Disparate cellular basis of improved liver repair in beta-catenin-overexpressing mice after long-term exposure to 3,5-diethoxycarbonyl-1,4-dihydrocollidine

Portal Fibrosis and the Ductular Reaction: Pathophysiological Role in the Progression of Liver Disease and Translational Opportunities

A consistent feature of chronic liver diseases and the hallmark of pathologic repair is the so-called "ductular reaction." This is a histologic abnormality characterized by an expansion of dysmorphic cholangiocytes inside and around portal spaces infiltrated by inflammatory, mesenchymal, and vascular cells. The ductular reaction is a highly regulated response based on the reactivation of morphogenetic signaling mechanisms and a complex crosstalk among a multitude of cell types. The nature and mechanism of these exchanges determine the difference between healthy regenerative liver repair and pathologic repair. An orchestrated signaling among cell types directs mesenchymal cells to deposit a specific extracellular matrix with distinct physical and biochemical properties defined as portal fibrosis. Progression of fibrosis leads to vast architectural and vascular changes known as "liver cirrhosis." The signals regulating the ecology of this microenvironment are just beginning to be addressed. Contrary to the tumor microenvironment, immune modulation inside this "benign" microenvironment is scarcely known. One of the reasons for this is that both the ductular reaction and portal fibrosis have been primarily considered a manifestation of cholestatic liver disease, whereas this phenomenon is also present, albeit with distinctive features, in all chronic human liver diseases. Novel human-derived cellular models and progress in "omics" technologies are increasing our knowledge at a fast pace. Most importantly, this knowledge is on the edge of generating new diagnostic and therapeutic advances. Here, we will critically review the latest advances, in terms of mechanisms, pathophysiology, and treatment prospects. In addition, we will delineate future avenues of research, including innovative translational opportunities.

Structural and functional effects of phosphopriming and scaffolding in the kinase GSK-3β

Glycogen synthase kinase 3β (GSK-3β) targets specific signaling pathways in response to distinct upstream signals. We used structural and functional studies to dissect how an upstream phosphorylation step primes the Wnt signaling component β-catenin for phosphorylation by GSK-3β and how scaffolding interactions contribute to this reaction. Our crystal structure of GSK-3β bound to a phosphoprimed β-catenin peptide confirmed the expected binding mode of the phosphoprimed residue adjacent to the catalytic site. An aspartate phosphomimic in the priming site of β-catenin adopted an indistinguishable structure but reacted approximately 1000-fold slower than the native phosphoprimed substrate. This result suggests that substrate positioning alone is not sufficient for catalysis and that native phosphopriming interactions are necessary. We also obtained a structure of GSK-3β with an extended peptide from the scaffold protein Axin that bound with greater affinity than that of previously crystallized Axin fragments. This structure neither revealed additional contacts that produce the higher affinity nor explained how substrate interactions in the GSK-3β active site are modulated by remote Axin binding. Together, our findings suggest that phosphopriming and scaffolding produce small conformational changes or allosteric effects, not captured in the crystal structures, that activate GSK-3β and facilitate β-catenin phosphorylation. These results highlight limitations in our ability to predict catalytic activity from structure and have potential implications for the role of natural phosphomimic mutations in kinase regulation and phosphosite evolution.

Ductular Reactions in Liver Injury, Regeneration, and Disease Progression-An Overview

Ductular reaction (DR) is a complex cellular response that occurs in the liver during chronic injuries. DR mainly consists of hyper-proliferative or reactive cholangiocytes and, to a lesser extent, de-differentiated hepatocytes and liver progenitors presenting a close spatial interaction with periportal mesenchyme and immune cells. The underlying pathology of DRs leads to extensive tissue remodeling in chronic liver diseases. DR initiates as a tissue-regeneration mechanism in the liver; however, its close association with progressive fibrosis and inflammation in many chronic liver diseases makes it a more complicated pathological response than a simple regenerative process. An in-depth understanding of the cellular physiology of DRs and their contribution to tissue repair, inflammation, and progressive fibrosis can help scientists develop cell-type specific targeted therapies to manage liver fibrosis and chronic liver diseases effectively.

Wnt-β-catenin in hepatobiliary homeostasis, injury, and repair

Wnt-β-catenin signaling has emerged as an important regulatory pathway in the liver, playing key roles in zonation and mediating contextual hepatobiliary repair after injuries. In this review, we will address the major advances in understanding the role of Wnt signaling in hepatic zonation, regeneration, and cholestasis-induced injury. We will also touch on some important unanswered questions and discuss the relevance of modulating the pathway to provide therapies for complex liver pathologies that remain a continued unmet clinical need.

Inhibiting Wnt Signaling Reduces Cholestatic Injury by Disrupting the Inflammatory Axis

BACKGROUND & AIMS

β-Catenin, the effector molecule of the Wnt signaling pathway, has been shown to play a crucial role in bile acid homeostasis through direct inhibition of farnesoid X receptor (FXR), which has pleiotropic effects on bile acid homeostasis. We hypothesize that simultaneous suppression of β-catenin signaling and activation of FXR in a mouse model of cholestasis will reduce injury and biliary fibrosis through inhibition of bile acid synthesis.

METHODS

To induce cholestasis, we performed bile duct ligation (BDL) on wild-type male mice. Eight hours after surgery, mice received FXR agonists obeticholic acid, tropifexor, or GW-4064 or Wnt inhibitor Wnt-C59. Severity of cholestatic liver disease and expression of target genes were evaluated after either 5 days or 12 days of treatment.

RESULTS

We found that although the FXR agonists worsened BDL-induced injury and necrosis after 5 days, Wnt-C59 did not. After 12 days of BDL, Wnt-C59 treatment, but not GW-4064 treatment, reduced both the number of infarcts and the number of inflammatory cells in liver. RNA sequencing analysis of whole livers revealed a notable suppression of nuclear factor kappa B signaling when Wnt signaling is inhibited. We then analyzed transcriptomic data to identify a cholangiocyte-specific signature in our model and demonstrated that Wnt-C59-treated livers were enriched for genes expressed in quiescent cholangiocytes, whereas genes expressed in activated cholangiocytes were enriched in BDL alone. A similar decrease in biliary injury and inflammation occurred in Mdr2 KO mice treated with Wnt-C59.

CONCLUSIONS

Inhibiting Wnt signaling suppresses cholangiocyte activation and disrupts the nuclear factor kappa B-dependent inflammatory axis, reducing cholestatic-induced injury.

Distinct hepatocyte identities in liver homeostasis and regeneration

The process of metabolic liver zonation is spontaneously established by assigning distributed tasks to hepatocytes along the porto-central blood flow. Hepatocytes fulfil critical metabolic functions, while also maintaining hepatocyte mass by replication when needed. Recent technological advances have enabled us to fine-tune our understanding of hepatocyte identity during homeostasis and regeneration. Subsets of hepatocytes have been identified to be more regenerative and some have even been proposed to function like stem cells, challenging the long-standing view that all hepatocytes are similarly capable of regeneration. The latest data show that hepatocyte renewal during homeostasis and regeneration after liver injury is not limited to rare hepatocytes; however, hepatocytes are not exactly the same. Herein, we review the known differences that give individual hepatocytes distinct identities, recent findings demonstrating how these distinct identities correspond to differences in hepatocyte regenerative capacity, and how the plasticity of hepatocyte identity allows for division of labour among hepatocytes. We further discuss how these distinct hepatocyte identities may play a role during liver disease.

Atypical cholangiocytes derived from hepatocyte-cholangiocyte transdifferentiation mediated by COX-2: a kind of misguided liver regeneration

BACKGROUND

Hepatocyte-cholangiocyte transdifferentiation (HCT) is a potential origin of proliferating cholangiocytes in liver regeneration after chronic injury. This study aimed to determine HCT after chronic liver injury, verify the impacts of HCT on liver repair, and avoid harmful regeneration by understanding the mechanism.

METHODS

A thioacetamide (TAA)-induced liver injury model was established in wild-type (WT-TAA group) and COX-2 panknockout (KO-TAA group) mice. HCT was identified by costaining of hepatocyte and cholangiocyte markers in vivo and in isolated mouse hepatocytes in vitro. The biliary tract was injected with ink and visualized by whole liver optical clearing. Serum and liver bile acid (BA) concentrations were measured. Either a COX-2 selective inhibitor or a β-catenin pathway inhibitor was administered in vitro.

RESULTS

Intrahepatic ductular reaction was associated with COX-2 upregulation in chronic liver injury. Immunofluorescence and RNA sequencing indicated that atypical cholangiocytes were characterized by an intermediate genetic phenotype between hepatocytes and cholangiocytes and might be derived from hepatocytes. The structure of the biliary system was impaired, and BA metabolism was dysregulated by HCT, which was mediated by the TGF-β/β-catenin signaling pathway. Genetic deletion or pharmaceutical inhibition of COX-2 significantly reduced HCT in vivo. The COX-2 selective inhibitor etoricoxib suppressed HCT through the TGF-β-TGFBR1-β-catenin pathway in vitro.

CONCLUSIONS

Atypical cholangiocytes can be derived from HCT, which forms a secondary strike by maldevelopment of the bile drainage system and BA homeostasis disequilibrium during chronic liver injury. Inhibition of COX-2 could ameliorate HCT through the COX-2-TGF-β-TGFBR1-β-catenin pathway and improve liver function.

Small molecule drugs promote repopulation of transplanted hepatocytes by stimulating cell dedifferentiation

Background & Aims

Hepatocyte transplantation has emerged as a possible treatment option for end-stage liver disease. However, an important obstacle to therapeutic success is the low level of engraftment and proliferation of transplanted hepatocytes, which do not survive long enough to exert therapeutic effects. Thus, we aimed to explore the mechanisms of hepatocyte proliferation in vivo and find a way to promote the growth of transplanted hepatocytes.

Methods

Hepatocyte transplantation was performed in Fah ^-/- mice to explore the mechanisms of hepatocyte proliferation in vivo. Guided by in vivo regeneration mechanisms, we identified compounds that promote hepatocyte proliferation in vitro. The in vivo effects of these compounds on transplanted hepatocytes were then evaluated.

Results

The transplanted mature hepatocytes were found to dedifferentiate into hepatic progenitor cells (HPCs), which proliferate and then convert back to a mature state at the completion of liver repopulation. The combination of two small molecules Y-27632 (Y, ROCK inhibitor) and CHIR99021 (C, Wnt agonist) could convert mouse primary hepatocytes into HPCs, which could be passaged for more than 30 passages in vitro. Moreover, YC could stimulate the proliferation of transplanted hepatocytes in Fah ^-/- livers by promoting their conversion into HPCs. Netarsudil (N) and LY2090314 (L), two clinically used drugs which target the same pathways as YC, could also promote hepatocyte proliferation in vitro and in vivo, by facilitating HPC conversion.

Conclusions

Our work suggests drugs promoting hepatocyte dedifferentiation may facilitate the growth of transplanted hepatocytes in vivo and may facilitate the application of hepatocyte therapy.

Impact and implications

Hepatocyte transplantation may be a treatment option for patients with end-stage liver disease. However, one important obstacle to hepatocyte therapy is the low level of engraftment and proliferation of the transplanted hepatocytes. Herein, we show that small molecule compounds which promote hepatocyte proliferation in vitro by facilitating dedifferentiation, could promote the growth of transplanted hepatocytes in vivo and may facilitate the application of hepatocyte therapy.

Cellular Homeostasis and Repair in the Biliary Tree

During biliary tree homeostasis, BECs are largely in a quiescent state and their turnover is slow for maintaining normal tissue homeostasis. BTSCs continually replenish new BECs in the luminal surface of EHBDs. In response to various types of biliary injuries, distinct cellular sources, including HPCs, BTSCs, hepatocytes, and BECs, repair or regenerate the injured bile duct. BEC, biliary epithelial cell; BTSC, biliary tree stem/progenitor cell; EHBD, extrahepatic bile ducts; HPC, hepatic progenitor cell.The biliary tree comprises intrahepatic bile ducts and extrahepatic bile ducts lined with epithelial cells known as biliary epithelial cells (BECs). BECs are a common target of various cholangiopathies for which there is an unmet therapeutic need in clinical hepatology. The repair and regeneration of biliary tissue may potentially restore the normal architecture and function of the biliary tree. Hence, the repair and regeneration process in detail, including the replication of existing BECs, expansion and differentiation of the hepatic progenitor cells and biliary tree stem/progenitor cells, and transdifferentiation of the hepatocytes, should be understood. In this paper, we review biliary tree homeostasis, repair, and regeneration and discuss the feasibility of regenerative therapy strategies for cholangiopathy treatment.

Human menstrual blood-derived stem cell transplantation suppresses liver injury in DDC-induced chronic cholestasis

BACKGROUND

Cholestatic liver injury can lead to serious symptoms and prognoses in the clinic. Currently, an effective medical treatment is not available for cholestatic liver injury. Human menstrual blood-derived stem cells (MenSCs) are considered as an emerging treatment in various diseases. This study aimed to explore the treatment effect of MenSCs in cholestatic liver injury.

METHODS

The treatment effect of MenSCs on chronic cholestatic liver injury was verified in 3,5-diethoxycarbonyl-1,4-dihydroxychollidine (DDC)-induced C57/BL6 mice. Pathological, fibrosis area in the liver tissue and serum liver enzymes were tested. Proteomics and western blot were used to explore the related targets and molecular mechanisms. Adeno-associated virus (AAV) 9-infected mice were applied for verification.

RESULTS

MenSCs markedly improved the survival rate of the DDC-treated mice (60% vs. 100%), and decreased the mouse serum aspartate aminotransferase (AST) (169.4 vs. 108.0 U/L, p < 0.001), alanine aminotransferase (ALT) (279.0 vs. 228.9 U/L, p < 0.01), alkaline phosphatase (ALP) (45.6 vs. 10.6 U/L, p < 0.0001), direct bilirubin (DBIL) (108.3 vs. 14.0 μmol/L, p < 0.0001) and total bilirubin (TBIL) (179.2 vs. 43.3 μmol/L, p < 0.0001) levels as well as intrahepatic cholestasis, bile duct dilation and fibrotic areas (16.12 vs. 6.57%, p < 0.05). The results further indicated that MenSCs repaired the DDC-induced liver tight junction (TJ) pathway and bile transporter (OATP2, BSEP and NTCP1) injury, thereby inhibiting COL1A1, α-SMA and TGF-β1 activation by upregulating liver β-catenin expression.

CONCLUSIONS

MenSC transplantation could be an effective treatment method for cholestatic liver injury in mice. MenSCs may exhibit therapeutic effects by regulating β-catenin expression.

WNT7B Regulates Cholangiocyte Proliferation and Function During Murine Cholestasis

We previously identified an up-regulation of specific Wnt proteins in the cholangiocyte compartment during cholestatic liver injury and found that mice lacking Wnt secretion from hepatocytes and cholangiocytes showed fewer proliferating cholangiocytes and high mortality in response to a 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) diet, a murine model of primary sclerosing cholangitis. In vitro studies demonstrated that Wnt7b, one of the Wnts up-regulated during cholestasis, induces proliferation of cholangiocytes in an autocrine manner and increases secretion of proinflammatory cytokines. We hypothesized that loss of Wnt7b may exacerbate some of the complications of cholangiopathies by decreasing the ability of bile ducts to induce repair. Wnt7b-flox mice were bred with Krt19-cre mice to deplete Wnt7b expression in only cholangiocytes (CC) or with albumin-Cre mice to delete Wnt7b expression in both hepatocytes and cholangiocytes (HC + CC). These mice were placed on a DDC diet for 1 month then killed for evaluation. Contrary to our expectations, we found that mice lacking Wnt7b from CC and HC + CC compartments had improved biliary injury, decreased cellular senescence, and lesser bile acid accumulation after DDC exposure compared to controls, along with decreased expression of inflammatory cytokines. Although Wnt7b knockout (KO) resulted in fewer proliferating cholangiocytes, CC and HC + CC KO mice on a DDC diet also had more hepatocytes expressing cholangiocyte markers compared to wild-type mice on a DDC diet, indicating that Wnt7b suppression promotes hepatocyte reprogramming. Conclusion: Wnt7b induces a proproliferative proinflammatory program in cholangiocytes, and its loss is compensated for by conversion of hepatocytes to a biliary phenotype during cholestatic injury.

The Thyromimetic Sobetirome (GC-1) Alters Bile Acid Metabolism in a Mouse Model of Hepatic Cholestasis

Chronic cholestasis results from bile secretory defects or impaired bile flow with few effective medical therapies available. Thyroid hormone triiodothyronine and synthetic thyroid hormone receptor agonists, such as sobetirome (GC-1), are known to impact lipid and bile acid (BA) metabolism and induce hepatocyte proliferation downstream of Wnt/β-catenin signaling after surgical resection; however, these drugs have yet to be studied as potential therapeutics for cholestatic liver disease. Herein, GC-1 was administered to ATP binding cassette subfamily B member 4 (Abcb4^-/-; Mdr2^-/-) knockout (KO) mice, a sclerosing cholangitis model. KO mice fed GC-1 diet for 2 and 4 weeks had decreased serum alkaline phosphatase but increased serum transaminases compared with KO alone. KO mice on GC-1 also had higher levels of total liver BA due to alterations in expression of BA detoxification, transport, and synthesis genes, with the net result being retention of BA in the hepatocytes. Interestingly, GC-1 does not induce hepatocyte proliferation or Wnt/β-catenin signaling in KO mice, likely a result of decreased thyroid hormone receptor β expression without Mdr2. Therefore, although GC-1 treatment induces a mild protection against biliary injury in the early stages of treatment, it comes at the expense of hepatocyte injury and is suboptimal because of lower expression of thyroid hormone receptor β. Thus, thyromimetics may have limited therapeutic benefits in treating cholestatic liver disease.

Liver Progenitors and Adult Cell Plasticity in Hepatic Injury and Repair: Knowns and Unknowns

The liver is a complex organ performing numerous vital physiological functions. For that reason, it possesses immense regenerative potential. The capacity for repair is largely attributable to the ability of its differentiated epithelial cells, hepatocytes and biliary epithelial cells, to proliferate after injury. However, in cases of extreme acute injury or prolonged chronic insult, the liver may fail to regenerate or do so suboptimally. This often results in life-threatening end-stage liver disease for which liver transplantation is the only effective treatment. In many forms of liver injury, bipotent liver progenitor cells are theorized to be activated as an additional tier of liver repair. However, the existence, origin, fate, activation, and contribution to regeneration of liver progenitor cells is hotly debated, especially since hepatocytes and biliary epithelial cells themselves may serve as facultative stem cells for one another during severe liver injury. Here, we discuss the evidence both supporting and refuting the existence of liver progenitor cells in a variety of experimental models. We also debate the validity of developing therapies harnessing the capabilities of these cells as potential treatments for patients with severe and chronic liver diseases.

Wnt-β-catenin signalling in liver development, health and disease

The canonical Wnt-β-catenin pathway is a complex, evolutionarily conserved signalling mechanism that regulates fundamental physiological and pathological processes. Wnt-β-catenin signalling tightly controls embryogenesis, including hepatobiliary development, maturation and zonation. In the mature healthy liver, the Wnt-β-catenin pathway is mostly inactive but can become re-activated during cell renewal and/or regenerative processes, as well as in certain pathological conditions, diseases, pre-malignant conditions and cancer. In hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCA), the two most prevalent primary liver tumours in adults, Wnt-β-catenin signalling is frequently hyperactivated and promotes tumour growth and dissemination. A substantial proportion of liver tumours (mainly HCC and, to a lesser extent, CCA) have mutations in genes encoding key components of the Wnt-β-catenin signalling pathway. Likewise, hepatoblastoma, the most common paediatric liver cancer, is characterized by Wnt-β-catenin activation, mostly as a result of β-catenin mutations. In this Review, we discuss the most relevant molecular mechanisms of action and regulation of Wnt-β-catenin signalling in liver development and pathophysiology. Moreover, we highlight important preclinical and clinical studies and future directions in basic and clinical research.

Genetic Lineage Tracing of Biliary Epithelial Cells

Lineage tracing of liver cells is a powerful tool to understand liver embryonic development, healthy liver cell homeostasis, tissue repair, and regeneration. Lineage tracing of biliary epithelial cells (BECs) in the adult liver has been used to assess the contribution of the biliary epithelium to liver injury, regeneration, and disease. These studies have shown the contribution of BECs to the expansion of ductular reaction (DR) and liver progenitor cells (LPCs) and eventually the generation of new hepatocytes. Few genetic lineage-tracing mouse models have been proved to trace BECs. This chapter is focused on lineage tracing of BECs in mouse models of liver injury and regeneration. First, we mention different existing approaches to trace the biliary epithelium based on proteins specifically expressed by BECs such as sex-determining region Y-box 9 (SOX9), osteopontin (OPN), and cytokeratin-19 (KRT19). Second, we describe mouse models that can be used to evaluate cell fate during liver injury and regeneration (i.e., partial hepatectomy (PHx), acute liver injury models, and chronic liver damage models such as 3,5-diethoxycarbonyl-1,4-dihydro-collidin (DDC) diet, choline-deficient ethionine-supplemented (CDE) diet, or chronic carbon tetrachloride (CCl₄) administration). Third, we suggest possible readouts to assess BECs fate based on immunofluorescence analysis.

Participation of liver stem cells in cholangiocarcinogenesis after aflatoxin B1 exposure of glutathione S-transferase A3 knockout mice

Aflatoxin B1, arguably the most potent human carcinogen, induces liver cancer in humans, rats, trout, ducks, and so on, but adult mice are totally resistant. This resistance is because of a detoxifying enzyme, mouse glutathione S-transferase A3, which binds to and inactivates aflatoxin B1 epoxide, preventing the epoxide from binding to DNA and causing mutations. Glutathione S-transferase A3 or its analog has not been detected in any of the sensitive species, including humans. The generation of a glutathione S-transferase A3 knockout (represented as KO or -/-) mice has allowed us to study the induction of liver cancer in mice by aflatoxin B1. In contrast to the induction of hepatocellular carcinomas in other species, aflatoxin B1 induces cholangiocarcinomas in GSTA3-/- mice. In other species and in knockout mice, the induction of liver cancer is preceded by extensive proliferation of small oval cells, providing additional evidence that oval cells are bipolar stem cells and may give rise to either hepatocellular carcinoma or cholangiocarcinoma depending on the nature of the hepatocarcinogen and the species of animal. The recent development of mouse oval cell lines in our laboratory from aflatoxin B1-treated GSTA3-/- mice should provide a new venue for study of the properties and potential of putative mouse liver stem cells.

Immortalized common marmoset (Callithrix jacchus) hepatic progenitor cells possess bipotentiality in vitro and in vivo

Common marmoset (Callithrix jacchus) is emerging as a clinically relevant nonhuman primate model for various diseases, but is hindered by the availability of marmoset cell lines, which are critical for understanding the disease pathogenesis and drug/toxicological screening prior to animal testing. Here we describe the generation of immortalized marmoset hepatic progenitor cells (MHPCs) by lentivirus-mediated transfer of the simian virus 40 large T antigen gene in fetal liver polygonal cells. MHPCs proliferate indefinitely in vitro without chromosomal alteration and telomere shortening. These cells possess hepatic progenitor cell-specific gene expression profiles with potential to differentiate into both hepatocytic and cholangiocytic lineages in vitro and in vivo and also can be genetically modified. Importantly, injected MHPCs repopulated the injured liver of fumarylacetoacetate hydrolase (Fah)-deficient mice with hepatocyte-like cells. MHPCs also engraft as cholangiocytes into bile ducts of 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-induced bile ductular injured mice. MHPCs provide a tool to enable efficient derivation and genetic modification of both hepatocytes and cholangiocytes for use in disease modeling, tissue engineering, and drug screening.

YAP triggers the Wnt/β-catenin signalling pathway and promotes enterocyte self-renewal, regeneration and tumorigenesis after DSS-induced injury

Impaired epithelial regeneration is a crucial pathophysiological feature of ulcerative colitis (UC). Yes-associated protein (YAP1) appears to control cell proliferation and differentiation. In this study, we sought to identify the roles of YAP in intestinal epithelial cell (IEC) self-renewal, regeneration and tumorigenesis. We first observed that YAP was significantly reduced in 62.5% (45/72) of human UC tissues and it was dramatically enhanced during epithelial regeneration in a murine colitis model. Using lentiviral infection, we established a YAP-overexpression (YAP^WT) mouse model. We then found that after tissue injury, YAP^WT mice had increased epithelial cell self-renewal capacity and drastically restored intestinal crypt structure. Strikingly, these mice were more susceptible to colitis-associated cancer (CAC) in chemically induced carcinoma. Mechanistically, YAP and β-catenin showed increased nuclear co-localization during regeneration after inflammation. Overexpressing YAP significantly improved IEC ‘wound-healing’ ability and increased the expression of both β-catenin and the transcriptional targets of Wnt signalling Lgr5 and cyclin D1, whereas silencing β-catenin in YAP^WT cells attenuated this effect. Remarkably, we observed that YAP could directly interact with β-catenin in the nucleus and formed a transcriptional YAP/β-catenin/TCF4 complex; Lgr5 and cyclin D1 were confirmed to be the target genes of this complex. In contrast, cancer cell proliferation and tumour development were suppressed by the phospho-mimetic YAP mutant. In summary, nuclear YAP-driven IEC proliferation could control epithelial regeneration after inflammation and may serve as a potential therapeutic target in UC. However, excessive YAP activation promoted CAC development.

Wnt/β-Catenin Signaling in Liver Development, Homeostasis, and Pathobiology

The liver is an organ that performs a multitude of functions, and its health is pertinent and indispensable to survival. Thus, the cellular and molecular machinery driving hepatic functions is of utmost relevance. The Wnt signaling pathway is one such signaling cascade that enables hepatic homeostasis and contributes to unique hepatic attributes such as metabolic zonation and regeneration. The Wnt/β-catenin pathway plays a role in almost every facet of liver biology. Furthermore, its aberrant activation is also a hallmark of various hepatic pathologies. In addition to its signaling function, β-catenin also plays a role at adherens junctions. Wnt/β-catenin signaling also influences the function of many different cell types. Due to this myriad of functions, Wnt/β-catenin signaling is complex, context-dependent, and highly regulated. In this review, we discuss the Wnt/β-catenin signaling pathway, its role in cell-cell adhesion and liver function, and the cell type-specific roles of Wnt/β-catenin signaling as it relates to liver physiology and pathobiology.

Wnt signaling regulates hepatobiliary repair following cholestatic liver injury in mice

Hepatic repair is directed chiefly by the proliferation of resident mature epithelial cells. Furthermore, if predominant injury is to cholangiocytes, the hepatocytes can transdifferentiate to cholangiocytes to assist in the repair and vice versa, as shown by various fate-tracing studies. However, the molecular bases of reprogramming remain elusive. Using two models of biliary injury where repair occurs through cholangiocyte proliferation and hepatocyte transdifferentiation to cholangiocytes, we identify an important role of Wnt signaling. First we identify up-regulation of specific Wnt proteins in the cholangiocytes. Next, using conditional knockouts of Wntless and Wnt coreceptors low-density lipoprotein-related protein 5/6, transgenic mice expressing stable β-catenin, and in vitro studies, we show a role of Wnt signaling through β-catenin in hepatocyte to biliary transdifferentiation. Last, we show that specific Wnts regulate cholangiocyte proliferation, but in a β-catenin-independent manner.

CONCLUSION

Wnt signaling regulates hepatobiliary repair after cholestatic injury in both β-catenin-dependent and -independent manners. (Hepatology 2016;64:1652-1666).

Rb and p53 Liver Functions Are Essential for Xenobiotic Metabolism and Tumor Suppression

The tumor suppressors Retinoblastoma (Rb) and p53 are frequently inactivated in liver diseases, such as hepatocellular carcinomas (HCC) or infections with Hepatitis B or C viruses. Here, we discovered a novel role for Rb and p53 in xenobiotic metabolism, which represent a key function of the liver for metabolizing therapeutic drugs or toxins. We demonstrate that Rb and p53 cooperate to metabolize the xenobiotic 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC). DDC is metabolized mainly by cytochrome P450 (Cyp)3a enzymes resulting in inhibition of heme synthesis and accumulation of protoporphyrin, an intermediate of heme pathway. Protoporphyrin accumulation causes bile injury and ductular reaction. We show that loss of Rb and p53 resulted in reduced Cyp3a expression decreased accumulation of protoporphyrin and consequently less ductular reaction in livers of mice fed with DDC for 3 weeks. These findings provide strong evidence that synergistic functions of Rb and p53 are essential for metabolism of DDC. Because Rb and p53 functions are frequently disabled in liver diseases, our results suggest that liver patients might have altered ability to remove toxins or properly metabolize therapeutic drugs. Strikingly the reduced biliary injury towards the oxidative stress inducer DCC was accompanied by enhanced hepatocellular injury and formation of HCCs in Rb and p53 deficient livers. The increase in hepatocellular injury might be related to reduce protoporphyrin accumulation, because protoporphrin is well known for its anti-oxidative activity. Furthermore our results indicate that Rb and p53 not only function as tumor suppressors in response to carcinogenic injury, but also in response to non-carcinogenic injury such as DDC.

Deletion of Wntless in myeloid cells exacerbates liver fibrosis and the ductular reaction in chronic liver injury

Background

Macrophages play critical roles in liver regeneration, fibrosis development and resolution. They are among the first responders to liver injury and are implicated in orchestrating the fibrogenic response via multiple mechanisms. Macrophages are also intimately associated with the activated hepatic progenitor cell (HPC) niche or ductular reaction that develops in parallel with fibrosis. Among the many macrophage-derived mediators implicated in liver disease progression, a key role for macrophage-derived Wnt proteins in driving pro-regenerative HPC activation towards a hepatocellular fate has been suggested. Wnt proteins, in general, however, have been associated with both pro- and anti-fibrogenic activities in the liver and other organs. We investigated the role of macrophage-derived Wnt proteins in fibrogenesis and HPC activation in murine models of chronic liver disease by conditionally deleting Wntless expression, which encodes a chaperone essential for Wnt protein secretion, in LysM-Cre-expressing myeloid cells (LysM-Wls mice).

Results

Fibrosis and HPC activation were exacerbated in LysM-Wls mice compared to littermate controls, in the absence of an apparent increase in myofibroblast activation or interstitial collagen mRNA expression, in both the TAA and CDE models of chronic liver disease. Increased Epcam mRNA levels paralleled the increased HPC activation and more mature ductular reactions, in LysM-Wls mice. Increased Epcam expression in LysM-Wls HPC was also observed, consistent with a more cholangiocytic phenotype. No differences in the mRNA expression levels of key pro-inflammatory and pro-fibrotic cytokines or the macrophage-derived HPC mitogen, Tweak, were observed. LysM-Wls mice exhibited increased expression of Timp1, encoding the key Mmp inhibitor Timp1 that blocks interstitial collagen degradation, and, in the TAA model, reduced expression of the anti-fibrotic matrix metalloproteinases, Mmp12 and Mmp13, suggesting a role for macrophage-derived Wnt proteins in restraining fibrogenesis during ongoing liver injury.

Conclusion

In summary, these data suggest that macrophage-derived Wnt proteins possess anti-fibrogenic potential in chronic liver disease, which may be able to be manipulated for therapeutic benefit.

Electronic supplementary material

The online version of this article (doi:10.1186/s13069-015-0036-7) contains supplementary material, which is available to authorized users.

Cyanidin-3-O-β-glucoside Purified from Black Rice Protects Mice against Hepatic Fibrosis Induced by Carbon Tetrachloride via Inhibiting Hepatic Stellate Cell Activation

This study investigated whether cyanidin-3-O-β-glucoside (Cy-3-G), a predominant anthocyanin, could exert a protective role on liver injury and its further mechanisms of the anti-fibrosis actions in mice. The results demonstrated that the treatment of Cy-3-G (800 mg/kg diet) for 8 weeks significantly attenuated hepatotoxicity and fibrosis in carbon tetrachloride (CCl4) administered mice. Cy-3-G strongly down-regulated the expression of α-smooth muscle actin (α-SMA), desmin, and matrix metalloproteinase (MMPs), which showed its suppression effect on the activation of hepatic stellate cells (HSCs). In addition, Cy-3-G favorably regulated oxidative stress and apoptosis in liver. Furthermore, Cy-3-G ameliorated the infiltration of inflammatory cells such as neutrophils and leukocytes and meanwhile suppressed the production of pro-inflammatory cytokines and growth factors. In conclusion, daily intake of Cy-3-G could prevent liver fibrosis progression in mice induced by CCl4 through inhibiting HSC activation, which provides a basis for clinical practice of liver fibrosis prevention.

Fibroblast growth factor signaling regulates the expansion of A6-expressing hepatocytes in association with AKT-dependent β-catenin activation

BACKGROUND & AIMS

Fibroblast Growth Factors (FGFs) promote the proliferation and survival of hepatic progenitor cells (HPCs) via AKT-dependent β-catenin activation. Moreover, the emergence of hepatocytes expressing the HPC marker A6 during 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-induced liver injury is mediated partly by FGF and β-catenin signaling. Herein, we investigate the role of FGF signaling and AKT-mediated β-catenin activation in acute DDC liver injury.

METHODS

Transgenic mice were fed DDC chow for 14days concurrent with either Fgf10 over-expression or inhibition of FGF signaling via expression of soluble dominant-negative FGF Receptor (R)-2IIIb.

RESULTS

After 14days of DDC treatment, there was an increase in periportal cells expressing FGFR1, FGFR2, and AKT-activated phospho-Serine 552 (pSer552) β-Catenin in association with up-regulation of genes encoding the FGFR2IIIb ligands, Fgf7, Fgf10, and Fgf22. In response to Fgf10 over-expression, there was an increase in the number of pSer552-β-Catenin((positive)+ive) periportal cells as well as cells co-positive for A6 and hepatocyte marker, Hepatocyte Nuclear Factor-4α (HNF4α). A similar expansion of A6(+ive) cells was observed after Fgf10 over-expression with regular chow and after partial hepatectomy during ethanol toxicity. Inhibition of FGF signaling increased the periportal A6(+ive)HNF4α(+ive) cell population while reducing centrolobular A6(+ive) HNF4α(+ive) cells. AKT inhibition with Wortmannin attenuated FGF10-mediated A6(+ive)HNF4α(+ive) cell expansion. In vitro analyses using FGF10 treated HepG2 cells demonstrated AKT-mediated β-Catenin activation but not enhanced cell migration.

CONCLUSIONS

During acute DDC treatment, FGF signaling promotes the expansion of A6-expressing liver cells partly via AKT-dependent activation of β-Catenin expansion of A6(+ive) periportal cells and possibly by reprogramming of centrolobular hepatocytes.

Sry HMG box protein 9-positive (Sox9+) epithelial cell adhesion molecule-negative (EpCAM-) biphenotypic cells derived from hepatocytes are involved in mouse liver regeneration

It has been shown that mature hepatocytes compensate tissue damages not only by proliferation and/or hypertrophy but also by conversion into cholangiocyte-like cells. We found that Sry HMG box protein 9-positive (Sox9(+)) epithelial cell adhesion molecule-negative (EpCAM(-)) hepatocyte nuclear factor 4α-positive (HNF4α(+)) biphenotypic cells showing hepatocytic morphology appeared near EpCAM(+) ductular structures in the livers of mice fed 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-containing diet. When Mx1-Cre:ROSA mice, which were injected with poly(I:C) to label mature hepatocytes, were fed with the DDC diet, we found LacZ(+)Sox9(+) cells near ductular structures. Although Sox9(+)EpCAM(-) cells adjacent to expanding ducts likely further converted into ductular cells, the incidence was rare. To know the cellular characteristics of Sox9(+)EpCAM(-) cells, we isolated them as GFP(+)EpCAM(-) cells from DDC-injured livers of Sox9-EGFP mice. Sox9(+)EpCAM(-) cells proliferated and could differentiate to functional hepatocytes in vitro. In addition, Sox9(+)EpCAM(-) cells formed cysts with a small central lumen in collagen gels containing Matrigel® without expressing EpCAM. These results suggest that Sox9(+)EpCAM(-) cells maintaining biphenotypic status can establish cholangiocyte-type polarity. Interestingly, we found that some of the Sox9(+) cells surrounded luminal spaces in DDC-injured liver while they expressed HNF4α. Taken together, we consider that in addition to converting to cholangiocyte-like cells, Sox9(+)EpCAM(-) cells provide luminal space near expanded ductular structures to prevent deterioration of the injuries and potentially supply new hepatocytes to repair damaged tissues.

Re-evaluation of liver stem/progenitor cells

Liver stem/progenitor cells (LPCs) are defined as cells that supply two types of liver epithelial cells, hepatocytes and cholangiocytes, during development, cellular turnover, and regeneration. Hepatoblasts, which are fetal LPCs derived from endoderm stem cells, robustly proliferate and differentiate into hepatocytes and cholangiocytes during fetal life. Between mid-gestation and the neonatal period, some cholangiocytes function as LPCs. Although LPCs in adult livers can be enriched in cells positive for cholangiocyte markers, their tissue localization and functions in cellular turnover remain obscure. On the other hand, it is well known that liver regeneration under conditions suppressing hepatocyte proliferation is supported by LPCs, though their origin has not been clearly identified. Recently many groups took advantage of new techniques including prospective isolation of LPCs by fluorescence-activated cell sorting and genetic lineage tracing to facilitate our understanding of epithelial supply in normal and injured livers. Those works suggest that, in normal livers, the turnover of hepatocytes mostly depends on duplication of hepatocytes. It is also demonstrated that liver epithelial cells as well as LPCs have great plasticity and flexible differentiation capability to respond to various types of injuries by protecting or repairing liver tissues.

Downregulation of miR-200a induces EMT phenotypes and CSC-like signatures through targeting the β-catenin pathway in hepatic oval cells

Hepatocellular carcinoma (HCC) can be derived from malignant transformed adult hepatic progenitor cells. However, the regulatory factors and molecular mechanisms underlying the process are not well defined. Our previous microRNA (miRNA) microarray analysis revealed a significant decrease of miR-200a level in F344 rat HCC side population (SP) fraction cells versus their normal counterparts. In the present study, we further investigated the effect of miR-200a on hepatic oval cell (HOC) phenotypes. We first confirmed downregulated miR-200a levels in rat hepatoma cells compared with WB-F344 cells. Next, by lentivirus-mediated loss-of-function studies, we showed that stable knockdown of miR-200a confers a mesenchymal phenotype to WB-F344 cells, including an elongated cell morphology, enhanced cell migration ability and expression of epithelial mesenchymal transition (EMT)-representative markers. Concomitantly, several cancer stem cell (CSC)-like traits appeared in these cells, which exhibit enhanced spheroid-forming capacity, express putative hepatic CSC markers and display superior resistance to chemotherapeutic drugs in vitro. Furthermore, bioinformatics analysis, luciferase assays and western blot analysis identified β-catenin (CTNNB1) as a direct and functional target of miR-200a. Knockdown of miR-200a partially activated Wnt/β-catenin signaling, and silencing of β-catenin functionally attenuated anti-miR-200a effects in vitro in WB-F344 cells. At length, in vivo xenograft assay demonstrated the acquisition of tumorigenicity of WB-F344 cells after miR-200a siliencing. Collectively, our findings indicate that miR-200a may function as an important regulatory factor in neoplastic transition of HOCs by targeting the β-catenin pathway.

Reprogramming fibroblasts into bipotential hepatic stem cells by defined factors

Recent studies have demonstrated direct reprogramming of fibroblasts into a range of somatic cell types, but to date stem or progenitor cells have only been reprogrammed for the blood and neuronal lineages. We previously reported generation of induced hepatocyte-like (iHep) cells by transduction of Gata4, Hnf1α, and Foxa3 in p19 Arf null mouse embryonic fibroblasts (MEFs). Here, we show that Hnf1β and Foxa3, liver organogenesis transcription factors, are sufficient to reprogram MEFs into induced hepatic stem cells (iHepSCs). iHepSCs can be stably expanded in vitro and possess the potential of bidirectional differentiation into both hepatocytic and cholangiocytic lineages. In the injured liver of fumarylacetoacetate hydrolase (Fah)-deficient mice, repopulating iHepSCs become hepatocyte-like cells. They also engraft as cholangiocytes into bile ducts of mice with DDC-induced bile ductular injury. Lineage conversion into bipotential expandable iHepSCs provides a strategy to enable efficient derivation of both hepatocytes and cholangiocytes for use in disease modeling and tissue engineering.

Functional consequences of WNT3/Frizzled7-mediated signaling in non-transformed hepatic cells

We have previously demonstrated that WNT3 and Frizzled7 (FZD7) expression levelswere upregulated in hepatocellular carcinoma (HCC) and that they directly interact to activate the canonical Wnt/β–catenin pathway in HCC cell lines. In this study, we investigated the functional consequences of WNT3 and FZD7 expression levels in non-transformed hepatic cells to address the question of whether WNT3/FZD7-mediated signal transduction could be involved in cellular transformation. After stable transfection of WNT3 and FZD7, the activation of the Wnt/β–catenin pathway was confirmed by western blot, immunostaining and quantitative real-time reverse transcriptase–PCR (qRT–PCR) analysis in two non-transformed hepatocyte-derived cell lines. In vitro characteristics of the malignant phenotype were measured, including cell proliferation, migration, invasion and anchorage-independent growth in soft agar. Stable expression of WNT3 and FZD7 in the two cell lines led to cellular accumulation of β-catenin and expression of downstream target genes activated by this pathway. In the stable WNT3/FZD7-expressing clones, hepatic cell proliferation, migration, invasion as well as soft agar colony formation were enhanced compared with the non-transformed control cells. The epithelial–mesenchymal transition (EMT) factors, Twist, Snail and Vimentin, were increased in cells expressing WNT3 and FZD7. However, the WNT3/FZD7-expressing cells did not form tumors in vivo. We conclude that activation of the WNT3/FZD7 canonical pathway has a role in the early stages of hepatocarcinogenesis by promoting the acquisition of a malignant phenotype with features of EMT.

Stem cells in liver diseases and cancer: recent advances on the path to new therapies

Stem cells have potential for therapy of liver diseases, but may also be involved in the formation of liver cancer. Recently, the American Association for the Study of Liver Diseases Henry M. and Lillian Stratton Basic Research Single Topic Conference "Stem Cells in Liver Diseases and Cancer: Discovery and Promise" brought together a diverse group of investigators to define the status of research on stem cells and cancer stem cells in the liver and identify problems and solutions on the path to clinical translation. This report summarizes the outcomes of the conference and provides an update on recent research advances. Progress in liver stem cell research includes isolation of primary liver progenitor cells (LPCs), directed hepatocyte differentiation of primary LPCs and pluripotent stem cells, findings of transdifferentiation, disease-specific considerations for establishing a therapeutically effective cell mass, and disease modeling in cell culture. Tumor-initiating stem-like cells (TISCs) that emerge during chronic liver injury share the expression of signaling pathways, including those organized around transforming growth factor beta and β-catenin, and surface markers with normal LPCs. Recent investigations of the role of TISCs in hepatocellular carcinoma have provided insight into the transcriptional and post-transcriptional regulation of hepatocarcinogenesis. Targeted chemotherapies for TISC are in development as a means to overcome cellular resistance and mechanisms driving disease progression in liver cancer.

Spontaneous repopulation of β-catenin null livers with β-catenin-positive hepatocytes after chronic murine liver injury

Prolonged exposure of mice to diet containing 0.1% 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) results in hepatobiliary injury, atypical ductular proliferation, oval cell appearance, and limited fibrosis. Previously, we reported that short-term ingestion of DDC diet by hepatocyte-specific β-catenin conditional knockout (KO) mice led to fewer A6-positive oval cells than wildtype (WT) littermates. To examine the role of β-catenin in chronic hepatic injury and repair, we exposed WT and KO mice to DDC for 80 and 150 days. Paradoxically, long-term DDC exposure led to significantly more A6-positive cells, indicating greater atypical ductular proliferation in KO, which coincided with increased fibrosis and cholestasis. Surprisingly, at 80 and 150 days in KO we observed a significant amelioration of hepatocyte injury. This coincided with extensive repopulation of β-catenin null livers with β-catenin-positive hepatocytes at 150 days, which was preceded by appearance of β-catenin-positive hepatocyte clusters at 80 days and a few β-catenin-positive hepatocytes at earlier times. Intriguingly, occasional β-catenin-positive hepatocytes that were negative for progenitor markers were also observed at baseline in the KO livers, suggesting spontaneous escape from cre-mediated recombination. These cells with hepatocyte morphology expressed mature hepatocyte markers but lacked markers of hepatic progenitors. The gradual repopulation of KO livers with β-catenin-positive hepatocytes occurred only following DDC injury and coincided with a progressive loss of hepatic cre-recombinase expression. A few β-catenin-positive cholangiocytes were observed albeit only after long-term DDC exposure and trailed the appearance of β-catenin-positive hepatocytes.

CONCLUSION

In a chronic liver injury model, β-catenin-positive hepatocytes exhibit growth and survival advantages and repopulate KO livers, eventually limiting hepatic injury and dysfunction despite increased fibrosis and intrahepatic cholestasis.

Beta-catenin signaling, liver regeneration and hepatocellular cancer: sorting the good from the bad

Among the adult organs, liver is unique for its ability to regenerate. A concerted signaling cascade enables optimum initiation of the regeneration process following insults brought about by surgery or a toxicant. Additionally, there exists a cellular redundancy, whereby a transiently amplifying progenitor population appears and expands to ensure regeneration, when differentiated cells of the liver are unable to proliferate in both experimental and clinical scenarios. One such pathway of relevance in these phenomena is Wnt/β-catenin signaling, which is activated relatively early during regeneration mostly through post-translational modifications. Once activated, β-catenin signaling drives the expression of target genes that are critical for cell cycle progression and contribute to initiation of the regeneration process. The role and regulation of Wnt/β-catenin signaling is now documented in rats, mice, zebrafish and patients. More recently, a regenerative advantage of the livers in β-catenin overexpressing mice was reported, as was also the case after exogenous Wnt-1 delivery to the liver paving the way for assessing means to stimulate the pathway for therapeutics in liver failure. β-Catenin is also pertinent in hepatic oval cell activation and differentiation. However, aberrant activation of the Wnt/β-catenin signaling is reported in a significant subset of hepatocellular cancers (HCC). While many mechanisms of such activation have been reported, the most functional means of aberrant and sustained activation is through mutations in the β-catenin gene or in AXIN1/2, which encodes for a scaffolding protein critical for β-catenin degradation. Intriguingly, in experimental models hepatic overexpression of normal or mutant β-catenin is insufficient for tumorigenesis. In fact β-catenin loss promoted chemical carcinogenesis in the liver due to alternate mechanisms. Since most HCC occur in the backdrop of chronic hepatic injury, where hepatic regeneration is necessary for maintenance of liver function, but at the same time serves as the basis of dysplastic changes, this Promethean attribute exhibits a Jekyll and Hyde behavior that makes distinguishing good regeneration from bad regeneration essential for targeting selective molecular pathways as personalized medicine becomes a norm in clinical practice. Could β-catenin signaling be one such pathway that may be redundant in regeneration and indispensible in HCC in a subset of cases?

Epiplakin1 is expressed in the cholangiocyte lineage cells in normal liver and adult progenitor cells in injured liver

We have previously identified Epiplakin1 (Eppk1) as a gene expressed in pancreatic progenitor cells. Here we studied the expression of Eppk1 in developing and regenerating livers in mice. Eppk1 is initially expressed in the early bipotential hepatoblasts and is later confined to the cholangiocytes. After birth, Eppk1 is expressed in the bile duct. In the livers of mice fed with a choline-deficient ethionine-supplemented (CDE) diet, Eppk1-positive cells dramatically increase in number. The Eppk1-positive cells express A6, thereby indicating that they are hepatic progenitor cells. Other cholangiocyte markers, such as Cytokeratins, E-cadherin, osteopontin and Sox9, are also co-expressed in the hepatic progenitor cells. Some of the Eppk1-positive cells express PCNA, a proliferation marker, thereby suggesting their identities as transient amplifying cells. In conclusion, we have shown that Eppk1 serves as a useful marker for detecting the hepatic progenitor population in the developing and adult liver. The use of Eppk1 as a marker will facilitate studies of mouse hepatic progenitor cells.

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.