Notch signaling controls hepatoblast differentiation by altering the expression of liver-enriched transcription factors

Bioartificial liver systems: current status and future perspective

Because the liver is a multifunctional and a vital organ for survival, the management of acute liver failure requires the support of a huge number of metabolic functions performed by the organ. Many early detoxification-based artificial liver techniques failed to treat the patients owing to the inadequate support of the many essential hepatic functions. For this reason, a bioartificial liver (BAL) comprising of viable hepatocytes on a mechanical support is believed to more likely provide these essential functions than a purely mechanical device. From 1990, nine clinical studies of various BAL systems have been reported, most of which utilize a hollow fiber technology, and a much larger number of various BAL systems have been suggested to show an enhanced performance. Safety issues such as immunological reactions, zoonosis and tumorgenicity have been successfully addressed for regulatory approval, but a recent report from a large-scale, randomized, and controlled phase III trial of a leading BAL system (HepatAssist) failed to meet our expectation of efficacy in terms of the overall survival rate. In this paper, we review the current BAL systems actively studied and discuss critical issues such as the hepatocyte bioreactor configuration and the hepatocyte source. On the basis of the insights gained from previously developed BAL systems and the rapid progress in stem cell technology, the short-term and long-term future perspectives of BAL systems are suggested.

Control of liver cell fate decision by a gradient of TGF beta signaling modulated by Onecut transcription factors

During liver development, hepatocytes and biliary cells differentiate from common progenitors called hepatoblasts. The factors that control hepatoblast fate decision are unknown. Here we report that a gradient of activin/TGFbeta signaling controls hepatoblast differentiation. High activin/TGFbeta signaling is required near the portal vein for differentiation of biliary cells. The Onecut transcription factors HNF-6 and OC-2 inhibit activin/TGFbeta signaling in the parenchyma, and this allows normal hepatocyte differentiation. In the absence of Onecut factors, the shape of the activin/TGFbeta gradient is perturbed and the hepatoblasts differentiate into hybrid cells that display characteristics of both hepatocytes and biliary cells. Thus, a gradient of activin/TGFbeta signaling modulated by Onecut factors is required to segregate the hepatocytic and the biliary lineages.

The emerging family of hepatoblastoma tumours: from ontogenesis to oncogenesis

The identification of distinct types and subtypes of hepatoblastoma has led to a successful classification of these lesions. In recent years, and particularly within large tumour trials, the spectrum of paediatric epithelial liver tumours has increased. This, together with the need for defining clinically relevant risk groups, will require a new approach to defining and classifying these cancers. Furthermore, an impressive amount of molecular biological information on liver ontogenesis and growth regulation of hepatic tumours has recently accumulated, which will allow the development of a comprehensive classification system with particular emphasis on prognostics. In this review, novel findings relating to these issues are discussed.

Transdifferentiation of mature rat hepatocytes into bile duct-like cells in vitro

We investigated the mechanism of phenotypic plasticity of hepatocytes in a three-dimensional organoid culture system, in which hepatocytic spheroids were embedded within a collagen gel matrix. Hepatocytes expressed several bile duct markers including cytokeratin (CK) 19 soon after culture and underwent branching morphogenesis within the matrix in the presence of insulin and epidermal growth factor. Cultured hepatocytes did not express Delta-like, a specific marker for oval cells and hepatoblasts. Furthermore, hepatocytes isolated from c-kit mutant rats (Ws/Ws), which are defective in proliferation of oval cells, showed essentially the same phenotypic changes as those isolated from control rats. The bile duct-like differentiation of hepatocytes was associated with increased expression of Jagged1, Jagged2, Notch1, and several Notch target genes. CK19 expression and branching morphogenesis were inhibited by dexamethasone, a mitogen-activated protein kinase kinase 1 (MEK1) inhibitor (PD98059), and a phosphatidyl inositol 3-kinase inhibitor (LY294002). After being cultured for more than 3 weeks within the gels, hepatocytes transformed into ductular structures surrounded by basement membranes. Our results suggest that hepatocytes might have the potential to transdifferentiate into bile duct-like cells without acquiring a stem-like phenotype and that this is mediated through specific protein tyrosine phosphorylation pathways.

Liver development update: new embryo models, cell lineage control, and morphogenesis

The three phases of liver development that are the focus of this review are: the specification of hepatoblasts within the endoderm, the lineage split of hepatoblasts into hepatocytes and biliary cells, and the interaction of these cells with different mesodermal cell derivatives during liver morphogenesis. Advances in these areas include new genes and experimental models for studying liver development, the role of HNF6 and HNF1beta transcription factors and notch signaling in the hepatocyte-biliary cell lineage decision, the identification of genomic targets for HNF4, and HNF4's role in controlling hepatic epithelial structure and the sinusoidal organization of the liver.