems during the drug discovery pathway is advantageous to minimize costs and improve patient safety. However, current cellular models are sub-optimal. This review addresses the potential use of pluripotent stem cells in the generation of hepatic cell lineages. It begins by highlighting the scale of the problem faced by the pharmaceutical industry, the precise nature of drug-induced liver injury and where in the drug discovery pathway the need for additional cell models arises. Current research is discussed, mainly for generating hepatocyte-like cells rather than other liver cell-types. In addition, an effort is made to identify where some of the major barriers remain in translating what is currently hypothesis-driven laboratory research into meaningful platform technologies for the pharmaceutical industry.