Sequential exposure to cytokines reflecting embryogenesis: the key for in vitro differentiation of adult bone marrow stem cells into functional hepatocyte-like cells

Hepatocyte differentiation of mesenchymal stem cells

BACKGROUND

Liver cell transplantation and bioartificial liver may provide metabolic support of liver function temporary and are prospective treatments for patients with liver failure. Mesenchymal stem cells (MSCs) are expected to be an ideal cell source for transplantation or liver tissue engineering, however the hepatic differentiation of MSCs is still insufficient for clinical application.

DATA SOURCES

A PubMed search on "mesenchymal stem cells", "liver cell" and "hepatocyte differentiation" was performed on the topic, and the relevant articles published in the past ten years were reviewed.

RESULTS

Hepatocyte-like cells differentiated from MSCs are a promising cell source for liver regeneration or tissue engineering. Although it is still a matter of debate as to whether MSC-derived hepatocytes may efficiently repopulate a host liver to provide adequate functional substitution, the majority of animal studies support that MSCs can become key players in liver-directed regenerative medicine. However the clinical application of human stem cells in the treatment of liver diseases is still in its infancy.

CONCLUSIONS

Future studies are required to improve the efficacy and consistency of hepatic differentiation from MSCs. It is necessary to better understand the mechanism to achieve transdifferentiation with high efficiency. More clinical trials are warranted to prove their efficacy in the management of patients with liver failure.

Comparison of in vitro hepatogenic differentiation potential between various placenta-derived stem cells and other adult stem cells as an alternative source of functional hepatocytes

Mesenchymal stem cells (MSCs) are powerful sources for cell therapy in regenerative medicine. The capability to obtain effective stem cell-derived hepatocytes would improve cell therapy for liver diseases. Recently, various placenta-derived stem cells (PDSCs) depending on the localization of placenta have been suggested as alternative sources of stem cells are similar to bone marrow-derived MSC (BM-MSCs) and adipose-derived MSC (AD-MSCs). However, comparative studies for the potentials of the hepatogenic differentiation among various MSCs largely lacking. Therefore, we investigated to compare the potentials for hepatogenic differentiation of PDSCs with BM-MSCs, AD-MSCs, and UCB-MSCs. Several MSCs were isolated from human term placenta, adipose tissue, and umbilical cord blood and characterized isolated MSCs and BM-MSCs was performed by quantitative reverse transcription-PCR (RT-PCR) and special stains after mesodermal differentiation. The hepatogenic potential of PDSCs was compared with AD-MSCs, UCB-MSCs, and BM-MSCs using RT-PCR, PAS stain, ICG up-take assays, albumin expression, urea production, and cytokine assays. MSCs isolated from different tissues all presented similar characteristics of MSCs. However, the proliferative potential of PDSCs and the expression of hepatogenic markers in differentiated PDSCs were higher than other MSCs. Interestingly, the expression of hepatocyte growth factor (HGF) increased in PDSCs after hepatogenic differentiation. Interestingly, stem cell factor (SCF) expression in chorionic plate-derived MSCs, one of the PDSCs, was significantly higher than in the other PDSCs. Taken together, the results of the present study suggest that MSCs isolated from various adult tissues can be induced to undergo hepatogenic differentiation in vitro, and that PDSCs may have the greatest potential for hepatogenic differentiation and proliferation. Therefore, PDSCs could be used as a stem cell source for cell therapy in liver diseases.

Organotypic liver culture models: meeting current challenges in toxicity testing

Prediction of chemical-induced hepatotoxicity in humans from in vitro data continues to be a significant challenge for the pharmaceutical and chemical industries. Generally, conventional in vitro hepatic model systems (i.e. 2-D static monocultures of primary or immortalized hepatocytes) are limited by their inability to maintain histotypic and phenotypic characteristics over time in culture, including stable expression of clearance and bioactivation pathways, as well as complex adaptive responses to chemical exposure. These systems are less than ideal for longer-term toxicity evaluations and elucidation of key cellular and molecular events involved in primary and secondary adaptation to chemical exposure, or for identification of important mediators of inflammation, proliferation and apoptosis. Progress in implementing a more effective strategy for in vitro-in vivo extrapolation and human risk assessment depends on significant advances in tissue culture technology and increasing their level of biological complexity. This article describes the current and ongoing need for more relevant, organotypic in vitro surrogate systems of human liver and recent efforts to recreate the multicellular architecture and hemodynamic properties of the liver using novel culture platforms. As these systems become more widely used for chemical and drug toxicity testing, there will be a corresponding need to establish standardized testing conditions, endpoint analyses and acceptance criteria. In the future, a balanced approach between sample throughput and biological relevance should provide better in vitro tools that are complementary with animal testing and assist in conducting more predictive human risk assessment.

Insulin-like growth factor 1 (IGF-I) improves hepatic differentiation of human bone marrow-derived mesenchymal stem cells

The ability of MSCs (mesenchymal stem cells) to differentiate between other cell types makes these cells an attractive therapeutic tool for cell transplantation. This project was designed to improve transdifferentiation of human MSCs into liver cells using IGF-I (insulin-like growth factor 1) which, despite its important role in liver development, has not been used for in vitro hepatic differentiation. In the present study, the MSCs derived from healthy human bone marrow samples were cultured and characterized by immunophenotyping and differentiation potential into osteoblast and adipocytes. Transdifferentiation into hepatocyte-like cells was performed in the presence/absence of IGF-I in combination with predefined hepatic differentiation cocktail. To evaluate transdifferentiation, morphological features, immuno-cytochemical staining of specific biological markers and hepatic functions were assessed. Morphological assessment and evaluation of glycogen content, albumin and AFP (α-feto protein) expression as well as albumin and urea secretion revealed statistically significant difference between experimental groups compared with the control. Morphology and function (albumin secretion) of IGF-I-treated cells were significantly better than IGF-I-free experimental group. To the best of our knowledge, our study is the first to demonstrate that the combination of IGF-I with the predefined hepatic differentiation cocktail will significantly improve the morphological features of the differentiated cells and albumin secretion.

Hepatogenic differentiation of mesenchymal stem cells induced by insulin like growth factor-I

AIM

To improve hepatic differentiation of human mesenchymal stem cell (MSC) using insulin growth factor 1 (IGF-I), which has important role in liver development, hepatocyte differentiation and function.

METHODS

Bone marrow of healthy donors was aspirated from the iliac crest. The adherent cells expanded rapidly and were maintained with periodic passages until a relatively homogeneous population was established. The identification of these cells was carried out by immunophenotype analysis and differentiation potential into osteocytes and adipocytes. To effectively induce hepatic differentiation, we designed a protocol based on a combination of IGF-I and liver specific factors (hepatocyte growth factor, oncostatin M and dexamethasone). Morphological features, hepatic functions and cytological staining were assessed to evaluate transdifferentiation of human marrow-derived MSCs.

RESULTS

Flow cytometric analysis and the differentiation potential into osteoblasts and adipocytes showed that more than 90% of human MSCs which were isolated and expanded were positive by specific markers and functional tests. Morphological assessment and evaluation of glycogen storage, albumin and α-feto protein expression, as well as albumin and urea secretion revealed a statistically significant difference between the experimental groups and control.

CONCLUSION

In vitro differentiated MSCs using IGF-I were able to display advanced liver metabolic functions, supporting the possibility of developing them as potential alternatives to primary hepatocytes.

Human amniotic fluid-derived stem cells can differentiate into hepatocyte-like cells in vitro and in vivo

Although human amniotic fluid is an attractive source of multipotent stem cells, the potential of amniotic fluid stem cells (AFSCs) to differentiate into hepatic cells has not been extensively evaluated. In this study, we examined whether human AFSCs can differentiate into a hepatic cell lineage in vitro and in vivo. After being treated with cytokines (fibroblast growth factor 4, basic fibroblast growth factor, hepatocyte growth factor, and oncostatin), AFSCs developed a morphology similar to that of hepatocytes. RT-PCR and immunofluorescence analysis showed that the treated AFSCs expressed the hepatocyte-specific markers albumin, cytokeratin 18, and alpha-fetoprotein. The differentiated cells also developed hepatocyte-specific functions, i.e., they secreted albumin, absorbed indocyanine green, and stored glycogen. When transplanted into CCl(4)-injured immunodeficient mice, undifferentiated AFSCs were integrated into the liver tissue, and they expressed markers characteristic of mature human hepatocytes. Although integration of AFSCs into the liver was limited (0.1-0.3% of hepatocytes), histological analysis showed that the recipient mice recovered more rapidly from CCl(4) injury than CCl(4)-injured mice that did not receive AFSCs. AFSCs can differentiate into hepatocyte-like cells in vitro and in vivo and can represent an easily accessible source of progenitor cells for hepatocyte regeneration and liver cell transplantation.

Enhanced differentiation of adult bone marrow-derived stem cells to liver lineage in aggregate culture

Hepatocyte-like cells derived from stem cells hold great potential for clinical and pharmaceutical applications, including high-throughput drug toxicity screening. We report a three-dimensional aggregate culture system for the directed differentiation of adult rat bone marrow-derived stem cells, rat multipotent adult progenitor cells, to hepatocyte-like cells. Compared to adherent monolayer cultures, differentiation in the aggregate culture system resulted in significantly higher expression level of liver-specific transcripts, including an increased albumin mRNA level, and higher levels of albumin and urea secretion. This coincides with the presence of significantly more cells that express intracellular albumin at levels found in primary hepatocytes. The differentiated cell aggregates exhibited cytochrome P450-mediated ethoxyresorufin-O-dealkylation and pentoxyresorufin-O-dealkylation activity. Consistent with these increased mature functions, cells within the aggregates were shown to have many ultrastructural features of mature hepatocytes by transmission electron microscopy. With the scalability of the aggregate culture system and the enhanced differentiation capability, this system may facilitate translation of generating hepatocytes from stem cells to technology.

Evaluation of the multipotent character of human adipose tissue-derived stem cells isolated by Ficoll gradient centrifugation and red blood cell lysis treatment

In the present study, the multipotent potential of two differential isolated human adipose-derived stem cell (hADSC) populations was evaluated. More specifically, hADSC isolated by means of classical Ficoll (F) gradient centrifugation were compared to hADSC isolated by means of red blood cell (RBC) lysis treatment and subsequent cultivation as 3D spheres. No significant difference in the genotypic expression of the multipotent markers Oct-4, Sox-2, Nanog, Klf-4 and cMyc could be observed between both isolation methods. Upon adipogenic and osteogenic differentiation, both hADSC populations showed lipid droplet accumulation and mineral deposition, respectively. Although, a more pronounced mineral deposition was observed in hADSC-RBC, suggesting a higher osteogenic potential. Upon exposure to keratinogenic media, both hADSC populations expressed the keratinocyte markers filaggrin and involucrin, evidencing a successful keratinogenic differentiation. Yet, no differences in expression were observed between the distinctive isolation procedures. Finally, upon exposure to neurogenic differentiation media, a significant difference in marker expression was observed. Indeed, hADSC-RBC only expressed vimentin and nestin, whereas hADSC-F expressed vimentin, nestin, NF-200, MBP and TH, suggesting a higher neurogenic potential. In summary, our data suggest that the choice of the most efficient isolation procedure of hADSC depends on the differentiated cell type ultimately required.

Alternative (non-animal) methods for cosmetics testing: current status and future prospects-2010

The 7th amendment to the EU Cosmetics Directive prohibits to put animal-tested cosmetics on the market in Europe after 2013. In that context, the European Commission invited stakeholder bodies (industry, non-governmental organisations, EU Member States, and the Commission's Scientific Committee on Consumer Safety) to identify scientific experts in five toxicological areas, i.e. toxicokinetics, repeated dose toxicity, carcinogenicity, skin sensitisation, and reproductive toxicity for which the Directive foresees that the 2013 deadline could be further extended in case alternative and validated methods would not be available in time. The selected experts were asked to analyse the status and prospects of alternative methods and to provide a scientifically sound estimate of the time necessary to achieve full replacement of animal testing. In summary, the experts confirmed that it will take at least another 7-9 years for the replacement of the current in vivo animal tests used for the safety assessment of cosmetic ingredients for skin sensitisation. However, the experts were also of the opinion that alternative methods may be able to give hazard information, i.e. to differentiate between sensitisers and non-sensitisers, ahead of 2017. This would, however, not provide the complete picture of what is a safe exposure because the relative potency of a sensitiser would not be known. For toxicokinetics, the timeframe was 5-7 years to develop the models still lacking to predict lung absorption and renal/biliary excretion, and even longer to integrate the methods to fully replace the animal toxicokinetic models. For the systemic toxicological endpoints of repeated dose toxicity, carcinogenicity and reproductive toxicity, the time horizon for full replacement could not be estimated.

Bone marrow mesenchymal stem cells as a treatment for acute liver failure

Bone marrow mesenchymal stem cells (BM-MSCs) are a kind of multipotent stem cells that have the capacity to undergo self-renewal and multi-lineage differentiation. In an appropriate microenvironment, BM-MSCs can differentiate into bone, cartilage, fat, nerve, liver or other cells. Based on this characteristic, BM-MSCs might be used as new seed cells for orthotopic liver transplantation and bioartificial liver support system. This paper reviews the recent advances in research on the use of BM-MSCs as a treatment for acute liver failure.

Hepatic differentiation of mesenchymal stem cells: in vitro strategies

Recently, evidence has been provided that mesenchymal stem/progenitor cells (MSC) from various sources (bone marrow, adipose tissue, skin, placenta, umbilical cord) could occasionally overcome lineage borders and differentiate into endodermal (hepatocytes) and ectodermal (neural cells) cell types in vitro. Whereas unidirectional differentiation into other mesenchymal cell types, including adipocytes, chondrocytes, and osteoblasts, readily occurs in the presence of a simple cocktail of growth factors and nutrients, successful bypassing of lineage borders mainly depends on multistep processes in a coordinated signaling network. Here, we provide a reproducible basic methodology to differentiate adult MSC into functional hepatocytes in a sequential and time-dependent way. In addition, focus lies on the functional characterization of MSC-derived hepatocyte-like cells. In particular, we provide a detailed modus operandi to measure the inducible cytochrome P450 (CYP)-dependent activity of MSC-derived hepatocyte-like cells.

Generation of adipose stromal cell-derived hepatic cells

The demand for primary human hepatocytes to test the toxicity of new drug candidates and to develop cell therapies for liver disease far exceed the number of hepatocytes that can be isolated from donated tissues. Less than 700 whole livers per year are available for research applications. The ability to utilize nonhepatic progenitor cells, such as adipose stromal cells (ASCs), to generate derivatives that mimic primary human hepatocytes would enable the scale-up production of cell products for bioartifical liver-assist devices, cell therapy, and drug discovery applications. ASC hepatogenesis is a rapidly evolving field with improved protocols continually being reported in the literature. In this chapter, current and effective protocols for the expansion, hepatic differentiation, and functional characterization of ASC-derived hepatic cells are outlined. Two major features distinguish optimized methodologies: (a) cytokine-mediated "reprogramming" of mesenchymal ASCs to enable transdifferentiation into endodermal cell lineages, and (b) treatment with sequential media formulations containing factors/extracellular matrices that mimic the temporal expression profiles seen during fetal liver development. Criteria for success are acquisition of hepatic functional activities, such as albumin/urea production and p450 CYP activities, at levels that approach those observed in primary human hepatocyte controls.

HNF-4α determines hepatic differentiation of human mesenchymal stem cells from bone marrow

AIM: To investigate the differentiation status and key factors to facilitate hepatic differentiation of human bone-marrow-derived mesenchymal stem cells (MSCs).

METHODS: Human MSCs derived from bone marrow were induced into hepatocyte-like cells following a previously published protocol. The differentiation status of the hepatocyte-like cells was compared with various human hepatoma cell lines. Overexpression of hepatocyte nuclear factor (HNF)-4α was mediated by adenovirus infection of these hepatocyte-like cells. The expression of interesting genes was then examined by either reverse transcription-polymerase chain reaction (RT-PCR) or real-time RT-PCR methods.

RESULTS: Our results demonstrated that the differentiation status of hepatocyte-like cells induced from human MSCs was relatively similar to poorly differentiated human hepatoma cell lines. Interestingly, the HNF-4 isoform in induced MSCs and poorly differentiated human hepatoma cell lines was identified as HNF-4γ instead of HNF-4α. Overexpression of HNF-4α in induced MSCs significantly enhanced the expression level of hepatic-specific genes, liver-enriched transcription factors, and cytochrome P450 (P450) genes.

CONCLUSION: Overexpression of HNF-4α improves the hepatic differentiation of human MSCs from bone marrow and is a simple way of providing better cell sources for clinical applications.

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.

Induction of hepatic differentiation of mouse bone marrow stromal stem cells by the histone deacetylase inhibitor VPA

Bone marrow stromal stem cells (BMSSCs) may have potential to differentiate in vitro and in vivo into hepatocytes. Here, we investigated the effects of valproic acid (VPA) involved in epigenetic modification, a direct inhibitor of histone deacetylase, on hepatic differentiation of mouse BMSSCs. Following the treatment of 2.5 mM VPA for 72 hrs, the in vitro expanded, highly purified and functionally active mouse BMSSCs from bone marrow were either exposed to some well-defined cytokines and growth factors in a sequential way (fibroblast growth factor-4 [FGF-4], followed by HGF, and HGF + OSM + ITS + dexamethasone, resembling the order of secretion during liver embryogenesis) or transplanted (caudal vein) in mice submitted to a protocol of chronic injury (chronic i.p. injection of CCl4). Additional exposure of the cells to VPA considerably improved the in vitro differentiation, as demonstrated by a more homogeneous cell population exhibited epithelial morphology, increasing expression of hepatic special genes and enhanced hepatic functions. Further more, in vivo results indicate that the pre-treatment of VPA significantly increased the homing efficiency of BMSSCs to the site of liver injury and, additionally, for supporting hepatic differentiation as well as in vitro. We have demonstrated the usefulness of VPA in the transdifferentiation of BMSSCs into hepatocytes both in vitro and in vivo, and regulation of fibroblast growth factor receptors (FGFRs) and c-Met gene expression through post-translational modification of core histones might be the primary initiating event for these effects. This mode could be helpful for liver engineering and clinical therapy.

Identification of cytokines involved in hepatic differentiation of mBM-MSCs under liver-injury conditions

AIM: To identify the key cytokines involved in hepatic differentiation of mouse bone marrow mesenchymal stem cells (mBM-MSCs) under liver-injury conditions.

METHODS: Abdominal injection of CCl₄ was adopted to duplicate a mouse acute liver injury model. Global gene expression analysis was performed to evaluate the potential genes involved in hepatic commitment under liver-injury conditions. The cytokines involved in hepatic differentiation of mBM-MSCs was functionally examined by depletion experiment using specific antibodies, followed by rescue experiment and direct inducing assay. The hepatic differentiation was characterized by the expression of hepatic lineage genes and proteins, as well as functional features.

RESULTS: Cytokines potentially participating in hepatic fate commitment under liver-injury conditions were initially measured by microarray. Among the up-regulated genes determined, 18 cytokines known to closely relate to liver growth, repair and development, were selected for further identification. The fibroblast growth factor-4 (FGF-4), hepatocyte growth factor (HGF) and oncostatin M (OSM) were finally found to be involved in hepatic differentiation of mBM-MSCs under liver-injury conditions. Hepatic differentiation could be dramatically decreased after removing FGF-4, HGF and OSM from the liver-injury conditioned medium, and could be rescued by supplementing these cytokines. The FGF-4, HGF and OSM play different roles in the hepatic differentiation of mBM-MSCs, in which FGF-4 and HGF are essential for the initiation of hepatic differentiation, while OSM is critical for the maturation of hepatocytes.

CONCLUSION: FGF-4, HGF and OSM are the key cytokines involved in the liver-injury conditioned medium for the hepatic differentiation of mBM-MSCs.

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.

In vitro hepatic differentiation of human bone marrow mesenchymal stem cells under differential exposure to liver-specific factors

Recent findings demonstrated that stem cells could be harvested from a patient and used to repair his or her own damaged liver. Additionally, stem cells may be manipulated in vitro to induce hepatic differentiation. The current study aims to determine the differentiation efficacy of various liver-specific factors (hepatocyte growth factor, Insulin-Transferrin-Selenium, dexamethasone, and nicotinamide) used for stem cell differentiation into hepatocyte-like cells. Human mesenchymal stem cells were exposed to different media containing these compounds added individually or in various combinations. Hepatic differentiation was assessed via quantitative reverse transcription-polymerase chain reaction and immunocytochemical staining for stemness or liver-specific genes and proteins, including albumin, cytokeratins 18 and 19, HepPar-1, alpha-fetoprotein, and nestin. In addition, functional tests for glycogen storage, urea production, glucose, and albumin synthesis were also performed. The expression profiles of albumin, alpha-fetoprotein, and cytokeratin 19 demonstrated that when hepatocyte growth factor, nicotinamide, or dexamethasone were added individually, incomplete hepatocyte differentiation was achieved; the obtained cell populations contained progenitors that expressed both hepatic (albumin) and biliary (cytokeratin 19) markers, as well as alpha-fetoprotein. Hepatocyte growth factor and nicotinamide were the factors with the most hepatogenic potential. When all factors were added to the culture, cells exhibited features that closely resembled human adult hepatocytes as determined by their gene expression patterns (albumin, HepPar-1, and alpha-fetoprotein, but not cytokeratin 19) and functional testing. These cells with hepatic function may become important tools for liver transplant procedures, liver development studies, and pharmacologic research.

Characterization and hepatic differentiation of skin-derived precursors from adult foreskin by sequential exposure to hepatogenic cytokines and growth factors reflecting liver development

In the present study, we investigated whether precursor cells isolated from the dermis of infant human foreskin are capable to differentiate into hepatocyte-like cells upon sequential and gradual exposure to hepatogenic factors [fibroblast growth factor (FGF)-4, hepatocyte growth factor (HGF), insulin-transferrin-selenite (ITS), dexamethasone and oncostatin M (OSM)], mimicking the liver embryogenesis in vivo. Undifferentiated human skin-derived precursors (hSKP) are characterized by a fibroblast-like shape. Yet, they already express typical hepatic proteins, including cytokeratin (CK)-18, hepatocyte nuclear factor (HNF)-4 and HNF-1alpha. Microarray analysis further reveals gene expression of (i) the stemness markers nestin, POU5F1 (OCT-4), telomerase reverse transcriptase (TERT) and thymocyte differentiation antigen (THY)-1, (ii) biliary CK14 and CK19, (iii) biliary/foetal hepatic connexin (Cx)-43, and (iv) adult hepatic CK18, HNF-4 and HNF-1alpha. Upon differentiation, cells undergo morphological and phenotypic changes. As such, hSKP adopt a more polygonal-to-cuboidal cell shape. At the protein level, Cx43 expression is downregulated whereas typical hepatic markers, including alfa-foetoprotein (AFP), prealbumin (TTR) and albumin (ALB), become expressed in accordance to in vivo patterns observed during hepatogenesis. In conclusion, these data show for the first time that hSKP are capable to "trans" differentiate into hepatocyte-like cells upon mimicking the in vivo micro-environment of developing hepatocytes in vitro.

In vitro differentiation of embryonic and adult stem cells into hepatocytes: state of the art

Stem cells are a unique source of self-renewing cells within the human body. Before the end of the last millennium, adult stem cells, in contrast to their embryonic counterparts, were considered to be lineage-restricted cells or incapable of crossing lineage boundaries. However, the unique breakthrough of muscle and liver regeneration by adult bone marrow stem cells at the end of the 1990s ended this long-standing paradigm. Since then, the number of articles reporting the existence of multipotent stem cells in skin, neuronal tissue, adipose tissue, and bone marrow has escalated, giving rise, both in vivo and in vitro, to cell types other than their tissue of origin. The phenomenon of fate reprogrammation and phenotypic diversification remains, though, an enigmatic and rare process. Understanding how to control both proliferation and differentiation of stem cells and their progeny is a challenge in many fields, going from preclinical drug discovery and development to clinical therapy. In this review, we focus on current strategies to differentiate embryonic, mesenchymal(-like), and liver stem/progenitor cells into hepatocytes in vitro. Special attention is paid to intracellular and extracellular signaling, genetic modification, and cell-cell and cell-matrix interactions. In addition, some recommendations are proposed to standardize, optimize, and enrich the in vitro production of hepatocyte-like cells out of stem/progenitor cells.

Current status of stem cell therapy for liver diseases

Liver failure is one of the main causes of death worldwide and is a growing health problem. Since the discovery of stem cell populations capable of differentiating into specialized cell types, including hepatocytes, the possibility of their utilization in the regeneration of the damaged liver has been a focus of intense investigation. A variety of cell types were tested both in vitro and in vivo, but the definition of a more suitable cell preparation for therapeutic use in each type of liver lesions is yet to be determined. Here we review the protocols described for differentiation of stem cells into hepatocytes, the results of cell therapy in animal models of liver diseases, as well as the available data of the clinical trials in patients with advanced chronic liver disease.

Sequential hepatogenic transdifferentiation of adipose tissue-derived stem cells: relevance of different extracellular signaling molecules, transcription factors involved, and expression of new key marker genes

Adipose tissue contains a mesenchymal stem cell (MSC) population known as adipose-derived stem cells (ASCs) capable of differentiating into different cell types. Our aim was to induce hepatic transdifferentiation of ASCs by sequential exposure to several combinations of cytokines, growth factors, and hormones. The most efficient hepatogenic protocol includes fibroblastic growth factors (FGF) 2 and 4 and epidermal growth factor (EGF) (step 1), hepatocyte growth factor (HGF), FGF2, FGF4, and nicotinamide (Nic) (step 2), and oncostatin M (OSM), dexamethasone (Dex), and insulin-tranferrin-selenium (step 3). This protocol activated transcription factors [GATA6, Hex, CCAAT/enhancer binding protein alpha and beta (CEBPalpha and beta), peroxisome proliferator-activated receptor-gamma, coactivator 1 alpha (PGC1alpha), and hepatocyte nuclear factor 4 alpha (HNF4alpha)], which promoted a characteristic hepatic phenotype, as assessed by new informative markers for the step-by-step hepatic transdifferentiation of hMSC [early markers: albumin (ALB), alpha-2-macroglobuline (alpha2M), complement protein C3 (C3), and selenoprotein P1 (SEPP1); late markers: cytochrome P450 3A4 (CYP3A4), apolipoprotein E (APOE), acyl-CoA synthetase long-chain family member 1 (ACSL1), and angiotensin II receptor, type 1 (AGTR1)]. The loss of adipose adult stem cell phenotype was detected by losing expression of Thy1 and inhibitor of DNA binding 3 (Id3). The reexpression of phosphoenolpyruvate corboxykinase (PEPCK), apolipoprotein C3 (APOCIII), aldolase B (ALDOB), and cytochrome P450 1A2 (CYP1A2) was achieved by transduction with a recombinant adenovirus for HNF4alpha and finally hepatic functionality was also assessed by analyzing specific biochemical markers. We conclude that ASCs could represent an alternative tool in clinical therapy for liver dysfunction and regenerative medicine.

Rapid hepatic fate specification of adipose-derived stem cells and their therapeutic potential for liver failure

BACKGROUND AND AIM

Multipotential mesenchymal stem cells (MSC), present in many organs and tissues, represent an attractive tool for the establishment of a successful stem cell-based therapy in the field of regeneration medicine. Adipose tissue mesenchymal stem cells (AT-MSC), known as adipose-derived stem cells (ASC) are especially attractive in the context of future clinical applications because of their high accessibility and minimal invasiveness during the procedure to obtain them. The goal of the present study was to induce human ASC into functional hepatocytes in vitro within a very short period of time and to check their therapeutic potential in vivo.

METHODS

In vitro generated ASC-derived hepatocytes were checked for hepatocyte-specific markers and functions. Afterwards, they were transplanted into nude mice with liver injury. Twenty-four hours after transplantation, biochemical parameters were evaluated in blood serum.

RESULTS

We have shown here that ASC can be differentiated into hepatocytes within 13 days and can reach the functional properties of primary human hepatocytes. After transplantation into mice with acute liver failure, ASC-derived hepatocytes can restore such liver functions as ammonia and purine metabolism. Markers of liver injury, alanine aminotransferase, aspartate aminotransferase, as well as ammonia, were decreased after ASC-derived hepatocyte transplantation.

CONCLUSIONS

Our data highlight the properties of ASC as having a special affinity for hepatocyte differentiation in vitro and liver regeneration in vivo. Thus, ASC may be a superior choice for the establishment of a therapy for injured liver.

Evolving concepts in liver tissue modeling and implications for in vitro toxicology

The development of human cell models stably expressing functional properties of the in vivo cells they are derived from for predicting toxicity of chemicals is a major challenge. For mimicking the liver, a major target of toxic chemicals, primary hepatocytes represent the most pertinent model. Their use is limited by interdonor functional variability and early phenotypic changes although their lifespan can be extended not only by culturing in a 2D dimension under sophisticated conditions but also by the use of synthetic and natural scaffolds as 3D supporting templates that allow cells to have a more stable microenvironment. Hepatocytes derived from stem cells could be the most appropriate alternative but up to now only liver progenitors/hepatoblasts are obtained in vitro. A few hepatocyte cell lines have retained a variable set of liver-specific functions. Among them are the human hepatoma HepaRG cells that express drug metabolism capacity at levels close to those found in primary hepatocytes making them a suitable model for both acute and chronic toxicity studies. New screening strategies are now proposed based on miniaturized and automated systems; they include the use of microfluidic chips and cell chips coupled with high content imaging analysis. Toxicogenomics technologies (particularly toxicotranscriptomics) have emerged as promising in vitro approaches for better identification and discrimination of cellular responses to chemicals. They should allow to discriminate compounds on the basis of the identification of a set of markers and/specific signaling pathways.

Xenotransplantation of Human Umbilical Cord Blood Mononuclear Cells to Rats with D-Galactosamine-Induced Hepatitis

Cord blood is an attractive cell source in regenerative medicine and represents an alternative to bone marrow. The aim of this study was to investigate whether human umbilical cord blood mononuclear (HUCBM) cells might be valuable in hepatic regenerative medicine. HUCBM cells differentiated in vitro into hepatocytes, as suggested by expression of albumin, cytokeratin-18, glutamine synthetase, α-fetoprotein, and cytochrome P450 3A4 at both mRNA and protein levels in a time-dependent fashion. In contrast, the hematopoietic phenotype was gradually lost, as demonstrated by disappearance of CD45 expression. The regenerative potential of HUCBM cells was tested by using a human-to-rat xenotransplant model in which HUCBM cells were intraportally injected into rats with D-galactosamine-induced hepatitis. Liver histology and biochemical markers of hepatic damage were determined. Presence of human cells was detected in blood and liver of both control and D-galactosamine-treated animals. Cell transplantation produced an improvement in both the histological damage and liver function, as demonstrated by plasma values of alkaline phosphatase, γ-glutamyl transferase, lactate dehydrogenase, and total and direct bilirubins. Results obtained suggest that HUCBM cells are capable of hepatic engraftment in this human-to-rat xenotransplant model and that transplantation of HUCBM cells may be a suitable therapy for liver disease.

In vitro differentiation of mouse bone marrow stromal stem cells into hepatocytes induced by conditioned culture medium of hepatocytes

The differentiation potential of adult stem cells has long been believed to be limited to the tissue or germ layer of their origin. However, recent studies have demonstrated that adult stem cells may encompass a greater potential than once thought. In the present study, we examined whether murine bone marrow derived stromal stem cells (BMSSCs) are able to differentiate into functional hepatocytes in vitro. BMSSCs were isolated from murine femora and tibiae, and the mesodermal multilineage differentiation potentials of these cells were functionally characterized. To effectively induce hepatic differentiation, we designed a novel protocol by using hepatocyte-conditioned medium. Hepatic differentiation from mouse BMSSCs was examined by a variety of assays at morphological and molecular levels. Morphologically, mouse BMSSCs became round and epithelioid, binucleated after induction. Differentiated cells were harvested on Days 0, 10, and 20 and subjected to examination of hepatocyte characteristics by reverse transcription-polymerase chain reaction (RT-PCR) and immunocytochemistry. We detected AFP, HNF-3beta, CK19, CK18, ALB, TAT, and G-6-Pase at the mRNA and/or protein levels, hepatocyte-like cells by culture in conditioned medium further demonstrated in vitro functions characteristic of liver cells, including glycogen storage, and urea secretion. Moreover, transplantation of the differentiated cells into liver-injured mice partially restored serum albumin level and significantly suppressed transaminase activity. Our findings indicated the transdifferentiation potential of mouse BMSSCs developing into the functional hepatocyte-like cells by conditioned culture medium and, hence, may serve as a model system for the study of mechanisms involved in the transdifferentiation, and a cell source for cell therapy of hepatic diseases.

Differentiation of neonatal rat epithelial cells from biliary origin into immature hepatic cells by sequential exposure to hepatogenic cytokines and growth factors reflecting liver development

It was investigated whether cryopreserved rat liver epithelial cells (RLEC) from biliary origin are capable of undergoing hepatic differentiation upon sequential exposure to liver-specific factors (fibroblast growth factor (FGF)-4, hepatocyte growth factor (HGF), insulin-transferrin-sodium-selenite (ITS), dexamethasone (Dex) and oncostatin M (OSM)), reflecting liver embryogenesis in vivo. As differentiation progressed, cells acquired a hepatic morphology (polygonal-to-cuboidal shaped, binucleated cells), corresponding well with the phenotypic changes observed. Biliary cytokeratin (CK)19 and connexin (Cx)43-expression both gradually decreased; CK19-expression disappeared even completely. In contrast, hepatic CK18-expression persisted throughout the culture time. Hepatocyte nuclear factor (HNF)3beta, alpha-foetoprotein (AFP), transthyretin (TTR), HNF4, albumin (ALB), HNF1alpha, multidrug resistance protein (MRP)2 and Cx32 were expressed at specific stages during RLEC-differentiation, thereby showing a progressive hepatic maturation. Indeed, immature AFP and mature ALB were sequentially expressed, in line with the in vivo liver embryogenesis. Expression of the early and mid-late 'liver-enriched' transcription factors (LETF) HNF3beta and HNF4 declined and translocated to the cytosol, respectively, while the late LETF HNF1alpha underwent a nuclear upregulation. In conclusion, RLEC are bipotent cells, capable of differentiation into immature hepatocytes in a hepatic-stimulating micro-environment. The robustness of the sequential conditions, developed before for hepatic 'transdifferentiation' of rat bone marrow stem cells (rBMSC), was hereby confirmed.

Chromatin remodeling agent trichostatin A: a key-factor in the hepatic differentiation of human mesenchymal stem cells derived of adult bone marrow

Background

The capability of human mesenchymal stem cells (hMSC) derived of adult bone marrow to undergo in vitro hepatic differentiation was investigated.

Results

Exposure of hMSC to a cocktail of hepatogenic factors [(fibroblast growth factor-4 (FGF-4), hepatocyte growth factor (HGF), insulin-transferrin-sodium-selenite (ITS) and dexamethasone)] failed to induce hepatic differentiation. Sequential exposure to these factors (FGF-4, followed by HGF, followed by HGF+ITS+dexamethasone), however, resembling the order of secretion during liver embryogenesis, induced both glycogen-storage and cytokeratin (CK)18 expression. Additional exposure of the cells to trichostatin A (TSA) considerably improved endodermal differentiation, as evidenced by acquisition of an epithelial morphology, chronological expression of hepatic proteins, including hepatocyte-nuclear factor (HNF)-3β, alpha-fetoprotein (AFP), CK18, albumin (ALB), HNF1α, multidrug resistance-associated protein (MRP)2 and CCAAT-enhancer binding protein (C/EBP)α, and functional maturation, i.e. upregulated ALB secretion, urea production and inducible cytochrome P450 (CYP)-dependent activity.

Conclusion

hMSC are able to undergo mesenchymal-to-epithelial transition. TSA is hereby essential to promote differentiation of hMSC towards functional hepatocyte-like cells.

Differential role of epigenetic modulators in malignant and normal stem cells: a novel tool in preclinical in vitro toxicology and clinical therapy

Adult stem cells are primitive cells that undergo asymmetric division, thereby giving rise to one clonogenic, self-renewing cell and one cell able to undergo multipotent differentiation. Disturbance of this controlled process by epigenetic alterations, including imbalance of histone acetylation/histone deacetylation and DNA methylation/demethylation, may result in uncontrolled growth, formation of self-renewing malignant stem cells and eventually cancer. In view of this notion, several epigenetic modulators, in particular those with histone deacetylase inhibiting activity, are currently being tested in phase I and II clinical trials for their promising chemotherapeutic properties in cancer therapy. As chromatin modulation is also involved in regulation of differentiation, normal development, embryonic and adult stem cell functions and maintenance of their plasticity during embryonic organogenesis, the question can be raised whether predestined cell fate can be modified through epigenetic interference. And if so, could this strategy enforce adult stem cells to differentiate into different types of functional cells? In particular, functional hepatocytes seem important for preclinical toxicity screening of candidate drugs. This paper reviews the potential use and relevance of epigenetic modifiers, including inhibitors of histone deacetylases and DNA methyltransferases (1) to change cell fate and 'trans'differentiate normal adult stem cells into hepatocyte-like cells and (2) to cure disorders, caused by uncontrolled growth of malignant stem cells.

Stem cell plasticity: Learning from hepatogenic differentiation strategies

Many studies on stem cell plasticity are challenging the concept that stem cells contain an intrinsically predefined, unidirectional differentiation program. This means that the developmental fate of a stem cell is dependent on the general potential of the cell (pre-determined stem cell fate) as well as on microenvironmental cues, such as stimuli from growth factors (stem cell niche). Here, we reviewed reports that examined the hepatocyte differentiation ability of stem cells from two different sources: embryonic stem cells and adult stem cells. All of those stem cells revealed the ability to give rise to hepatocyte-like cells using different induction strategies. However, it is still not clear which of those stem cells would be the best source for hepatocyte replacement or which would be the best protocol. We herein present the current knowledge regarding available protocols and factors used in order to obtain functional hepatocytes from stem cells.