Hepatocytes deficient in CCAAT/enhancer binding protein alpha (C/EBP alpha) exhibit both hepatocyte and biliary epithelial cell character

Robust protein-based engineering of hepatocyte-like cells from human mesenchymal stem cells

BACKGROUND

Cells of interest can be prepared from somatic cells by forced expression of lineage-specific transcription factors, but it is required to establish a vector-free system for their clinical use. Here, we report a protein-based artificial transcription system for engineering hepatocyte-like cells from human umbilical cord-derived mesenchymal stem cells (MSCs).

METHODS

MSCs were treated for 5 days with 4 artificial transcription factors (4F), which targeted hepatocyte nuclear factor (HNF)1α, HNF3γ, HNF4α, and GATA-binding protein 4 (GATA4). Then, engineered MSCs (4F-Heps) were subjected to epigenetic analysis, biochemical analysis and flow cytometry analysis with antibodies to marker proteins of mature hepatocytes and hepatic progenitors such as delta-like homolog 1 (DLK1) and trophoblast cell surface antigen 2 (TROP2). Functional properties of the cells were also examined by injecting them to mice with lethal hepatic failure.

RESULTS

Epigenetic analysis revealed that a 5-day treatment of 4F upregulated the expression of genes involved in hepatic differentiation, and repressed genes related to pluripotency of MSCs. Flow cytometry analysis detected that 4F-Heps were composed of small numbers of mature hepatocytes (at most 1%), bile duct cells (~19%) and hepatic progenitors (~50%). Interestingly, ~20% of 4F-Heps were positive for cytochrome P450 3A4, 80% of which were DLK1-positive. Injection of 4F-Heps significantly increased survival of mice with lethal hepatic failure, and transplanted 4F-Heps expanded to more than 50-fold of human albumin-positive cells in the mouse livers, well consistent with the observation that 4F-Heps contained DLK1-positive and/or TROP2-positive cells.

CONCLUSION

Taken together with observations that 4F-Heps were not tumorigenic in immunocompromised mice for at least 2 years, we propose that this artificial transcription system is a versatile tool for cell therapy for hepatic failures.

Hepatobiliary Differentiation: Principles from Embryonic Liver Development

Hepatocytes and biliary epithelial cells (BECs), the two endodermal cell types of the liver, originate from progenitor cells called hepatoblasts. Based principally on in vitro data, hepatoblasts are thought to be bipotent stem cells with the potential to produce both hepatocytes and BECs. However, robust in vivo evidence for this model has only recently emerged. We examine the molecular mechanisms that stimulate hepatoblast differentiation into hepatocytes or BECs. In the absence of extrinsic cues, the default fate of hepatoblasts is hepatocyte differentiation. Inductive cues from the hepatic portal vein, however, initiate transcription factor expression in hepatoblasts, driving biliary specification. Defining the mechanisms of hepatobiliary differentiation provides important insights into congenital disorders, such as Alagille syndrome, and may help to better characterize the poorly understood hepatic lineage relationships observed during regeneration from liver injury.

Self-Organizing Human Induced Pluripotent Stem Cell Hepatocyte 3D Organoids Inform the Biology of the Pleiotropic TRIB1 Gene

Establishment of a physiologically relevant human hepatocyte‐like cell system for in vitro translational research has been hampered by the limited availability of cell models that accurately reflect human biology and the pathophysiology of human disease. Here we report a robust, reproducible, and scalable protocol for the generation of hepatic organoids from human induced pluripotent stem cells (hiPSCs) using short exposure to nonengineered matrices. These hepatic organoids follow defined stages of hepatic development and express higher levels of early (hepatocyte nuclear factor 4A [HNF4A], prospero‐related homeobox 1 [PROX1]) and mature hepatic and metabolic markers (albumin, asialoglycoprotein receptor 1 [ASGR1], CCAAT/enhancer binding protein α [C/EBPα]) than two‐dimensional (2D) hepatocyte‐like cells (HLCs) at day 20 of differentiation. We used this model to explore the biology of the pleiotropic TRIB1 (Tribbles‐1) gene associated with a number of metabolic traits, including nonalcoholic fatty liver disease and plasma lipids. We used genome editing to delete the TRIB1 gene in hiPSCs and compared TRIB1‐deleted iPSC‐HLCs to isogenic iPSC‐HLCs under both 2D culture and three‐dimensional (3D) organoid conditions. Under conventional 2D culture conditions, TRIB1‐deficient HLCs showed maturation defects, with decreased expression of late‐stage hepatic and lipogenesis markers. In contrast, when cultured as 3D hepatic organoids, the differentiation defects were rescued, and a clear lipid‐related phenotype was noted in the TRIB1‐deficient induced pluripotent stem cell HLCs. Conclusion: This work supports the potential of genome‐edited hiPSC‐derived hepatic 3D organoids in exploring human hepatocyte biology, including the functional interrogation of genes identified through human genetic investigation.

Oct3/4 is potentially useful for the suppression of the proliferation and motility of hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) cells are immature compared with healthy mature hepatocytes. Transcription factors serve a role in hepatocyte differentiation. The expression levels of transcription factors in HCC cell lines have been investigated to determine potential therapeutic targets. In the present study, the HLE, HLF, PLC/PRF/5, Huh-7, Hep3B, Huh-6 and HepG2 HCC cell lines were subjected to reverse-transcription polymerase chain reaction (RT-PCR) of transcription factors, including NANOG, Oct3/4, GATA binding protein 4 (GATA4), GATA6 and hematopoietically expressed homeobox (HHEX). In addition, these cell lines were analyzed using RT-quantitative PCR (RT-qPCR) of NANOG and Oct3/4. The 201B7 human induced pluripotent stem cells were evaluated as a model of pluripotent cells. The HLF cells were transfected with Oct3/4 small interfering RNA (siRNA) and used in an MTS colorimetric assay and a scratch assay. NANOG was not expressed in any of the cell lines. However, GATA4, GATA6 and HHEX were expressed in the majority of the HCC cell lines. In addition, NANOG and Oct3/4 were expressed in 201B7 cells. Oct3/4 was expressed in HLE, HLF and Hep3B cells; however, its expression levels were significantly reduced compared with those in 201B7 cells. RT-qPCR demonstrated that the expression of Oct3/4 siRNA suppressed the proliferation and motility of HLF cells. Oct3/4 siRNA may be a potentially effective therapy for the suppression of the proliferation and motility of HCC cells.

Hepatocyte selection medium-enriched hepatocellular carcinoma cells are positive for α-fetoprotein and CD44

Tissues surrounding hepatocellular carcinomas (HCCs) lack glucose. Hepatocyte selection medium (HSM) is deficient in glucose and is supplemented with galactose. HCC cells were cultured in HSM to investigate the stem cell markers α-fetoprotein (AFP) and cluster of differentiation 44 (CD44). HCC cells (HLF and PLC/PRF/5 cells) were cultured in HSM. Viable cell numbers were determined on days 0 and 7 following culture in HSM. RNA was isolated and subjected to reverse transcription-quantitative PCR (RT-qPCR) to analyze the mRNA expression levels of AFP and CD44. Immunostaining was performed to analyze the protein levels of AFP and CD44. The number of viable cells was significantly decreased on day 7 following culture in HSM. The expression levels of AFP and CD44 increased on day 7 as assessed using RT-qPCR. Immunostaining confirmed the results of RT-qPCR analysis. The number of viable HCC cells was decreased in HSM, whereas the expression levels of AFP and CD44 increased. Therefore, HSM is potentially useful for the enrichment of HCC cells with cancer stem cell characteristics.

CCAAT/enhancer-binding protein α decreases the viability of gastric cancer cells

CCAAT/enhancer-binding protein (C/EBP) α, C/EBPβ and C/EBPδ are involved in inflammation and cell differentiation. In the present study, their roles in human gastric cancer cells were investigated. The human gastric cancer cell lines MKN45 and MKN74 were subjected to the reverse transcription-quantitative polymerase chain reaction (RT-qPCR) to analyze the expression levels of C/EBPα, C/EBPβ and C/EBPδ. The cells were transfected with expression plasmids for either C/EBPα or C/EBPδ, and subjected to a 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium inner salt (MTS) assay and RT-qPCR for analysis of cyclin D1 expression. Expression levels of C/EBPα and C/EBPδ were decreased in MKN45 and MKN74 cells compared with in normal gastric tissue. Expression levels of C/EBPβ were decreased in MKN45 cells and increased in MKN74 cells. Viability of MKN45 cells was decreased by C/EBPα and C/EBPδ. Viability of MKN74 cells was decreased by C/EBPα, but increased by C/EBPδ. Expression levels of cyclin D1 were decreased in association with C/EBPα and C/EBPδ overexpression in MKN45 cells. Expression levels of cyclin D1 were decreased in association with C/EBPα overexpression, but increased in association with C/EBPδ overexpression, in MKN74 cells. The results of the present study indicate that C/EBPα is potentially useful for the treatment of gastric cancer.

Expression and regulation of C/EBPα in normal myelopoiesis and in malignant transformation

One of the most studied transcription factors in hematopoiesis is the leucine zipper CCAAT-enhancer binding protein α (C/EBPα), which is mainly involved in cell fate decisions for myeloid differentiation. Its involvement in acute myeloid leukemia (AML) is diverse, with patients frequently exhibiting mutations, deregulation of gene expression, or alterations in the function of C/EBPα. In this review, we emphasize the importance of C/EBPα for neutrophil maturation, its role in myeloid priming of hematopoietic stem and progenitor cells, and its indispensable requirement for AML development. We discuss that mutations in the open reading frame of CEBPA lead to an altered C/EBPα function, affecting the expression of downstream genes and consequently deregulating myelopoiesis. The emerging transcriptional mechanisms of CEBPA are discussed based on recent studies. Novel insights on how these mechanisms may be deregulated by oncoproteins or mutations/variants in CEBPA enhancers are suggested in principal to reveal novel mechanisms of how CEBPA is deregulated at the transcriptional level.

Improved Survival and Initiation of Differentiation of Human Induced Pluripotent Stem Cells to Hepatocyte-Like Cells upon Culture in William's E Medium followed by Hepatocyte Differentiation Inducer Treatment

BACKGROUND

Hepatocyte differentiation inducer (HDI) lacks both glucose and arginine, but is supplemented with galactose and ornithine, and is added together with other reagents such as apoptosis inhibitor and oncostatin M. Although human induced pluripotent stem (iPS) cells initiate hepatocyte differentiation, most die within 7 days. In this study, we investigated both HDI and conventional media for their potential to improve cell survival.

MATERIALS AND METHODS

201B7 iPS cells were cultured in conventional media. This consisted of three cycles of 5-day culture in William's E (WE) medium, followed by a 2-day culture in HDI.

RESULTS

Expression levels of α-feto protein (AFP) were higher in cells cultured in WE and in Dulbecco's Modified Eagle's Medium/Nutrient F-12 Ham (DF12). 201B7 cells expressed the highest AFP and albumin (ALB) when cultured in HDI for 2 days following 7-day culture in WE. After three cycles of 5-day culture in WE followed by 2 days in HDI, 201B7 cells expressed AFP and ALB 54 ± 2.3 (average ± standard deviation) and 73 ± 15.1 times higher, respectively, than those cultured in ReproFF (feeder-free condition).

CONCLUSION

201B7 cells survived culture in WE for 7 days followed HDI for 2 days. After three cycles of culture under these conditions, hepatocyte differentiation was enhanced, as evidenced by increased AFP and ALB expression.

An Optimal Medium Supplementation Regimen for Initiation of Hepatocyte Differentiation in Human Induced Pluripotent Stem Cells

Human induced pluripotent stem (hiPS) cells are an ideal source for hepatocytes. Glucose and arginine are necessary for cells to survive. Hepatocytes have galactokinase (GALK), which metabolizes galactose for gluconeogenesis, and ornithine transcarbamylase (OTC), which converts ornithine to arginine in the urea cycle. Hepatocyte selection medium (HSM) lacks both glucose and arginine, but contains galactose and ornithine. Although human primary hepatocytes survive in HSM, all the hiPS cells die in 3 days. The aim of this study was to modify HSM so as to initiate hepatocyte differentiation in hiPS cells within 2 days. Hepatocyte differentiation initiating medium (HDI) was prepared by adding oncostatin M (10 ng/ml), hepatocyte functional proliferation inducer (10 nM), 2,2'-methylenebis (1,3-cyclohexanedione) (M50054) (100 μg/ml), 1× non-essential amino acid, 1× sodium pyruvate, nicotinamide (1.2 mg/ml), L-proline (30 ng/ml), and L-glutamine (0.3 mg/ml) to HSM. HiPS cells (201B7 cells) were cultured in HDI for 2 days. RNA was isolated, used as template for cDNA, and subjected to real-time quantitative polymerase chain reaction. Alpha-fetoprotein, γ-glutamyl transpeptidase, and delta-like 1 were upregulated. Expression of albumin was not observed. Expression of transcription factors specific to hepatocytes was upregulated. The expression of GALK2, OTC, and CYP3A4 were increased. In conclusion, differentiation of 201B7 cells to hepatoblast-like cells was initiated in HDI. Limitations were small number of cells were obtained, and the cells with HDI were not mature hepatocytes.

Transcription Factors and Medium Suitable for Initiating the Differentiation of Human-Induced Pluripotent Stem Cells to the Hepatocyte Lineage

Transcription factors and culture media were investigated to determine the condition to initiate the differentiation of human-induced pluripotent stem (iPS) cells most efficiently. The expression of genes in human adult liver was compared with that in 201B7 cells (iPS cells) using cDNA microarray analysis. Episomal plasmids expressing transcription factors were constructed. 201B7 cells were transfected with the episomal plasmids and cultured in ReproFF (feeder-free media maintaining pluripotency), Leibovitz-15 (L15), William's E (WE), or Dulbecco's modified Eagle medium/Nutrient F-12 Ham (DF12) for 7 days. RNA was isolated and subjected to real-time quantitative PCR to analyze the expression of alpha-feto protein (AFP) and albumin. cDNA microarray analysis revealed 16 transcription factors that were upregulated in human adult liver relative to that in 201B7 cells. Episomal plasmids expressing these 16 genes were transfected into 201B7 cells. CCAAT/enhancer-binding protein alpha (CEBPA), CCAAT/enhancer-binding protein beta (CEBPB), forkhead box A1 (FOXA1), and forkhead box A3 (FOXA3) up-regulated AFP and down-regulated Nanog. These four genes were further analyzed. The expression of AFP and albumin was the highest in 201B7 cells transfected with the combination of CEBPA, CEBPB, FOXA1, and FOXA3 and cultured in WE. The combination of CEBPA, CEBPB, FOXA1, and FOXA3 was suitable for 201B7 cells to initiate differentiation to the hepatocyte lineage and WE was the most suitable medium for culture after transfection. J. Cell. Biochem. 117: 2001-2009, 2016. © 2016 Wiley Periodicals, Inc.

Role of transcription factor CCAAT/enhancer-binding protein alpha in human fetal liver cell types in vitro

AIM

The transcription factor CCAAT/enhancer-binding protein alpha (C/EBPα) has been shown to play an important role in liver development, cell proliferation and differentiation. It is, however, largely unknown if C/EBPα regulates cell differentiation and proliferation differently in the diverse cell types of the human liver. We investigated the role of C/EBPα in primary human fetal liver cells and liver cell subpopulations in vitro using a 3-D perfusion bioreactor as an advanced in vivo-like human organ culture model.

METHODS

Human fetal liver cells were investigated in vitro. C/EBPα gene expression was knocked down using siRNA or overexpressed by plasmid transfection. Cell type-specific gene expression was studied, cell populations and their proliferation were investigated, and metabolic parameters were analyzed.

RESULTS

When C/EBPα gene expression was knocked down, we observed a significantly reduced expression of typical endothelial, hematopoietic and mesenchymal genes such as CD31, vWF, CD90, CD45 and α-smooth muscle actin in fetal cells. The intracellular expression of hepatic proteins and genes for liver-specific serum proteins α-fetoprotein and albumin were reduced, their protein secretion was increased. Fetal endothelial cell numbers were reduced and hepatoblast numbers were increased. C/EBPα overexpression in fetal cells resulted in increased endothelial numbers, but did not affect mesenchymal cell types or hepatoblasts.

CONCLUSION

We demonstrated that the effects of C/EBPα are specific for the different human fetal liver cell types, using an advanced 3-D perfusion bioreactor as a human in vivo-like model.

Liver maturation deficiency in p57(Kip2)-/- mice occurs in a hepatocytic p57(Kip2) expression-independent manner

Fetal hepatic stem/progenitor cells, hepatoblasts, are highly proliferative cells and the source of both hepatocytes and cholangiocytes. In contrast, mature hepatocytes have a low proliferative potency and high metabolic functions. Cell proliferation is regulated by cell cycle-related molecules. However, the correlation between cell cycle regulation and hepatic maturation are still unknown. To address this issue, we revealed that the cell cycle inhibitor p57(Kip2) was expressed in the hepatoblasts and mesenchymal cells of fetal liver in a spatiotemporal manner. In addition, we found that hepatoblasts in p57(Kip2)-/- mice were highly proliferative and had deficient maturation compared with those in wild-type (WT) mice. However, there were no remarkable differences in the expression levels of cell cycle- and bipotency-related genes except for Ccnd2. Furthermore, p57(Kip2)-/- hepatoblasts could differentiate into mature hepatocytes in p57(Kip2)-/- and WT chimeric mice, suggesting that the intrinsic activity of p57(Kip2) does not simply regulate hepatoblast maturation.

Endodermal and mesenchymal cross talk: a crossroad for the maturation of foregut organs

The developmental stages of each foregut organ are intimately linked to the development of the other foregut organs such that the ultimate function of any one foregut organ, such as the metabolic function of the liver, depends on organizational changes associated with the maturation of multiple foregut organs. These changes include: (i) proliferation of the intrahepatic bile ducts and hepatoblasts within the liver coinciding with parenchymal expansion, (ii) elongation of extrahepatic bile ducts, which allows for proper gallbladder (GB) formation, and (iii) duodenal elongation and rotation, which coincides with all of the above to connect the intrahepatic, extrahepatic, and pancreatic ductal systems with the intestine. It is well established that cross talk between endodermal and mesenchymal components of the foregut occurs, particularly regarding the vascularization of developing organs. Furthermore, genetic mutations in mesenchymal and hepatic compartments of the developing foregut result in similar foregut pathologies: hypoplastic liver, absence of GB, biliary atresia (intrahepatic and/or extrahepatic), and failure of gut elongation and rotation. Finally, these shared pathologies can be linked to deficiencies in genes specific to the septum transversum mesenchyme (Hes1, Hlx, and Foxf1) or liver (Hhex and Hnf6), illustrating the complexity of such cross talk.

CCAAT/enhancer binding protein-mediated regulation of TGFβ receptor 2 expression determines the hepatoblast fate decision

Human embryonic stem cells (hESCs) and their derivatives are expected to be used in drug discovery, regenerative medicine and the study of human embryogenesis. Because hepatocyte differentiation from hESCs has the potential to recapitulate human liver development in vivo, we employed this differentiation method to investigate the molecular mechanisms underlying human hepatocyte differentiation. A previous study has shown that a gradient of transforming growth factor beta (TGFβ) signaling is required to segregate hepatocyte and cholangiocyte lineages from hepatoblasts. Although CCAAT/enhancer binding proteins (c/EBPs) are known to be important transcription factors in liver development, the relationship between TGFβ signaling and c/EBP-mediated transcriptional regulation in the hepatoblast fate decision is not well known. To clarify this relationship, we examined whether c/EBPs could determine the hepatoblast fate decision via regulation of TGFβ receptor 2 (TGFBR2) expression in the hepatoblast-like cells differentiated from hESCs. We found that TGFBR2 promoter activity was negatively regulated by c/EBPα and positively regulated by c/EBPβ. Moreover, c/EBPα overexpression could promote hepatocyte differentiation by suppressing TGFBR2 expression, whereas c/EBPβ overexpression could promote cholangiocyte differentiation by enhancing TGFBR2 expression. Our findings demonstrated that c/EBPα and c/EBPβ determine the lineage commitment of hepatoblasts by negatively and positively regulating the expression of a common target gene, TGFBR2, respectively.

Single-step protocol for the differentiation of human-induced pluripotent stem cells into hepatic progenitor-like cells

Induced pluripotent stem (iPS) cells are ideal sources of hepatocyte for transplantation into patients experiencing hepatic failure. Growth and transcription factors were analyzed to design a single-step protocol for the differentiation of iPS cells into hepatocytes. The expression of transcription factors was analyzed using reverse transcription-polymerase chain reaction (RT-PCR) and compared among iPS cells, as well as fetal and adult liver cells. iPS cells were cultured with growth factors and RT-PCR was performed to analyze the expression of transcription factors. iPS cells were introduced with transcription factors, cultured with growth factors and subjected to real-time quantitative PCR. Indocyanine green (ICG) was added to the medium as a hepatocyte marker. Sox17, GATA4, GATA6, FoxA2, HEX, HNF4α and C/EBPα were expressed in fetal and adult liver cells, but not in iPS cells. Sox17, GATA6 and HNF4α were expressed after exposure a combination of oncostatin M, epidermal growth factor, retinoic acid, dexamethasone and ITS (OERDITS). When iPS cells were introduced with FoxA2, GATA4, HEX and C/EBPα and cultured with OERDITS for 8 days, the cells expressed α-fetoprotein, δ-like (Dlk)-1 and γ-glutamyl transpeptidase (GTP), and ICG uptake was observed. Exposure to FoxA2, GATA4, HEX and C/EBPα and culturing with OERDITS supplementation potentially serves as a single-step inducer for the differentiation of iPS cells into hepatic progenitor-like cells within 8 days.

Transcriptional control of hepatocyte differentiation

The liver is the largest glandular organ in the body and plays a central role in controlling metabolism. During hepatogenesis, complex developmental processes must generate an array of cell types that are spatially arranged to generate a hepatic architecture that is essential to support liver function. The processes that control the ultimate formation of the liver are diverse and complex and in many cases poorly defined. Much of the focus of research during the past three decades has been on understanding how hepatocytes, which are the predominant liver parenchymal cells, differentiate during embryogenesis. Through a combination of mouse molecular genetics, embryology, and molecular biochemistry, investigators have defined a myriad of transcription factors that combine to control formation and function of hepatocytes. Here, we will review the major discoveries that underlie our current understanding of transcriptional regulation of hepatocyte differentiation.

Beta-catenin signaling in hepatic development and progenitors: which way does the WNT blow?

The Wnt/β-catenin pathway is an evolutionarily conserved signaling cascade that plays key roles in development and adult tissue homeostasis and is aberrantly activated in many tumors. Over a decade of work in mouse, chick, xenopus, and zebrafish models has uncovered multiple functions of this pathway in hepatic pathophysiology. Specifically, beta-catenin, the central component of the canonical Wnt pathway, is implicated in the regulation of liver regeneration, development, and carcinogenesis. Wnt-independent activation of beta-catenin by receptor tyrosine kinases has also been observed in the liver. In liver development across various species, through regulation of cell proliferation, differentiation, and maturation, beta-catenin directs foregut endoderm specification, hepatic specification of the foregut, and hepatic morphogenesis. Its role has also been defined in adult hepatic progenitors or oval cells especially in their expansion and differentiation. Thus, beta-catenin undergoes tight temporal regulation to exhibit pleiotropic effects during hepatic development and in hepatic progenitor biology.

Bone morphogenetic protein-4 induced rat hepatic progenitor cell (WB-F344 cell) differentiation toward hepatocyte lineage

Hepatic progenitor cells are local stem cells in the liver and they can be differentiated into either hepatocytes or cholangiocytes depending on different stimulations. These stimulations include extracellular growth factors and intracellular transcription factors. Bone morphogenetic protein 4 (BMP4) is a member of transforming growth factor beta (TGF-beta) superfamily and was first identified as growth factor to induce ectopic bone formation from skeletal muscle. Role of BMP4 in the liver is still unclear especially its role in hepatic progenitor cells (HPCs) differentiation. BMP4 was used to stimulate rat HPCs (WB-F344 cells) and differentiation of WB-F344 cells was investigated by reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot analysis. Both adenovirus delivered BMP4 and recombinant BMP4 were able to induce expression of hepatocyte markers such as albumin, TAT-1, and G6Pase but not cholangiocyte markers such as beta4-integrin and CK19. BMP4 induced differentiation of WB-F344 cells toward hepatocytes was mediated by increase in phosphorylation of Smad1 and ERK1/2. Moreover, BMP4 also stimulated expression of transcription factor--C/EBP-alpha, which involved in differentiation of WB-F344 cells toward hepatocytes. BMP4 is able to stimulate WB-F344 cells differentiation toward hepatocyte lineage.

Tbx3 promotes liver bud expansion during mouse development by suppression of cholangiocyte differentiation

After specification of the hepatic endoderm, mammalian liver organogenesis progresses through a series of morphological stages that culminate in the migration of hepatocytes into the underlying mesenchyme to populate the hepatic lobes. Here, we show that in the mouse the transcriptional repressor Tbx3, a member of the T-box protein family, is required for the transition from a hepatic diverticulum with a pseudo-stratified epithelium to a cell-emergent liver bud. In Tbx3-deficient embryos, proliferation in the hepatic epithelium is severely reduced, hepatoblasts fail to delaminate, and cholangiocyte rather than hepatocyte differentiation occurs. Molecular analyses suggest that the primary function of Tbx3 is to maintain expression of hepatocyte transcription factors, including hepatic nuclear factor 4a (Hnf4a) and CCAAT/enhancer binding protein (C/EBP), alpha (Cebpa), and to repress expression of cholangiocyte transcription factors such as Onecut1 (Hnf6) and Hnf1b.

CONCLUSION

Tbx3 controls liver bud expansion by suppressing cholangiocyte and favoring hepatocyte differentiation in the liver bud.

Tbx3 controls the fate of hepatic progenitor cells in liver development by suppressing p19ARF expression

Although the T-box family of transcription factors function in many different tissues, their role in liver development is unknown. Here we show that Tbx3, the T-box gene that is mutated in human ulnar-mammary syndrome, is specifically expressed in multipotent hepatic progenitor cells, ;hepatoblasts', isolated from the developing mouse liver. Tbx3-deficient hepatoblasts presented severe defects in proliferation as well as uncontrollable hepatobiliary lineage segregation, including the promotion of cholangiocyte (biliary epithelial cell) differentiation, which thereby caused abnormal liver development. Deletion of Tbx3 resulted in the increased expression of the tumor suppressor p19(ARF) (Cdkn2a), which in turn induced a growth arrest in hepatoblasts and activated a program of cholangiocyte differentiation. Thus, Tbx3 plays a crucial role in controlling hepatoblast proliferation and cell-fate determination by suppressing p19(ARF) expression and thereby promoting liver organogenesis.

Insulin-like growth factor (IGF)-II regulates CCAAT/enhancer binding protein alpha expression via phosphatidyl-inositol 3 kinase in human hepatoblastoma cell lines

To reveal growth factor and its signal pathway to CCAAT/enhancer binding protein alpha (C/EBPalpha) in hepatocyte differentiation, we used Huh-6 and HepG2, human hepatoblastoma (HBL) cell lines that maintain the expression of genes in hepatoblasts and remain at that stage of differentiation. Insulin-like growth factor (IGF)-II, hepatocyte growth factor (HGF), and dexamethasone (Dex) stimulated HBL cells for Northern blot analysis. Bromodeoxyuridine (BrdU) up-take assay and Western blot analysis on albumin was performed to unveil proliferation and differentiation activity of IGF-II. C/EBPalpha and phosphorylation of Akt were analyzed by Western blot analysis. LY294002 and wortmannin, specific inhibitors of PI3 kinase, and PD98059, a specific inhibitor of mitogen-activated protein (MAP) kinase, were used to examine the signaling pathway of C/EBPalpha upregulated by IGF-II. Luciferase assay was performed to study the promoter activity of C/EBPalpha. Actinomycin D was used to analyze half-life of C/EBPalpha mRNA. IGF-II up-regualted C/EBPalpha by Northern blot and Western blot while HGF and Dex did not by Northern blot. IGF-II promoted proliferation and differentiation by BrdU up-take assay and Western blot analysis on albumin. Akt phosphorylated by IGF-II, suggested that phosphatidyl-inositol (PI) 3 kinase mediated the signaling pathway of IGF-II. LY294002 and wortmannin suppressed expression of C/EBPalpha. IGF-II activated the promoter activity and prolonged half-life of mRNA, suggesting that IGF-II activated promoter and stabilized mRNA. LY294002 and wortmannin suppressed the promoter activity of C/EBPalpha while PD98059 did not, suggesting that activation of the promoter was mediated by PI3 kinase.

The C/EBP family of transcription factors: a paradigm for interaction between gene expression and proliferation control

In recent years, a link between the transcriptional regulators of lineage-specific gene expression and progenitor proliferation control has emerged. A main exponent of this phenomenon is the CCAAT/enhancer binding protein (C/EBP) family of basic region-leucine zipper proteins. These transcription factors control the differentiation of a range of cell types, and have key roles in regulating cellular proliferation through interaction with cell cycle proteins. More recently, their position at the crossroads between proliferation and differentiation has made them strong candidate regulators of tumorigenesis, and C/EBPs have been described as both tumor promoters and tumor suppressors.

Transcription factor sumoylation and factor YY1 serve to modulate mouse amelogenin gene expression

Amelogenin proteins are essential in the control of enamel biomineralization and the amelogenin gene therefore is spatiotemporally regulated to ensure proper amelogenin protein expression. In this study, we examined the role of sumoylation to alter CCAAT/enhancer-binding protein alpha (C/EBPalpha) activity, and performed a search using a protein/DNA array system for other proteins that act co-operatively with C/EBPalpha to alter amelogenin expression. We observed that C/EBPalpha was modified by sumoylation, and that this modification played an indirect inhibitory role on the regulation of C/EBPalpha activity which appeared to act through other transcription factors. The protein/DNA array allowed us to single out the transcription factor, YY1, which acts in the absence of direct DNA binding to repress both the basal amelogenin promoter activity and C/EBPalpha-mediated transactivation. Taken together, these pathways may account for part of the physiological modulation of the amelogenin gene expression in accordance with tooth developmental and enamel biomineralization requirements.

C/EBPalpha: a tumour suppressor in multiple tissues?

The CCATT/enhancer binding protein alpha, C/EBPalpha, is a key transcription factor involved in late differentiation events of several cell types. Besides acting as a classical transcription factor, C/EBPalpha is also a well-characterized inhibitor of mitotic growth in most cell lines tested. In line with its anti-mitotic properties, C/EBPalpha has been shown to interact with, and alter the activities of, several cell cycle related proteins and a number of models as to the mechanistics of C/EBPalpha-mediated growth repression have been proposed. More recently, several reports have indicated that C/EBPalpha acts as a tumour suppressor in the hematopoietic system and that mutation within C/EBPalpha is sufficient to induce tumourigenesis. Here, we will review these data and probe the possibility that C/EBPalpha also act as a tumour suppressor in other C/EBPalpha-expressing tissues.

The proline-histidine-rich CDK2/CDK4 interaction region of C/EBPalpha is dispensable for C/EBPalpha-mediated growth regulation in vivo

The C/EBPalpha transcription factor regulates growth and differentiation of several tissues during embryonic development. Several hypotheses as to how C/EBPalpha inhibits cellular growth in vivo have been derived, mainly from studies of tissue culture cells. In fetal liver it has been proposed that a short, centrally located, 15-amino-acid proline-histidine-rich region (PHR) of C/EBPalpha is responsible for the growth-inhibitory function of the protein through its ability to interact with CDK2 and CDK4, thereby inhibiting their activities. Homozygous Cebpa(DeltaPHR/DeltaPHR) (DeltaPHR) mice, carrying a modified cebpa allele lacking amino acids 180 to 194, were born at the Mendelian ratio, reached adulthood, and displayed no apparent adverse phenotypes. When fetal livers from the DeltaPHR mice were analyzed for their expression of cell cycle markers, bromodeoxyuridine incorporation, cyclin-dependent kinase 2 kinase activity, and global gene expression, we failed to detect any cell cycle or developmental differences between the DeltaPHR mice and their control littermates. These in vivo data demonstrate that any C/EBPalpha-mediated growth repression via the PHR as well as the basic region is dispensable for proper embryonic development of, and cell cycle control in, the liver. Surprisingly, control experiments performed in C/EBPalpha null fetal livers yielded similar results.

Expression of neuritin during liver maturation and regeneration

Cell surface molecules are not only important for cell-cell interactions but also useful for a marker to define cell types and differentiation stages. Unlike hematopoietic system in which numerous such antigens have been identified, only a few cell surface molecules have been used to define differentiation stage of hepatocytes. In order to identify such cell surface molecules, we performed DNA microarray analysis using mRNA from fetal hepatocytes in E12.5 and E17.5 mice and cDNAs encoding a membrane protein were selected. Northern blot analysis was employed to confirm the genes upregulated during maturation of fetal hepatocytes and neuritin, a GPI-anchored protein, was found as a membrane protein expressed in hepatocytes, but not in nonparenchymal cells. Its expression increased along with liver development and the maximum expression was achieved from the neonatal to adult stage. The neuritin protein was localized in sinusoidal lumen of hepatocytes in adult liver. Partial hepatectomy transiently downregulated the expression of neuritin. The expression of neuritin mRNA in C/EBPalpha deficient liver was reduced to about 50% of that of wild type mice. Thus, neuritin expression is well correlated to the maturation of hepatocytes and can be a useful tool to define the differentiation stage of hepatocytes.

Suppression of C/EBP alpha expression in biliary cell differentiation from hepatoblasts during mouse liver development

BACKGROUND/AIMS

Intrahepatic biliary cell differentiation takes place in periportal hepatoblasts under the influence of the subjacent mesenchyme, which leads to the suppression of mature hepatocyte marker expression. This study was undertaken to analyze C/EBP alpha and beta expression, which may govern transcription of mature hepatocyte marker genes, during mouse liver development with special attention given to biliary differentiation.

METHODS

Expression of C/EBP alpha and beta was immunohistochemically examined. Expression of alpha-fetoprotein, albumin and urea cycle enzymes, the genes of which have CCAAT motifs in their upstream regulatory sequences, was examined immunohistochemically or by using in situ hybridization.

RESULTS

C/EBP alpha started to be expressed in endodermal cells of 9.5-day liver primordium, and continued to be expressed in hepatoblasts and hepatocytes throughout development. Although biliary cell progenitors transiently expressed mature hepatocyte markers, their expression of C/EBP alpha was weak or totally absent. The signals of C/EBP beta in hepatocytes were weak in fetal liver, but became stronger with postnatal development. Differentiated epithelial cells of intrahepatic biliary structures did not express C/EBP alpha.

CONCLUSIONS

These data suggest that the suppression of C/EBP alpha expression may be prerequisite to biliary cell differentiation in the hepatoblast population and one of its earliest signs.

CCAAT/enhancer-binding protein alpha (C/EBPalpha) activates transcription of the human microsomal epoxide hydrolase gene (EPHX1) through the interaction with DNA-bound NF-Y

Microsomal epoxide hydrolase (mEH) plays a central role in xenobiotic metabolism as well as mediating the sodium-dependent uptake of bile acids into the liver, where these compounds regulate numerous biological processes such as cholesterol metabolism and hepatocyte signaling pathways. Little is known, however, about the factors that control the constitutive and inducible expression of the mEH gene (EPHX1) that is altered during development and in response to numerous xenobiotics. In previous studies we have established that GATA-4 binding to the EPHX1 core promoter is critical for EPHX1 expression. The -80/+25 bp core promoter also contained a reversed CCAAT box (-5/-1 bp), integrity of which was required for maximal basal EPHX1 transcription in HepG2 cells. Transient transfection of CCAAT/enhancer-binding protein alpha (C/EBPalpha) substantially stimulated EPHX1 promoter activity. Electrophoretic mobility shift assays, however, revealed that nuclear factor Y (NF-Y), but not C/EBPalpha, directly bound to this site although increased expression of NF-Y had no effect on EPHX1 promoter activity. These results suggested that C/EBPalpha activated EPHX1 expression through its interaction with NF-Y bound to the CCAAT box. The existence of a C/EBPalpha[NF-Y] complex was supported by electrophoretic mobility shift assays using antibodies against NF-Y and C/EBPalpha as well as by the ability of a dominant-negative NF-Y expression vector to inhibit promoter activity. The interaction between these transcription factors was established by co-immunoprecipitation analysis and glutathione S-transferase pull-down assays, whereas the association of the two factors and the interaction of NF-Y with the CCAAT box in vivo was confirmed by chromatin immunoprecipitation assays. C/EBPalpha-dependent EPHX1 activation was also supported by reconstitution studies in HeLa cells that lack this protein. These results establish that EPHX1 expression is regulated by C/EBPalpha interacting with DNA-bound NF-Y.

CCAAT/enhancer-binding proteins: structure, function and regulation

CCAAT/enhancer binding proteins (C/EBPs) are a family of transcription factors that all contain a highly conserved, basic-leucine zipper domain at the C-terminus that is involved in dimerization and DNA binding. At least six members of the family have been isolated and characterized to date (C/EBP alpha[bond]C/EBP zeta), with further diversity produced by the generation of different sized polypeptides, predominantly by differential use of translation initiation sites, and extensive protein-protein interactions both within the family and with other transcription factors. The function of the C/EBPs has recently been investigated by a number of approaches, including studies on mice that lack specific members, and has identified pivotal roles of the family in the control of cellular proliferation and differentiation, metabolism, inflammation and numerous other responses, particularly in hepatocytes, adipocytes and haematopoietic cells. The expression of the C/EBPs is regulated at multiple levels during several physiological and pathophysiological conditions through the action of a range of factors, including hormones, mitogens, cytokines, nutrients and certain toxins. The mechanisms through which the C/EBP members are regulated during such conditions have also been the focus of several recent studies and have revealed an immense complexity with the potential existence of cell/tissue- and species-specific differences. This review deals with the structure, biological function and the regulation of the C/EBP family.

Mutations in the gene encoding the transcription factor CCAAT/enhancer binding protein alpha in myelodysplastic syndromes and acute myeloid leukemias

The CCAAT/enhancer binding protein alpha (C/EBPalpha) protein is essential for proper lung and liver function and granulocytic and adipose tissue differentation. It was hypothesized that abnormalties in C/EBPalpha function contribute to the development of malignancies in a variety of tissues. To test this, genomic DNA from 408 patient samples and 5 cell lines representing 11 different cancers was screened for mutations in the C/EBPalpha gene. Two silent polymorphisms termed P1 and P2 were present at frequencies of 13.5% and 2.2%, respectively. Of the 12 mutations detected in 10 patients, silent changes were identified in one nonsmall cell lung cancer, one prostate cancer, and one acute myelogenous leukemia (AML) subtype M4. The 9 remaining mutations were detected in 1 of 92 (1.1%) myelodysplastic syndrome (MDS) samples and 6 of 78 (7.7%) AML (AML-M2 and AML-M4) samples. Some mutations truncated the predicted protein with loss of the DNA-binding (basic region) and dimerization (leucine zipper [ZIP]) domains by either deletions or nonsense codons. Also, inframe deletions or insertions in the fork region located between the leucine zipper and basic region, or within the leucine zipper, disrupted the alpha-helical phase of the bZIP domain. The inframe deletion and insertion mutations abrogated the transcriptional activation function of C/EBPalpha on the granulocyte colony-stimulating factor receptor promoter. These mutants localized properly to the nucleus, but were unable to bind to the C/EBP site in the promoter and did not possess dominant-negative activity. The mutations in the MDS patient and one AML-M2 patient were biallelic, indicating a loss of C/EBPalpha function. These results suggest that mutation of C/EBPalpha is involved in specific subtypes of AML and in MDS, but may occur rarely in other types of leukemias or nonhematologic malignancies.