Deriving and identifying hepatocytes from embryonic stem cells

Proteome alteration of early-stage differentiation of mouse embryonic stem cells into hepatocyte-like cells

To explore the molecular basis of inducible differentiation of embryonic stem cells into hepatocyte-like cells, a proteomic strategy was utilized to examine the global protein expression alterations after early-stage differentiation of a mouse D3 embryonic stem (ES) cell line along hepatic lineage. The undifferentiated D3 cells were treated stepwise with combinations of defined chemicals and growth factors. The differentiated cells were identified by hepatocyte-like morphology, expressed liver-specific markers as well as the evidence of glycogen storage. The subsequent proteomic separation and identification were performed with 2-DE followed by MALDI-TOF-MS/MS analysis. Of the 119 differentially displayed protein spots analyzed, 90 spots presenting 64 distinct proteins were finally identified. The interested protein expressions were validated by Western blotting such as albumin and cytokeratin-8. Bioinformatic annotations indicated that this set of proteins was enriched with transcription, translation regulation and protein processing, energy/metabolism and chaperone functions. A part of them had been found to be involved in the differentiation of mouse ES cells. Interestingly, approximately 40% of these proteins had been previously reported as being dysregulated in hepatocellular carcinoma. It suggested that these changed proteins may be candidate regulators of ES cell differentiation, some of them may be specific to hepatic differentiation.

Transient gene delivery for functional enrichment of differentiating embryonic stem cells

There is a critical need for new sources of hepatocytes, both clinically to provide support for patients with liver failure and in drug discovery for toxicity, metabolic and pharmacokinetic screening of new drug entities. We have reported previously a variety of methods for differentiating murine embryonic stem (ES) cells into hepatocyte-like cells. One major challenge of our work and others in the field has been the ability to selectively purify and enrich these cells from a heterogeneous population. Traditional approaches for inserting new genes (e.g., stable transfection, knock-in, retroviral transduction) involve permanent alterations in the genome. These approaches can lead to mutations and involve the extra costs and time of developing, validating and maintaining new cell lines. We have developed a transient gene delivery system that uses fluorescent gene reporters for purification of the cells. Following a transient transfection, the cells are purified through a fluorescence-activated cell sorter (FACS), re-plated in secondary culture and subsequent phenotypic analysis is performed. In an effort to test the ability of the reporters to work in a transient environment for our differentiation system, we engineered two non-viral plasmid reporters, the first driven by the mouse albumin enhancer/promoter and the second by the mouse cytochrome P450 7A1 (Cyp7A1) promoter. We optimized the transfection efficiency of delivering these genes into spontaneously differentiated ES cells and sorted independent fractions positive for each reporter 17 days after inducing differentiation. We found that cells sorted based on the Cyp7A1 promoter showed significant enrichment in terms of albumin secretion, urea secretion and cytochrome P450 1A2 detoxification activity as compared to enrichment garnered by the albumin promoter-based cell sort. Development of gene reporter systems that allow us to identify, purify and assess homogeneous populations of cells is important in better understanding stem cell differentiation pathways. And engineering cellular systems without making permanent gene changes will be critical for the generation of clinically acceptable cellular material in the future.

Vascular endothelial growth factor promotes proliferation and function of hepatocyte-like cells in embryoid bodies formed from mouse embryonic stem cells

BACKGROUND/AIMS

Embryoid bodies (EBs) formed from embryonic stem cells (ESCs) differentiate into hepatocyte-like cells (HLCs), and are thus thought to be a useful cell source for drug testing and bioartificial liver. The aim of this study was to induce proliferation and function of ESC-derived HLCs in EBs using HLC-endothelial cell interaction.

METHODS

EBs were cultured in the presence of vascular endothelial growth factor (VEGF) and/or VEGF receptor (VEGFR) inhibitors. To reproduce HLC-endothelial cell interaction, we overexpressed VEGF in ESC-derived HLCs under the control of Cyp7a1 gene in EBs.

RESULTS

VEGF added to the cultured EBs increased the proliferation of ESC-derived endothelial cells, resulting in the promotion of proliferation and function of ESC-derived HLCs. In EBs, the VEGFR2 inhibitor suppressed expression of albumin and endothelial cell marker genes, whereas the inhibitor for both VEGFR1 and VEGFR2 suppressed expression of Cyp7a1 and hepatocyte growth factor (Hgf) genes. Upon exposure to VEGF, the endothelial cells in EBs increased Hgf mRNA expression. Forced VEGF expression in ESC-derived HLCs in EBs induced angiogenesis around the HLCs and resulted in an increase in the amount of HLCs.

CONCLUSIONS

VEGF indirectly induces the proliferation and function of ESC-derived HLCs through VEGFR1 and VEGFR2 signaling in endothelial cells.

Embryonic stem cell biology

ES cell research represents an exploding field of exploration. Initially predicted to provide rapid cures for numerous human diseases, the clinical usefulness of ES cell-derived cells remains untested in humans. However, ES cells have rapidly expanded our knowledge of human development and the molecular details of differentiation. Our ability to generate relatively pure populations of specifically differentiated cells for transplantation has markedly improved. It is hoped that soon researchers will overcome the biologic impediments to successful treatment of human disease with ES cell-derived cells.