High-ratio differentiation of embryonic stem cells into hepatocytes in vitro

Stem Cell-Based Therapies for Liver Diseases: An Overview and Update

BACKGROUND

Liver disease is one of the top causes of death globally. Although liver transplantation is a very effective treatment strategy, the shortage of available donor organs, waiting list mortality, and high costs of surgery remain huge problems. Stem cells are undifferentiated cells that can differentiate into a variety of cell types. Scientists are exploring the possibilities of generating hepatocytes from stem cells as an alternative for the treatment of liver diseases.

METHODS

In this review, we summarized the updated researches in the field of stem cell-based therapies for liver diseases as well as the current challenges and future expectations for a successful cell-based liver therapy.

RESULTS

Several cell types have been investigated for liver regeneration, such as embryonic stem cells, induced pluripotent stem cells, liver stem cells, mesenchymal stem cells, and hematopoietic stem cells. In vitro and in vivo studies have demonstrated that stem cells are promising cell sources for the liver regeneration.

CONCLUSION

Stem cell-based therapy could be a promising therapeutic method for patients with end-stage liver disease, which may alleviate the need for liver transplantation in the future.

The similarities between smDCs and regDCs in alleviating the immune injury caused by transplantation of hepatocytes differentiated from ESCs

Background

This study aimed to investigate the tolerogenic mechanisms induced by semimature dendritic cells (smDCs) and regulatory dendritic cells (regDCs) after transplantation of hepatocytes differentiated from mouse embryonic stem cells (ESCs) and to confirm the low immunogenicity of hepatocytes differentiated from ESCs.

Methods

Green fluorescent protein-labeled ESCs collected from 129 mice were cultured to differentiate into hepatocytes. smDCs and regDCs were cultured in vitro. The hepatocytes were cultured after being extracted from the livers of 129 mice. After injecting smDCs or regDCs 3 days in advance, these differentiated hepatocytes and normal hepatocytes were transplanted into the livers of BALB/c mice separately. Subsequently, the histopathological features and cytokines in transplant tissues as well as the Foxp3 expression in peripheral blood CD4⁺ T cells of the recipients were examined.

Results

The morphological phenotypes of smDCs and regDCs were similar. They both expressed medium levels of MHC-II, CD40, CD80, and CD86, high levels of TGF-β and IL-10, and low levels of IL-2. The survival of differentiated hepatocytes was prolonged and inflammatory infiltration in transplant tissues was reduced in both the smDC and regDC groups. Foxp3 expression in peripheral blood CD4⁺ T cells of the smDC group increased to 5.38% and that of the regDC group also rose to 3.87%. Moreover, the inflammatory infiltration in the tissues receiving transplanted hepatocytes was more obvious.

Conclusions

smDCs and regDCs were similar tolerogenic dendritic cells. They both could alleviate the immune injury by inducing CD4⁺CD25⁺Foxp3⁺ regulatory T cells through the medium expression of MHC-II, CD40, CD80, and CD86 and the appropriate secretion of cytokines. Hepatocytes differentiated from ESCs displayed low immunogenicity.

Efficient large-scale generation of functional hepatocytes from mouse embryonic stem cells grown in a rotating bioreactor with exogenous growth factors and hormones

Introduction

Embryonic stem (ES) cells are considered a potentially advantageous source of hepatocytes for both transplantation and the development of bioartificial livers. However, the efficient large-scale generation of functional hepatocytes from ES cells remains a major challenge, especially for those methods compatible with clinical applications.

Methods

In this study, we investigated whether a large number of functional hepatocytes can be differentiated from mouse ES (mES) cells using a simulated microgravity bioreactor. mES cells were cultured in a rotating bioreactor in the presence of exogenous growth factors and hormones to form embryoid bodies (EBs), which then differentiated into hepatocytes.

Results

During the rotating culture, most of the EB-derived cells gradually showed the histologic characteristics of normal hepatocytes. More specifically, the expression of hepatic genes and proteins was detected at a higher level in the differentiated cells from the bioreactor culture than in cells from a static culture. On further growing, the EBs on tissue-culture plates, most of the EB-derived cells were found to display the morphologic features of hepatocytes, as well as albumin synthesis. In addition, the EB-derived cells grown in the rotating bioreactor exhibited higher levels of liver-specific functions, such as glycogen storage, cytochrome P450 activity, low-density lipoprotein, and indocyanine green uptake, than did differentiated cells grown in static culture. When the EB-derived cells from day-14 EBs and the cells’ culture supernatant were injected into nude mice, the transplanted cells were engrafted into the recipient livers.

Conclusions

Large quantities of high-quality hepatocytes can be generated from mES cells in a rotating bioreactor via EB formation. This system may be useful in the large-scale generation of hepatocytes for both cell transplantation and the development of bioartificial livers.

Mouse A6-positive hepatic oval cells derived from embryonic stem cells

Oval cells have a potential to differentiate into a variety of cell lineages including hepatocytes and biliary epithelia. Several models have been established to activate the oval cells by incorporating a variety of toxins and carcinogens, alone or combined with surgical treatment. Those models are obviously not suitable for the study on human hepatic oval cells. It is necessary to establish a new and efficient model to study the human hepatic oval cells. In this study, the hepatocyte growth factor (HGF) and epidermal growth factor (EGF) were used to induce differentiation of mouse embryonic stem (ES) cells into hepatic oval cells. We first confirmed that hepatic oval cells derived from ES cells, which are bipotential, do exist during the course of mouse ES cells' differentiation into hepatic parenchymal cells. RT-PCR and transmission electron microscopy were applied in this study. The ratio of Sca-1+/CD34+ cells sorted by FACS in the induction group was increased from day 4 and reached the maximum on the day 8, whereas that in the control group remained at a low level. The differentiation ratio of Sca-1+/CD34+ cells in the induction group was significantly higher than that in the control group. About 92.48% of the sorted Sca-1+/CD34+ cells on the day 8 were A6 positive. Highly purified A6+/Sca-1+/CD34+ hepatic oval cells derived from ES cells could be obtained by FACS. The differentiation ratio of hepatic oval cells in the induction group (up to 4.46%) was significantly higher than that in the control group. The number of hepatic oval cells could be increased significantly by HGF and EGF. The study also examined the ultrastructures of ES-derived hepatic oval cells' membrane surface by atomic force microscopy. The ES-derived hepatic oval cells cultured and sorted by our protocols may be available for the future clinical application.

Epithelial-mesenchymal transition delayed by E-cad to promote tissue formation in hepatic differentiation of mouse embryonic stem cells in vitro

Hepatic differentiation of embryonic stem cells (ESCs) usually results in a single cell lineage, and the formation of liver tissues remains difficult. Here, we examine the role of epithelial-mesenchymal transition (EMT) that is regulated by epithelial cadherin (E-cad) expression in hepatic tissue formation from ESCs. E-cad was transfected into mouse ESCs to enable a stable expression of E-cad. Hepatic differentiation of ESCs was then induced by hepatic growth factors. Wnt/β-catenin signaling and EMT speed were examined to determine the differentiation process. Hepatic and angiogenesis markers, as well as differentiated cell-adhesive force were also examined to identify the hepatic tissue differentiation. In our results, E-cad expression gradually decreased in normal ESC (N-ESC) differentiation, but remained stable in the E-cad transfected ESC (EC-ESC) group. In EC-ESC differentiation, expressions of cytoplastic β-catenin and EMT were much lower and significantly prolonged. Angiogenesis markers vascular endothelial growth factor receptor-1 (VEGFR-1) and CD31/PECAM-1 were expressed only on day 5-13 in N-ESC differentiation, whereas VEGFR-1 and CD31/PECAM-1 were expressed prolonged on day 5-17 in the EC-ESC group and were coincident with the expression of hepatic markers. Finally, EC-ESC differentiation maintained multilayer-growth patterns, and abundant vascular network structures appeared and migrated in albumin-positive cell areas. The cellular adhesion forces between embryonic body cells in EC-ESC differentiation during day 13-17 were similar to those of mouse liver tissue. In conclusion, accelerated EMT due to the decreased E-cad expression may partially contribute to the failure of hepatic tissue formation in N-ESC differentiation. E-cad can act in synergy with hepatic growth factors and facilitate the early-stage formation of hepatic tissues through down-regulating Wnt/β-catenin signaling and delaying EMT. This work provides a new insight into hepatic tissue differentiation that is mediated by E-cad from ESC.

Awakened by cellular stress: isolation and characterization of a novel population of pluripotent stem cells derived from human adipose tissue

Advances in stem cell therapy face major clinical limitations, particularly challenged by low rates of post-transplant cell survival. Hostile host factors of the engraftment microenvironment such as hypoxia, nutrition deprivation, pro-inflammatory cytokines, and reactive oxygen species can each contribute to unwanted differentiation or apoptosis. In this report, we describe the isolation and characterization of a new population of adipose tissue (AT) derived pluripotent stem cells, termed Multilineage Differentiating Stress-Enduring (Muse) Cells, which are isolated using severe cellular stress conditions, including long-term exposure to the proteolytic enzyme collagenase, serum deprivation, low temperatures and hypoxia. Under these conditions, a highly purified population of Muse-AT cells is isolated without the utilization of cell sorting methods. Muse-AT cells grow in suspension as cell spheres reminiscent of embryonic stem cell clusters. Muse-AT cells are positive for the pluripotency markers SSEA3, TR-1-60, Oct3/4, Nanog and Sox2, and can spontaneously differentiate into mesenchymal, endodermal and ectodermal cell lineages with an efficiency of 23%, 20% and 22%, respectively. When using specific differentiation media, differentiation efficiency is greatly enhanced in Muse-AT cells (82% for mesenchymal, 75% for endodermal and 78% for ectodermal). When compared to adipose stem cells (ASCs), microarray data indicate a substantial up-regulation of Sox2, Oct3/4, and Rex1. Muse-ATs also exhibit gene expression patterns associated with the down-regulation of genes involved in cell death and survival, embryonic development, DNA replication and repair, cell cycle and potential factors related to oncogenecity. Gene expression analysis indicates that Muse-ATs and ASCs are mesenchymal in origin; however, Muse-ATs also express numerous lymphocytic and hematopoietic genes, such as CCR1 and CXCL2, encoding chemokine receptors and ligands involved in stem cell homing. Being highly resistant to severe cellular stress, Muse-AT cells have the potential to make a critical impact on the field of regenerative medicine and cell-based therapy.

Liver regenerative medicine: advances and challenges

Liver transplantation is the standard care for many end-stage liver diseases. However, donor organs are scarce and some people succumb to liver failure before a donor is found. Liver regenerative medicine is a special interdisciplinary field of medicine focused on the development of new therapies incorporating stem cells, gene therapy and engineered tissues in order to repair or replace the damaged organ. In this review we consider the emerging progress achieved in the hepatic regenerative medicine within the last decade. The review starts with the characterization of liver organogenesis, fetal and adult stem/progenitor cells. Then, applications of primary hepatocytes, embryonic and adult (mesenchymal, hematopoietic and induced pluripotent) stem cells in cell therapy of liver diseases are considered. Current advances and challenges in producing mature hepatocytes from stem/progenitor cells are discussed. A section about hepatic tissue engineering includes consideration of synthetic and natural biomaterials in engineering scaffolds, strategies and achievements in the development of 3D bioactive matrices and 3D hepatocyte cultures, liver microengineering, generating bioartificial liver and prospects for fabrication of the bioengineered liver.

Strategies to produce hepatocytes and hepatocyte-like cells from pluripotent stem cells

Human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) are a potent source for unlimited production of hepatocytes and hepatocyte-like cells that may replace primary human hepatocytes in a variety of fields including liver cell therapy, liver tissue engineering, manufacturing bioartificial liver, modeling inherited and chronic liver diseases, drug screening and toxicity testing. Human ESCs are able to spontaneously form embryoid bodies, which then spontaneously differentiate to various tissue-specific cell lineages containing a total of 10-30% albumin-producing hepatocytes and hepatocyte-like cells. Enrichment of embryoid bodies with the definitive endoderm, from which hepatocytes arise, yields increasing the final ratio of hepatocyte population up by 50-65%. Current strategies of the directed differentiation of human ESCs (and iPSCs) to hepatocytes that reproduce liver embryogenesis by sequential stimulation of culturing ESCs with tissue-specific growth factors result in achieving the differentiation rate up to 60-80%. In the future, directed differentiation of human ESCs and iPSCs to hepatocytes should be further optimized towards generating homogeneous cultures of hepatocytes in order to avoid expensive procedures of separation and isolation of hepatocytes and hepatocyte-like cells.

Progress in stem cell biology in regenerative medicine for liver disease

Regenerative medicine using stem cells has attracted much attention, since stem cells are responsible for highly proliferative activity and multipotential ability of differentiation. Induced pluripotent stem cells and embryonic stem cells or the adult stem cells such as bone marrow-derived stem cells and adipose tissue-derived stem cells have been expected as a cell source of regenerative medicine. Since differentiating methods of human stem cells into the defined lineage of cells remains to be developed, we focus on the differentiating strategies of pluripotent stem cells and mesenchymal stem cells into liver lineage, especially on cytokine function and gene expression during hepatic differentiation. The survey of previously published papers discloses that the protocols that mimic the liver developmental process seem to be effective in obtaining functional hepatocytes. However, in order to develop hepatic regenerative medicine that is useful in a clinical setting, more effective and potent strategies that obtain mature hepatocytes are required.

Differentiation of mouse embryonic stem cells into hepatocytes induced by a combination of cytokines and sodium butyrate

There is increasing evidence to suggest that embryonic stem cells (ESCs) are capable of differentiating into hepatocytes in vitro. In this study, we used a combination of cytokines and sodium butyrate in a novel three-step procedure to efficiently direct the differentiation of mouse ESCs into hepatocytes. Mouse ESCs were first differentiated into definitive endoderm cells by 3 days of treatment with Activin A. The definitive endoderm cells were then differentiated into hepatocytes by the addition of acidic fibroblast growth factor (aFGF) and sodium butyrate to the culture medium for 5 days. After 10 days of further in vitro maturation, the morphological and phenotypic markers of hepatocytes were characterized using immunohistochemistry, immunoblotting, and reverse transcription-polymerase chain reaction (RT-PCR). Furthermore, the cells were tested for functions associated with mature hepatocytes, including glycogen storage and indocyanine green uptake and release, and the ratio of hepatic differentiation was determined by counting the percentage of albumin-positive cells. In the presence of medium containing cytokines and sodium butyrate, numerous epithelial cells resembling hepatocytes were observed, and approximately 74% of the cells expressed the hepatic marker, albumin, after 18 days in culture. RT-PCR analysis and immunohistochemistry showed that these cells expressed adult liver cell markers, and had the abilities of glycogen storage and indocyanine green uptake and release. We have developed an efficient method for directing the differentiation of mouse ESCs into cells that exhibit the characteristics of mature hepatocytes. This technique will be useful for research into the molecular mechanisms underlying liver development, and could provide a source of hepatocytes for transplantation therapy and drug screening.

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.

Directing hepatic differentiation of embryonic stem cells with protein microarray-based co-cultures

Embryonic stem cells hold considerable promise in tissue engineering and regenerative medicine as a source of tissue-specific cells. However, realizing this promise requires novel methods for guiding lineage-specific differentiation of stem cells. In this study, we developed a micropatterned co-culture platform for stimulating hepatic differentiation of mouse embryonic stem cells (mESCs). Studies of mESC and hepatic cell adhesion preferences revealed that mESCs required fibronectin for attachment, while hepatic cells (HepG2) preferred collagen (I) substrate and did not adhere to fibronectin. Printing columns of collagen (I) and fibronectin spots (300 microm diameter), followed by sequential seeding of the two cell types, allowed the positioning of clusters of mESCs adjacent to groups of hepatic cells within the same microarray. These micropatterned co-cultures were maintained for up to two weeks in hepatic differentiation media supplemented. To examine the differentiation, mESCs were selectively extracted from the co-culture using laser microdissection and analyzed using real-time reverse transcriptase (RT)-polymerase chain reaction (PCR). These analyses revealed that mESCs co-cultured with HepG2 cells showed a decrease in pluripotency gene expression concomitant with up-regulation of endodermal genes. In addition, the co-culture format induced a significant increase in the expression of liver genes compared to mESCs cultured alone. In conclusion, micropatterned co-cultures of mESCs and hepatic cells showed a significant promise in driving stem cell differentiation towards hepatic phenotype. In the future, this cell culture platform will be further enhanced to enable efficient conversion of mouse and human ESCs to hepatocytes.

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

Human embryonic stem cells (hESCs) have the potential to be a promising source of liver cells, hepatocytes, for regenerative medicine given their unlimited proliferative and pluripotent differentiative capacity. However, the inefficient embryoid body process and limited understanding of molecular signals potentiating cell-specific differentiation plague the use of hESCs as a hepatic source. In this study, we describe an efficient growth factor-based process for directed differentiation of hESCs that bypasses embryoid body development. The system involves adherent hESC culture exposure to activin A treatment followed by incorporation of various growth factor combinations composed of dexamethasone, oncostatin M, hepatocyte growth factor, and Wnt3A. The hESC-derived hepatocyte-like cells resulting from optimal growth factor combinations exhibit characteristic hepatocyte morphology, express hepatocyte markers, and possess hepatospecific functional activity. The differentiated cultures express hepatic-related genes shown by reverse transcription-polymerase chain reaction and immunofluorescence analysis revealed binucleated cells with coexpression of albumin/cytokeratin 18. Furthermore, the hESC-derived hepatocyte-like cells exhibit functional hepatic characteristics, such as indocyanine green uptake and release, albumin secretion, and inducible cytochrome P450 activity. This directed differentiation of adherent hESCs offers an efficient process to produce hepatocyte-like cells in vitro for hepatocyte differentiation studies and organotypic cultures for diagnostic and therapeutic applications.

Liver-enriched transcription factors are critical for the expression of hepatocyte marker genes in mES-derived hepatocyte-lineage cells

Background

Induction of stem cell differentiation toward functional hepatocytes is hampered by lack of knowledge of the hepatocyte differentiation processes. The overall objective of this project is to characterize key stages in the hepatocyte differentiation process.

Results

We established a mouse embryonic stem (mES) cell culture system which exhibited changes in gene expression profiles similar to those observed in the development of endodermal and hepatocyte-lineage cells previously described in the normal mouse embryo. Transgenic mES cells were established that permitted isolation of enriched hepatocyte-lineage populations. This approach has isolated mES-derived hepatocyte-lineage cells that express several markers of mature hepatocytes including albumin, glucose-6-phosphatase, tyrosine aminotransferase, cytochrome P450-3a, phosphoenolpyruvate carboxykinase and tryptophan 2,3-dioxygenase. In addition, our results show that the up-regulation of the expression levels of hepatocyte nuclear factor-3α, -4α, -6, and CCAAT-enhancer binding protein-β might be critical for passage into late-stage differentiation towards functional hepatocytes. These data present important steps for definition of regulatory phenomena that direct specific cell fate determination.

Conclusion

The mES cell culture system generated in this study provides a model for studying transition between stages of the hepatocyte development and has significant potential value for studying the molecular basis of hepatocyte differentiation in vitro.

Enhanced differentiation of embryonic stem cells using co-cultivation with hepatocytes

We examined the effects of co-cultivated hepatocytes on the hepatospecific differentiation of murine embryonic stem (ES) cells. Utilizing an established mouse ES cell line expressing high or low levels of E-cadherin, that we have previously shown to be responsive to hepatotrophic growth factor stimulation (Dasgupta et al., 2005. Biotechnol Bioeng 92(3):257-266), we compared co-cultures of cadherin-expressing ES (CE-ES) cells with cultured rat hepatocytes, allowing for either paracrine interactions (indirect co-cultures) or both juxtacrine and paracrine interactions (direct co-cultures, random and patterned). Hepatospecific differentiation of ES cells was evaluated in terms of hepatic-like cuboidal morphology, heightened gene expression of late maturation marker, glucose-6-phosphatase in relation to early marker, alpha-fetoprotein (AFP), and the intracellular localization of albumin. Hepatocytes co-cultured with growth factor primed CE-ES cells markedly enhanced ES cell differentiation toward the hepatic lineage, an effect that was reversed through E-cadherin blockage and inhibited in control ES cells with reduced cadherin expression. Comparison of single ES cell cultures versus co-cultures show that direct contact co-cultures of hepatocytes and CE-ES cells maximally promoted ES cell commitment towards hepatodifferentiation, suggesting cooperative effects of cadherin-based juxtacrine and paracrine interactions. In contrast, E-cadherin deficient mouse ES (CD-ES) cells co-cultured with hepatocytes failed to show increased G6P expression, confirming the role of E-cadherin expression. To establish whether albumin expression in CE-ES cells was spatially regulated by co-cultured hepatocytes, we co-cultivated CE-ES cells around micropatterned, pre-differentiated rat hepatocytes. Albumin localization was enhanced "globally" within CE-ES cell colonies and was inhibited through E-cadherin antibody blockage in all but an interfacial band of ES cells. Thus, stem cell based cadherin presentation may be an effective tool to induce hepatotrophic differentiation by leveraging both distal/paracrine and contact/juxtacrine interactions with primary cells of the liver.

Selective enrichment of hepatocytes from mouse embryonic stem cells with a culture system containing cholestatic serum

AIM

There is increasing evidence indicating that embryonic stem (ES) cells are capable of differentiating into hepatocyte-like cells in vitro. However, it is necessary to improve the differentiation efficiency so as to promote the clinical application. Here, we report an efficient culture system to support hepatocyte differentiation from ES cells by utilizing cholestatic serum.

METHODS

One week after the induction of E14 mouse ES cells into hepatocytes with sodium butyrate, cholestatic serum was added into the culture system at various concentrations and hepatocyte-like cells were induced to proliferate. The morphological and phenotypic markers of hepatocytes were characterized using light microscopy, immunocytochemistry, and RT-PCR, respectively. The function of glycogen storage of the differentiated cells was detected by Periodic acid-Schiff (PAS) reaction, and the ratio of hepatic differentiation was determined by counting the albumin and PAS-positive cells.

RESULTS

In the presence of conditional selective medium containing cholestatic serum, numerous epithelial cells resembling hepatocytes were observed. The RT-PCR analysis showed that undifferentiated ES cells did not express any hepatic-specific markers; however, in the presence of sodium butyrate and conditional selective medium containing cholestatic serum, hepatic differentiation markers were detected. Immunofluorescence staining showed that those ES-derived hepatocytes were alpha-fetoprotein, albumin, and cytokeratin 18 positive, with the ability of storing glycogen. Further determination of the hepatic differentiation ratio showed that the application of cholestatic serum efficiently enriched ES-derived hepatocyte-like cells by inducing lineage differentiation and enhancing lineage proliferation.

CONCLUSION

The conditional selective medium containing cholestatic serum is optimal to selectively enrich hepatocyte-like cells from mixed differentiated ES cells, which may provide a novel method to improve the hepatic differentiation ratio of ES cells.

Differentiation of embryoid-body cells derived from embryonic stem cells into hepatocytes in alginate microbeads in vitro

AIM

Embryonic stem (ES) cells are being widely investigated as a promising source of hepatocytes with their proliferative, renewable, and pluripotent capacities. However, controlled and scalable ES cell differentiation culture into functional hepatocytes is challenging. In this study, we examined the differentiating potential of embryoid-body cells derived from ES cells into hepatocytes in alginate microbeads containing exogenous growth factors in vitro.

METHODS

Embryoid bodies were formed from ES cells by suspension methods. Embryoid bodies cultured for 5 d were treated with trypsin-EDTA. The disaggregated cells were encapsulated in alginate microbeads and stimulated with exogenous growth factors to induce hepatic differentiation. In the course of cell differentiation, cell morphology and viability were observed, and the expression patterns of some genes of the hepatocyte were confirmed by RT-PCR. An immunofluorescence analysis revealed the expression of albumin (ALB) and cytokeratin-18 (CK18). Hepatocyte functional assays were confirmed by the secretion of ALB and urea.

RESULTS

We showed that embryoid-body cells could maintain cell viability in alginate microbeads in vitro. We also found that directed differentiated cells expressed several hepatocyte genes including alpha-fetoprotein (AFP), ALB, Cyp7a1, CK18, transthyretin (TTR) and tyrosine aminotransferase (TAT) and produced ALB and urea in alginate microbeads. The directed differentiated cells expressed ALB and CK18 proteins on d 14. However, embryoid-body cells could not form hepatocytes without exogenous growth factors in alginate microbeads.

CONCLUSION

The differentiation of embryoid-body cells into hepatocytes containing exogenous growth factors in alginate microbeads gives rise to functional hepatocytes and may develop scalable stem cell differentiation strategies for bioartificial livers and hepatocyte transplantation.

Microfabrication-based modulation of embryonic stem cell differentiation

Embryonic stem (ES) cells form spontaneous aggregates during differentiation, and cell-cell communication in the aggregates plays an important role in differentiation. The development of a controlled differentiation scheme for ES cells has been hindered by the lack of a reliable method to produce uniform aggregate sizes. Conventional techniques, such as hanging drop and suspension cultures, do not allow precise control over size of ES cell aggregates. To surmount this problem, we microfabricated adhesive stencils to make mouse ES (mES) cell aggregates of specific sizes ranging from 100 microm to 500 microm in diameter. With this technique, we studied the effect of the initial aggregate size on ES cell differentiation. After 20 days of induction of differentiation, we analyzed the stem cell populations using gene and protein expression assays as well as biochemical functions. Notably, we found that germ layer differentiation depends on the initial size of the ES cell aggregate. Among the ES cell aggregate sizes tested, the aggregates with 300 microm diameter showed similar differentiation profiles of three germ layers as embryoid bodies made using the "hanging drop" technique. The smaller (100 microm) aggregates showed the increased expression of ectodermal markers compared to the larger (500 microm) aggregates, while the 500 microm aggregates showed the increased expression of mesodermal and endodermal markers compared to the 100 microm aggregates. These results indicate that the initial size of the aggregate is an important factor for ES cell differentiation, and can affect germ layer selection as well as the extent of differentiation.

Deriving and identifying hepatocytes from embryonic stem cells

The generation of hepatocytes from embryonic stem cells (ESCs) holds considerable promise for basic and applied research. However, the unequivocal identification of hepatocytes in ESC differentiation strategies has been hampered by a lack of hepatocyte-specific markers. Recent studies are beginning to address this issue with the identification of hepatocyte-specific genes and the production of hepatocytes from intermediate cell types like definitive endoderm. Assuming the successful identification of ESC-derived hepatocytes, the next challenge will be in balancing the proliferation and differentiation of these cells in order to generate usable numbers of functional hepatocytes in vitro.

Embryonic stem cells develop into hepatocytes after intrasplenic transplantation in CCl4-treated mice

AIM: To transplant undifferentiated embryonic stem (ES) cells into the spleens of carbon tetrachloride (CCl₄)-treated mice to determine their ability to differentiate into hepatocytes in the liver.

METHODS: CCl₄, 0.5 mL/kg body weight, was injected into the peritoneum of C57BL/6 mice twice a week for 5 wk. In group 1 (n = 12), 1 x 10⁵ undifferentiated ES cells (0.1 mL of 1 x 10⁶/mL solution), genetically labeled with GFP, were transplanted into the spleens 1 d after the second injection. Group 2 mice (n = 12) were injected with 0.2 mL of saline twice a week, instead of CCl₄, and the same amount of ES cells was transplanted into the spleens. Group 3 mice (n = 6) were treated with CCl₄ and injected with 0.1 mL of saline into the spleen, instead of ES cells. Histochemical analyses of the livers were performed on post-transplantation d (PD) 10, 20, and 30.

RESULTS: Considerable numbers of GFP-immunopositive cells were found in the periportal regions in group 1 mice (CCl₄-treated) on PD 10, however, not in those untreated with CCl₄ (group 2). The GFP-positive cells were also immunopositive for albumin (ALB), alpha-1 antitrypsin, cytokeratin 18, and hepatocyte nuclear factor 4 alpha on PD 20. Interestingly, most of the GFP-positive cells were immunopositive for DLK, a hepatoblast marker, on PD 10. Although very few ES-derived cells were demonstrated immunohistologically in the livers of group 1 mice on PD 30, improvements in liver fibrosis were observed. Unexpectedly, liver tumor formation was not observed in any of the mice that received ES cell transplantation during the experimental period.

CONCLUSION: Undifferentiated ES cells developed into hepatocyte-like cells with appropriate integration into tissue, without uncontrolled cell growth.

Molecular Mechanism of Liver Development and Regeneration

The liver is the central organ for metabolism and has strong regenerative capability. Although the liver has been studied mostly biochemically and histopathologically, genetic studies using gene-targeting technology have identified a number of cytokines, intracellular signaling molecules, and transcription factors involved in liver development and regeneration. In addition, various in vitro systems such as fetal liver explant culture and primary culture of fetal liver cells have been established, and the combination of genetic and in vitro studies has accelerated investigation of liver development. Identification of the cell-surface molecules of liver progenitors has made it possible to identify and isolate liver progenitors, making the liver a unique model for stem cell biology. In this review, we summarize progresses in understanding liver development and regeneration.

Promoted differentiation of cynomolgus monkey ES cells into hepatocyte-like cells by co-culture with mouse fetal liver-derived cells

AIM: To explore whether a co-culture of cynomolgus monkey embryonic stem (cES) cells with embryonic liver cells could promote their differentiation into hepatocytes.

METHODS: Mouse fetal liver-derived cells (MFLCs) were prepared as adherent cells from mouse embryos on embryonic d (ED) 14, after which undifferentiated cES cells were co-cultured with MFLCs. The induction of cES cells along a hepatic lineage was examined in MFLC-assisted differentiation, spontaneous differentiation, and growth factors (GF) and chemicals-induced differentiations (GF-induced differentiation) using retinoic acid, leukemia inhibitory factor (LIF), FGF2, FGF4, hepatocyte growth factor (HGF), oncostatin M (OSM), and dexamethasone.

RESULTS: The mRNA expression of α-fetoprotein, albumin (ALB), α-1-antitrypsin, and hepatocyte nuclear factor 4α was observed earlier in the differentiating cES cells co-cultured with MFLCs, as compared to cES cells undergoing spontaneous differentiation and those subjected to GF-induced differentiation. The expression of cytochrome P450 7a1, a possible marker for embryonic endoderm-derived mature hepatocytes, was only observed in cES cells that had differentiated in a co-culture with MFLCs. Further, the disappearance of Oct3/4, a representative marker of an undifferentiated state, was noted in cells co-cultured with MFLCs, but not in those undergoing spontaneous or GF-induced differentiation. Immunocytochemical analysis revealed an increased ratio of ALB-immunopositive cells among cES cells co-cultured with MFLCs, while glycogen storage and urea synthesis were also demonstrated.

CONCLUSION: MFLCs showed an ability to induce cES cells to differentiate toward hepatocytes. The co-culture system with MFLCs is a useful method for induction of hepatocyte-like cells from undifferentiated cES cells.

Study of hepatocyte differentiation using embryonic stem cells

The liver has many crucial functions including metabolizing dietary molecules, detoxifying compounds, and storing glycogen. The hepatocytes, comprising most of the liver organ, progressively modify their gene expression profile during the fetal development according to their roles in the different phases of development. Embryonic stem (ES) cells serve as a major tool in understanding liver development. These cells may also serve as a source of hepatic cells for cellular therapy. In this review, we aim to summarize the research that has been performed in the field of hepatocyte differentiation from mouse and human ES cells. We discuss the various methodologies for the differentiation of ES cells towards hepatic cells using either spontaneous or directed differentiation protocols. Although many protocols for differentiating ES cells to hepatic cells have been developed, the analysis of their status is not trivial and can lead to various conclusions. Hence, we discuss the issues of analyzing hepatocytes by means of the specificity of the markers for hepatocytes and the status of the cells as fetal or adult hepatocytes.

Embryoid body-mediated differentiation of mouse embryonic stem cells along a hepatocyte lineage: insights from gene expression profiles

Pluripotent embryonic stem (ES) cells represent a promising renewable cell source for the generation of functional differentiated cells. Previous studies incorporating embryoid body (EB)-mediated stem cell differentiation have, either spontaneously or after growth factor and extracellular matrix protein supplementation, yielded populations of hepatocyte lineage cells expressing mature hepatocyte markers such as albumin (ALB). In an effort to promote ES cell commitment to the hepatocyte lineage, we have evaluated the effects of four culture conditions on albumin and gene expression in differentiating ES cells. Quantitative in situ immunofluorescence and cDNA microarray analyses were used to describe not only lineage specificity but also to provide insights into the effects of disparate culture environments on the mechanisms of differentiation. The results of these studies suggest that spontaneous and collagen-mediated differentiation induce cells with the highest levels of ALB expression but mature liver specific genes were only expressed in the spontaneous condition. Further analysis of gene expression profiles indicated that two distinct mechanisms may govern spontaneous and collagen-mediated differentiation.

Prometheus' challenge: molecular, cellular and systemic aspects of liver regeneration

The fascinating aspect of the liver is the capacity to regenerate after injury or resection. A variety of genes, cytokines, growth factors, and cells are involved in liver regeneration. The exact mechanism of regeneration and the interaction between cells and cytokines are not fully understood. There seems to exist a sequence of stages that result in liver regeneration, while at the same time inhibitors control the size of the regenerated liver. It has been proven that hepatocyte growth factor, transforming growth factor, epidermal growth factor, tumor necrosis factor-alpha, interleukins -1 and -6 are the main growth and promoter factors secreted after hepatic injury, partial hepatectomy and after a sequence of different and complex reactions to activate transcription factors, mainly nuclear factor kappaB and signal transduction and activator of transcription-3, affects specific genes to promote liver regeneration. Unraveling the complex processes of liver regeneration may provide novel strategies in the management of patients with end-stage liver disease. In particular, inducing liver regeneration should reduce morbidity for the donor and increase faster recovery for the liver transplantation recipient.

Hepatocyte differentiation from embryonic stem cells and umbilical cord blood cells

With the development of regeneration medicine, many researchers have attempted hepatic differentiation from nonhepatic-origin cell sources. The differentiation of embryonic stem (ES) cells into hepatocyte-like cells has been reported in several papers. Mouse ES cells have shown a potential to develop into hepatocyte-like cells in vitro on the basis of hepatic gene expression after adding several growth factors. We transplanted cultured embryoid body (EB) cells (male) into female mice. A liver specimen of the recipient was examined by immunohistochemical staining for albumin and fluorescence in situ hybridization for the Y chromosome after transplantation. Both Y chromosome- and albumin-positive cells were recognized in the recipient female liver, and were considered to be hepatocyte-like cells derived from ES cells containing the Y chromosome. Many groups, including ourselves, have studied hepatocyte-like cell differentiation from umbilical cord blood cells (UBCs). We cultured nucleated cells isolated from UBCs. Using immunostaining, ALB-positive and CK-19-positive cells were recognized in the culture. Dual staining of ALB and CK-19 demonstrated that ALB was coexpressed with CK-19, suggesting the existence of hepatic progenitors. In this review, we consider recent studies of the differentiation of hepatocytes from nonhepatic origins, especially ES cells and umbilical cord blood.

E-cadherin synergistically induces hepatospecific phenotype and maturation of embryonic stem cells in conjunction with hepatotrophic factors

Since effective cell sourcing is a major challenge for the therapeutic management of liver disease and liver failure, embryonic stem (ES) cells are being widely investigated as a promising source of hepatic-like cells with their proliferative and pluripotent capacities. Cell-cell interactions are crucial in embryonic development modulating adhesive and signaling functions; specifically, the cell-cell adhesion ligand, cadherin is instrumental in gastrulation and hepatic morphogenesis. Inspired by the role of cadherins in development, we investigated the role of expression of E-cadherin in cultured murine ES cells on the induction of hepatospecific phenotype and maturation. The cadherin-expressing embryonic stem (CE-ES) cells intrinsically formed pronounced cell aggregates and cuboidal morphology whereas cadherin-deficient cadherin-expressing embryonic stem (CD-ES) cells remained more spread out and corded in morphology. Through controlled stimulation with single or combined forms of hepatotrophic growth factors; hepatocyte growth factor (HGF), dexamethasone (DEX) and oncostatin M (OSM), we investigated the progressive maturation of CE-ES cells, in relation to the control, CD-ES cells. Upon growth factor treatment, the CE-ES cells adopted a more compacted morphology, which exhibited a significant hepatocyte-like cuboidal appearance in the presence of DEX-OSM-HGF. In contrast, the CD-ES cells exhibited a mixed morphology and appeared to be more elongated in the presence of DEX-OSM-HGF. Reverse-transcriptase polymerase chain reaction was used to delineate the most differentiating condition in terms of early (alpha-fetoprotein (AFP)), mid (albumin), and late-hepatic (glucose-6-phosphatase) markers in relation to growth factor presentation for both CE-ES and CD-ES cells. We report that following the most differentiating condition of DEX-OSM-HGF stimulation, CE-ES cells expressed increased levels of albumin and glucose-6-phosphatase, whereas the CD-ES cells showed low levels of AFP and marginal levels of albumin and glucose-6-phosphatase. These trends suggest that the membrane expression of E-cadherin in ES cells can elicit a marked response to growth factor stimulation and lead to the induction of later stages of hepatocytic maturation. Thus, cadherin-engineered ES cells could be used to harness the cross-talk between the hepatotrophic and cadherin-based signaling pathways for controlled acceleration of ES hepatodifferentiation.